Ergonomic Design

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Anthropometrics, Human
Factors & Ergonomics
Technological Design
What are Human Factors,
Ergonomics & Anthropometrics?
Anthropometrics
• Anthropometrics;
Anthropometrics is the data which concerns the
dimensions of human beings.
• Designers need to makes sure that the products they
design are the right size for the user and therefore
comfortable to use. Designers have access to books of
drawings like these which state measurements of human
beings of all sizes.
• Examples at work…
Knowing about percentiles is an important part of becoming a responsible designer.
Human Factors
• Human Factors;
Human factors involves the study of all aspects of the
way humans relate to the world around them, with the
aim of improving operational performance, safety,
through life costs and/or adoption through improvement
in the experience of the end user.
• The science of understanding the properties of human
capability (Human Factors Science).
• Examples at work…
Did you know that the U.S. military is responsible for the majority of data on
Human Factors!
It was a result out of WWII aircraft design and engineering.
Ergonomics
• Ergonomics;
•
Ergonomics is the scientific discipline concerned with designing according to
the human needs, and the profession that applies theory, principles, data
and methods to design in order to optimize human well-being and overall
system performance. The field is also called human engineering, and
human factors engineering.
•
Ergonomic research is primarily performed by ergonomists, who study
human capabilities in relationship to their work demands. Information
derived from ergonomists contributes to the design and evaluation of tasks,
jobs, products, environments and systems in order to make them
compatible with the needs, abilities and limitations of people
•
Examples at work..
A poorly designed work station can produce long term medical conditions.
Why should designers be aware of
anthropometrics, human factors and
ergonomics?
• Allows designers to accommodate various percentiles of
the population so the majority of people can use and
interact with the product or service being designed.
• Designers must be aware of human factors,
anthropometrics and ergonomics to ensure their product
or service is safe and socially responsible. (designing
public places is especially sensitive to these conditions)
The Impact of Human Factors,
Ergonomics & Anthropometry on Design
• A designer can use Human Factors,
Ergonomics & Anthropometry to their
advantage or these things may work
against their design. Good design
observes these qualities first because no
one wants to use or own a product or
service which carries out the task poorly or
dangerously.
Consider the following….
• A Toilet designed by a fashion designer
Fashion designers work to a fantasy of
what the human body looks like. They are
taught how to draw human figures in a
distorted, idealized way.
The impact designers can have
on society;
• The two figures in the middle are typical of
fashion design drawings. Designs are based on
these oddly proportioned, fantasy, body shapes.
• The figures on either side were statistical
averages from a series of anthropometrics
studies done with US military personnel. Whilst
limited to a select age range and profession,
these nonetheless are based on measurable
and observable reality. These are real body
shapes. ( From Human Dimension & Interior
Space by Julius Panero and Martin Zelnilk)
As illustrated in the two middle sketches of the human form.
The Result
• If a product designer were to work off the same
fantasy body shapes that fashion designers do,
a typical toilet would look like this.
• None of us would willingly climb a stepladder
every time we need to use our toilet - how silly
would that be? And yet, why is it that we
continue to try and fit into clothes that were not
designed for our bodies to begin with, or shoes
that are uncomfortable and damage our feet?
• This is most peculiar.
The result is a tall, narrow and most uncomfortable toilet.
Where can we find information on body
sizes, shapes, standard furniture sizes,
etc.?
• Human Factor Texts
• Resource Manuals
• Internet
• Making your own anthropometric data
Henry Dreyfuss, one of America’s first
Industrial Designers was instrumental in
using human dimensions to
Improve the products people interact with
on a daily basis.
Henry Dreyfuss; One of America’s First
Industrial Designers
• Dreyfuss was born in Brooklyn, New York. As
one of the celebrity industrial designers of the
1930s and 1940s, Dreyfuss dramatically
improved the look, feel, and usability of dozens
of consumer products. As opposed to Raymond
Loewy and other contemporaries, Dreyfuss was
not a stylist: he applied common sense and a
scientific approach to design problems. His work
both popularized the field for public
consumption, and made significant contributions
to the underlying fields of ergonomics,
anthropometrics, and human factors.
Some of Dreyfuss’ Designs...
Did you know that John Deere hired Dreyfuss to “Modernize” the look of the
tractor.
What do we do with all of this data
on the human form?
• In the first slide we observed that there are individual
differences in human characteristics. These follow a
normal distribution. This is true with anthropometric
measurements.
• You may have heard the expression "to design for the
5th percentile female to the 95th percentile male." This
means that for the selected anthropometric measure,
such as height, the lower limit of our range is the height
of a 5th percentile female and the upper limit is the
height of a 95th percentile male. This range
accommodates 90% of the population for that one
selected measure.
Population Variance
• We again use the concept of "population." This
is important in anthropometrics as there are
differences in size and body segment
proportions due to age, gender, and ethnicity.
So, to properly select the data to use, we must
know something about our population
composition, and we must know what
percentage of the population we wish to
accommodate. The anthropometric range will
be much different if we are designing products
for male, professional basketball players than if
we are designing for the general public.
Application of the Anthropometric Data
• In choosing the proper anthropometric
measurements to use, we must know not only
the user population, but also the specific
application or design problem. If we are
designing overhead luggage racks for public
transportation, accommodating 90% of the rider
population is probably sufficient. However, if we
are determining the position of an emergency
button, we should design to accommodate 99%
of the rider population, including wheelchair
users.
A Guide to Designing with Human
Factors in mind.
Step 1. Understand Organizational/Mission Need
Step 2.Understand and Define Context of Use
Step 3.Perform Function Analysis
Step 4.Allocate Functions
Step 5.Analyze and Design Tasks
Step 6.Design Human-to-System Interfaces &
Workstations
User/Human-Centered Design Steps
References, Resources & Links
•
•
http://www.ergonomics4schools.com/lzone/anthropometry.htm
www.baddesigns.com
(a great website to help illustrate bad design when thinking of
human factors)
• http://www.k-state.edu/udlearnsite/Lesson4.htm
• http://www.hf.faa.gov/Webtraining/HFModel/Variance/anthropometri
cs1.htm
(an excellent site with a quiz)
• http://www.sil.si.edu/exhibitions/doodles/cf/convincing.cfm
Human factors and ergonomics
Something , somewhere went terribly wrong
Dr. Samaresh Das
Ergonomics
Ergonomics
o Ergonomics is the science and the art of fitting the
job and the workplace to workers’ needs.
o It is the study of work & way to make jobs / tasks
in a better way
o It is a way to make work easier
Why Ergonomics?
To reduce the risk of
•Accidents
•Injury
• Ill health due to poor ergonomics
Reduce Sickness absence / Costs
Increase Performance / Output
Everyone in any organization is at risk and it is not just
“heavy” or “physical” jobs that cause injury
So our aim should be FEEL BETTER ,
WORK SMARTER
Assessments……
By assessing all aspects of:
o Individuals and the jobs they perform e.g. Their
physical capabilities, tasks, equipment ,tools and
working environment
oTo design work systems that are safe, flexible,
efficient and productive.
Health Issues Associated with
Poor Ergonomics
Work related
upper limb disorder
Back pain / Injuries
Psychological problems (Stress)
Musculoskeletal Disorders- MSDs
o Musculoskeletal Disorders affect the muscles, nerves
and tendons. They are:
o Carpal Tunnel Syndrome
o Tendinitis
o Rotator cuff injuries (shoulder problem)
o Epicondylitis (elbow problem)
o Muscle strains and low back pain
Back Pain & Injuries
oBending, Twisting, and Lifting
o Incorrect Posture
o Prolonged Sitting and/or Standing
o Slips & Fall
o Exposure to Vibration
Work Related Upper Limb Disorders
continued “over use” can lead to permanent damage through:
oRepetitive actions
oFrequent applications of force
oUnnatural postures/positions
oInadequate rest and recovery
oInadequate physical preparation (warming up)
Psychological Factors
Person under too much “pressure” may be more at risk at
o Physical / Ergonomic Injury
o Fatigue
o Accidents ( mistakes, inattention, saving time, shortcut’s)
o General ill health (run down / poor condition)
o Substance Abuse
The Worker & Ergonomics
Two Basic Objectives
o Match the requirements of a task to the individual
o Optimise the design of the task to the individual to reduce the
risk of injury, ill health and discomfort.
o E.g.: Work stations may need to be adjustable so that they
suit a range of people.
The Worker & Ergonomics
The Worker
Each worker is unique:
oSize & Shape
o Age and Gender
o Race and Language
o Physical Ability (Health & Fitness)
o Limitations Vulnerabilities, Disabilities, Mental Ability
o Experience
oIt is very difficult to optimise a task or a workplace to suit
everyone
People and Comfort….
Different views about
• Temperature
• Ventilation
• Lighting
• Background Noise
• Isolation
• Overcrowding
• Communication
Discomfort will influence how a person will work
The Risk of Injury….
o Doing something too frequently without break
o Work in awkward position/ angles
o Workstation is not “fit”
o Working under discomfort and significant pressure
Office Ergonomics
Office of horrors
Ideal Office
Good ergonomics
Ergonomics
Good ergonomics
o
o
o
o
o
One of the biggest injury risk factors is static posture.
Try to spend at least 5 minutes /hr hour away from your computer.
Remember to ONLY stretch to the point of mild tension.
Try to incorporate the stretches into your daily routine.
This slide provides some illustrations of simple active stretches to
perform at the office.
Hand Exercises
o Tightly clench your hand into a fist and release, fanning out the
fingers. Repeat 3 times
Back and Shoulder Exercises
Stand up straight, place your right hand on your left
shoulder and move your head back gently. Do the
same thing for the right shoulder
Head and Neck Exercises…
o Move head sideways from left to right and back to
left
o Move head backwards and then forward
Stretches…..
Stretches Cont……
Stretches Cont……
Home Ergonomics
oIt is about making home more comfortable, efficient and user-friendly
living space.
oThe ergonomics of home greatly affect body and overall health.
Kitchen
oInstall a cushioned mat to stand
o Use oven that is mounted near chest height, eliminating
the need to bend over.
o Choose a refrigerator that has a bottom-mounted freezer,
which reduces the need to bend over when accessing the
main body of the fridge.
Living Room
o Furniture should be easy to move
o Avoid couches that are too low and without a proper
lumbar support
o Avoid sitting in front of the television in a position
where neck is maintaining an upward tilt
o When eating in front of the television, place food on a
surface that is high enough to eliminate the need to
bend over to eat.
Bedroom
oUse cervical pillow that supports the natural curve of
neck.
o Use products that properly support your neck while
reading or watching television in bed
o Use a mattress that supports your spine
Bathroom
oUse bath, floor mats and install hand bars to prevent slips
and falls.
oBathroom sinks and showerheads not be too low
Driving Ergonomics
Driving Ergonomics
For back Support
o Choose
a vehicle that sits high - an SUV instead of a
sports car
oEnter the car first by sitting down and then swinging
your legs under the wheel
o To leave your vehicle, slide the car seat back before
swinging your legs out and planting your feet on the
ground.
oLook for cars with automatic transmissions and power
steering.
oUse a lumbar support cushion and add foam wedges to
the seat to elevate pelvis
For Neck and shoulder
o Avoid
leaning forward when sit in the driver’s seat
o Position the car seat comfortable and not stretching
o Make sure there is sufficient room between head and
the roof of the car
Optimal car seat
o Choose
a comfortable and supportive seat
o Confirm all adjustment mechanisms are easy to use
o Make sure the seat material does not create discomfort
and all parts of the seat provide adequate support
Ergo Driving Break
o To
reset spine and alleviate pressure caused by
prolonged sitting, take advantage of red lights or sitting
in traffic by doing some simple stretches
oHolding a steering wheel in awkward postures or too
tightly can cause carpal tunnel syndrome
Ergo Checklist
Thanks
Anatomy and Posture
Ergonomics
BMMD 3553
Introduction
• Understand the structure of the body – Human Anatomy
• Describe the posture risk of neck pain, headaches and arm pain
• Describe the causes and biomechanics of lower back
Human Anatomy
What isanatomy
• Study of the structure and
relationshipbetween body parts
Basic Human BodyAnatomy
• Using a rather crude analogy: The tent analogy
Canvas
Tent pole
Guy ropes
Basic Human BodyAnatomy
• The skeleton can be likened to an articulated tent pole with guy
ropes (postural muscles) on every side.
• The fabric of the tent corresponds to the soft tissues of the body.
• Ligaments can be likened to the springs and rubber fittings that
stabilise the articulations of the tent pole,
Basic Human BodyAnatomy
The tendons are the ends of the guy ropes where they insert into the poles.
Tendons : connects muscles to thebone
Ligaments : connects bones to bones
Conclusion: Postural Stress can cause pain and discomfort!
Anatomy of the spine
Spine is made up of 33 individual bones called vertebrae, which are stacked
together by different joints to form a column.
There are three natural curves in our spinal column that allow us to transfer
loads and distribute stress very efficiently.
three natural curves
Anatomy of the spine
Five regions of
the spinal column
Anatomy of thespine
Intervertebral discs
Each vertebra in your spine is separated and cushioned by an intervertebral
disc, keeping the bones from rubbing together.
Facet joints
The facet joints of the spine allow forward and back motion. Each
vertebra has four facet joints, one pair that connects to the vertebra above
(superior facets) and one pair that connects to the vertebra below (inferior facets)
Anatomy of theskeletal and muscular
Force and Stress in Human Body
• Human body is only able to withstand a finite amount of force
and stress
• May be imposed externally or internally
• Can be induced by:
• The posture
• Type of task
Task and Posture
Task and Posture
• Task and postural stress can vary independently of
each other
• Ex: Lifting a barbell, can be high in task stress but
can be performed in non stressful postures.
What is the problem with poor posture?
•
Poor posture results from changes (accentuating or
minimizing) in the natural curves of spine which places a
lot of abnormal stress and strain on the joints, muscles and
nerves.
•
How does this happen? Well it all comes down to
biomechanics –
Posture Risk for Neck Pain, Headaches and Arm Pain
• The average adult human head weighs approximately 4.5 – 5.4kg.
In normal posture the cervical spine is effective at withstanding this
weight.
• Poor posture facilitates a forward head position that actually makes
your head “heavier”.
• This leads to muscle strains and irritated joints because your
muscles have to work harder to stabilize the spine.
• If this occurs for prolonged periods, it can lead to headaches and
neck pain.
19.0 kg
14.5 kg
5.4 kg
Forward head posture makes your head “heavier”.
• Forward head posture and rounded shoulders also puts
a lot of compression on the nerves that originate from
the neck and supply the upper limbs.
Shoulder
Arm
Forearm
Hand
• If nerves get compressed, they get irritated which can
lead to pain, numbness, tingling and/or weakness in the
arms, hands and fingers.
Low Back Pain
Posture Risk for Low Back Pain (LBP)
•
Lumbar spine (aka low back) is predisposed to a phenomenon
called “creep” from sitting with a slumped posture. Creep is when
the soft tissues (ie. muscles, ligaments) adapt from the stress of
being in a flexed posture for too long by becoming longer.
•
This is a problem because muscles become weaker and more
prone to straining, spinal joints become prone to irritation and
discs are at risk of being injured
slumped posture
upright posture
Low Back Pain
• Canbe caused by many things
• Most common cause for limiting people fromwork
• Afflicts 80%of people sometime in their lives
• Surprisingly….. “ In the long run, surgery, chiropractic
care etc are considered no more effective than no
treatment in reducing low back pain”
Chiropractic:
a form of alternative medicine that emphasizes diagnosis, treatment and prevention of mechanical disorders of the
musculoskeletal system
Low BackPain
•75%-80% of Americans experience low back pain sometime during life.
•Second only to the common cold in causing absence in the workplace.
• Mechanical stress & psychosocial.
• Low back and shoulder girdle pain are major problems in the industrialized
world.
• Acute pain is usually gone, but chronic LBP may need medical intervention
Causes ofLBP
• Can be very elusive for clinicians even!
• Not from the disc itself, since itdoes not have nerve endings
• Could possibly come from the posterior ligaments and backmuscles
• Irritated by mechanical trauma due damage or
degeneration of bony structures
•Nerve compression may also be the source of pain.
• Can also be caused by non-work related such as kidneys
• Back pain is a complex problem (Waddell, 1982) and detailed investigation
of back problems is best left to expertclinicians.
• Our job is to search for causes in theworkplace.
Risk factors forLBP
• Mechanical loading
• Whole body vibration (fork lift drivers)
• Twisting upper body
• Constrained sitting for 8 hours perday
Common Injuries of The Back
• Low Back Pain
• Soft Tissue Injuries
• Acute Fractures
• Stress Fractures
• Disc Herniations
• Whiplash Injuries
Diagnostic Devices for Muscle Problems e.g; Low Back Pain
• Performed via electromyography readings of the backmuscles as depicted in
below figure.
Display monitor
Data acquisition
system
EMG sensors
Application
• Electromyography (EMG) is a diagnostic procedure that evaluates
the health condition of muscles and the nerve cells that control them.
Biomechanics of LowBack
• Your back extensor muscles works very, very hard
• They help to maintain your backposture
• Example:
An object is held 50cm in front of yourbody. The object weighs 30kg.
• Your back muscles have to exerta force equivalent to 300kg !
Biomechanics of LowBack
• The total compressive force acting on the spine (Ct), is calculated as
follows:
• Ct = (Compressive force due to upper bodyweight)
• + (Compressive force due to load)
• + (Compressive force due to back muscle contraction needed to maintain posture)
Forces Acting on the Spine
• Forces acting on the spine include:
•
•
•
•
Body weight
Tension in the spinal ligaments
Tension in the surrounding muscles
Intra abdominal pressure
• The major form of loading on the spine is:
• Axial loading
Upright Position
• Spinal compression
•
Resulting from:
Body weight + Weight held by arms and hands.
• When standing upright
•
•
Total body center of gravity is anterior to the
spinal column.
Spine is placed under constant forward bending
moment.
Torque
• Defined: The rotary effect of a force about an axis of
rotation, measured as the producer of the force and the
perpendicular distance between the force’s line of action
and the axis.
• To maintain an upright position
- Torque is counteracted by tension in the back
extensor muscles.
Spinal Muscles Role In Lifting
• Spinal muscles have small moment arms with respect
to the vertebral joints.
• Have to generate large forces to counteract the
torque produced about the spine by body weight and
objects being lifted.
Erector Spinae
Muscles
moment arms
Spinal Erector Muscle
• The erector spinae or spinal erectors is a
set of muscles that straighten and rotate the
back
Why Lift WithThe Legs?
• Back muscles, with a moment arm
of approximately 6 cm, must counter
the torque produced by the weights
of the body plus any external loads.
• T= F x r
(r : moment arm = 6 cm)
Solutions
Bending moment
• Tp = Fp x Lp
(Lp : moment arm = 0.4 m)
= 200 x 0.4 (Fp : load force= 200 N)
= 112.5 Nm
Muscle force
TM = Fm x Lm
192.5 = Fm x 0.05
Fm = 192.5 / 0.05
• Tw = Fw x Lw
(Lw : moment arm = 0.25 m)
= 450 x 0.25 (Fw : body force= 400 N)
= 80 Nm
• TM = Tp + Tw
TM = 192.5 Nm
Fm = 3850 N
Compressive Forces on the Spine
Lumbosacral (L5-S1 disc)
• Definition: Of or relating to or near the small of the back and the back part
of the pelvis between the hips. The lumbosacral junction consist of the
L5 vertebral body articulating with the first sacral vertebral body. In the
seated position the lumbosacral discs are loaded three times more than
standing.
• The L5-S1 disc is deep in the pelvis, and does not have much motion. It is
also connected to the sacrum by a large ligament (sacral-ala ligament)
which helps limit motion.
Sacrum
vertebrae
Lumbar vertebrae
Question: How much torque is developedby
the erector spinae muscles with a Fm 6 cm?
• 1 lb. = 4.448 Newtons
• Segment Weight Moment Arm
Head
Trunk
Arms
Box
58 N
328 N
81 N
111 N
• Torque at L5-S1=
• Force?
• Fm=
25 cm
10 cm
20 cm
40 cm
1 lb. = 4.448 Newtons
Segment
Head
Trunk
Arms
Box
Weight
13 lbs. (58N)
73.75 lbs.(328N)
18.2 lbs. (81N)
24.95 lbs. (111N)
Moment Arm
25 cm
10 cm
20 cm
40 cm
Torque at L5-S1=
(328N)(10cm) + (81N)(20cm) + (58N)(25cm) + (111N)(40cm) = ?
Ans:
Torque at L5-S1 = 10,790 Ncm
Force?
Ans:
0 = (Fm)(6cm) - 10,790 In static position, sum of the torques acting at any point is zero.
Fm = 1798.33 N or (404.30 lbs.)
Problem for a 61.23kg Person
• How much force must be developed by the erector spinae
with a moment arm of 6 cm. From the L5-S1 joint center to
maintain the body in a lifting position with segment moment
arms as specified?
• Segment
• Head
• Trunk
• Arms
• Box Lifted
• Torque ?
Weight
50 N
280 N
65 N
100 N
Moment Arm
22 cm.
12 cm.
25 cm.
42 cm.
Segment
Weight
Moment Arm
• Head
50 N
22 cm.
• Trunk
280 N
12 cm.
• Arms
65 N
25 cm.
• BoxLifted
100 N
42 cm.
Torque at L5-S1=
(50N)(22cm) + (280N)(12cm) + (65N)(25cm) + (100N)(42cm) = ?
Ans:
Torque at L5-S1 = 10,285 Ncm
Force?
Ans:
0 = (Fm)(6cm) - 10,285 In static position, sum of the torques acting at any point is zero.
Fm = 1714 N
A cadaver, also referred to as a corpse (singular) in medical, literary, and legal usage, or when
intended for dissection, is a deceased body.
What Does The Research Show?
• % Load Compression on L3 during the upright
standing, lying down, and sitting.
• Compression increases more with spinal
flexion, and increases still further with a
slouched sitting position.
Posture
Spinal compression tolerancelimits (SCTL)
• Much is now known about the strength of spinal motion
segments and their component discs and intervertebral bodies,
owing to the in-vitro studies of researchers.
• These studies typically involve the removal of motion segments
from cadavers and testing them to failure in a compression
testing machine.
In vitro (meaning: in the glass) studies are performed with microorganisms, cells, or biological molecules outside their normal biological
context. Colloquially called "test-tube experiments", these studies in biology and its subdisciplines are traditionally done in labware such as test
tubes, flasks, Petri dishes, and microtiter plates. Studies conducted using components of an organism that have been isolated from their usual
biological surroundings permit a more detailed or more convenient analysis than can be done with whole organisms; however, results obtained
from in vitro experiments may not fully or accurately predict the effects on a whole organism.
Spinal compression tolerancelimits
• The spinal compression tolerance limit (SCTL) is the
maximum compressive load to which a specified
motion segment can be exposed without failure
• If the estimated load exceeds the SCTL, then the tasks
must be redesigned, by reducing either the load or the
load moment.
Spinal compression tolerancelimits
• Ayoub and Mital (1997) quote SCTLs of 6700 N for people
under 40 years of age and 3400 N for people over 60.
• Summary = SCTL decreases with age.
Can low back pain beprevented?
• In general, the evidence that low back pain can be prevented in
the general population is not promising.
• There seems to be some evidence that exercise is beneficial,
The mechanisms by which exercise may help are unknown.
• It is of interest that Stevenson et al. (2001) found that personal
fitness is an important defence against low back pain.
THE END
Biomechanics of Works
Introduction
“Low Back Pain remains the most prevalent
and costly work-related injury.”
[Liberty Mutual Research Center Research
Report, 1998]
Objectives for today
Describe what happens to the lower back
in term of biomechanics;
 Understand the common pitfalls
associated with lifting;
 Describe the appropriate manual
handling design techniques.

Introduction
 In the USA, about 500 000 workers, suffer
some type of overexertion injury per year.
 In the UK, more than 25% of accidents
involve handling goods in one way or another
(Health and Safety Commission, 1991).
 In Malaysia, 1,111 cases involving back
injuries were reported in 2007 (SOCSO Report)
Introduction

Hoogendoorn et al. (2000) found an
increased risk of low back pain in workers
who lifted a 25 kg load more than 15 times
per day.
 Magora (1972) found that low back
symptoms were more common in workers
who regularly lifted weights of 3 kg or
more than in those who sometimes lifted
such weights.
Introduction
 When carrying out manual handling
tasks, the weight of the load being lifted
is transferred to the spinal column in the
form of compression and shear forces.
 The compression and shear are greater
when the load is lifted quickly because
higher forces are needed to accelerate
the mass
from rest, according to
Newton’s laws of motion.
Introduction

Spinal compression is increased
when loads are lifted and is increased
even more when they are lifted
quickly and
when the posture is
imbalanced.
 Lifting technique does influence
manual handling efficiency
Introduction
 According to Grieve and Pheasant (1982), the
trunk can fail in these ways when a weight is lifted:
 1. The muscles and ligaments of the back can fail under
excessive tension.
 2. The intervertebral disc may herniate as the
nucleus is extruded under excessivecompression.
 These injuries are often referred to colloquially as
‘muscle strains or tears’,
‘slipped discs’ and
‘hernias’.
Introduction

A catastrophic injury such as a disc
prolapse is not simply caused by a
sudden event such as lifting a heavy
weight.

It is usually the end product of years of
degeneration of the disc and surrounding
structures.
Prevention of manual handling
injuries in the workplace
Prevention of manual handling
injuries in the workplace
Prevention of manual handling
injuries in the workplace

The most common approach (as well as most
useless!) in most industries is to train workers
to lift safely.
 The
notion that it is safer to ‘lift with the knees
and not with the back’, that people can be
trained to lift safely and that injury will be
prevented, is deeply ingrained (difficult to
change).
Prevention of manual handling
injuries in the workplace

Despite the large number of studies that have
shown no benefits of the training, training is a
popular approach.

Snook et al. (1978) compared three
approaches to low back injury prevention:



preemployment/pre-placement selection
training in lifting techniques
job design
Prevention of manual handling
injuries in the workplace

The findings showed :no difference in the
proportion of injuries in companies that
did or did not train their workers in lifting
techniques,

Nor were there any effects due to
selection based on medical screening.
Prevention of manual handling
injuries in the workplace
But………significantly fewer back injuries were
found in companies where the loads were
acceptable to more than 75% of the
workforce.
Prevention of manual handling
injuries in the workplace

Snook et al concluded that workers are 3
times more likely to hurt their backs when
performing exertions acceptable to less
than 75% of the workforce.
Strength Testing:
Useful/Useless?

It has been noted in some literature that
strength testing for worker selection can
reduce the risk of injury.

Theidea : Job Demand =Physical Ability

But , in general strength testing will not
prevent the occurrence of low back pain.
Safe lifting
techniques.
Dangerous
assumptions
about manual
handling safety

Assumption # 1
Assumption No. 1. The techniques being
taught are safer, inpractice

Although there is some evidence that lifting
from a squatting position is safer than lifting
from a stooping position,
a. Squat lifting

b. Stoop lifting
squat lifting uses weights specially designed to
be lifted from a squatting position


Most weights in industry are not
designed to be lifted from a squatting
position
In some situations, as in lifting an unstable
load squat lifting techniques may actually
increase the load moment or they may
be completely impractical.
Assumption # 2

Assumption No. 2. ‘Safe’ techniques are
usable and have no ‘Hidden Costs’


Squat lifting techniques require greater
coordination and control than the
alternatives and also place a higher load
on the cardiovascular system and the
knees.
For one-off lifts, the additional demands
may be acceptable, but for repetitive lifting
they soon take their toll.

The knees weaken rapidly beyond about 60
degrees of knee flexion and the knee
ligaments are at increasing risk of rupture
(see Grieve and Pheasant, 1982, for further
discussion)
 Rabinowitz
et al. (1998) found that:
1.stoop lifting was associated with greater
back pain and
2.squat lifting with greater knee pain.

Repetitive squat lifting for 15 minutes placed
an escalating cardiovascular load of an extra
26 heart beats/min compared with stoop
lifting. People rated the task as ‘somewhat
hard’ compared with stoop lifting, which was
rated as ‘light’.
Assumption # 3

Assumption No. 3. The training will transfer
to the work situation


The author knows of only one study that
demonstrates long-term (6-month) change
in lifting technique as a result of manual
handling training
Although you can teach an old dog new
tricks, the old ones persist in long-term
memory, and will dominate their behavior

Ergonomists have long understood the
principle that well-learnt behaviors cannot
be ‘unlearnt’.

As soon as we cease to monitor our
performance, old habits tend to return.
Assumption # 4

Assumption No. 4. Any reductions in risk
are large enough to protect people from
injury


Training people to ‘make more use of the
legs’ does not guarantee lower back stress.
Although squat lifting may reduce back
stress by lowering the load moment, it is
less
clear whether the reduction is
sufficient to prevent injury.

Spinal tissues have a compression
tolerance limit or threshold (Genaidy et
al., 1993)

A lowering of the load will only bring
about a reduction in injury rates if the
compressive forces are bought below
threshold: if the absolute level of risk is
reduced to a safe level
A reduction in risk is not the same
as an improvement in safety. This
is clearly recognized in the
European Union manual handling
guidelines, which state that manual
handling should be avoided as
much as possible.
Assumption # 5

Assumption No. 5. There are no
perverse outcomes associated with the
use of ‘Safe’ handling techniques
There is experimental evidence that people
will lift heavier weights when they feel
safe than when they feel unsafe (McCoy et
al., 1988).
 Estimates of MAWL were over 50% higher
when
told
that
squat
lifting
was
implemented
(Bridger
and
Friedberg
(1999)


The implication is that manual handling
training could act as a barrier to change
by creating the impression that
‘something has been done’.
Designing handling tasks

3 principles of industrial medicine:



Remove the threat;
Remove the operator;
Protect the operator.
Designing handling tasks
Design of handling task
Design of handling task

The characteristic of the load is
also important:


20kg of lead is considered “lighter” than
20 kg of feathers since it can be held
closer to body.
Containers for one- or two-handed
handling should be designed as small as
possible so that the load is kept close to
the body.
Abdominal Belts :Healthor Hoax?

The practice of wrapping materials around the
waist with the aim of improving posture and
poise is found throughout history and across
cultures.

Shah (1993), for example, reports that in Nepal
most people who lift and carry heavy weights wrap
a 5-metre length of cloth (called a ‘Patuka’) around
the waist before work
Abdominal Belts
Abdominal Belts :Healthor Hoax?

Abdominal belts are thought to protect
workers by restricting undue flexion or
rotation of the spine

By increasing the Intra Abdominal Pressure
(IAP) the spine is protected indirectly.
 But
the scientific evidence is yet to be
there about back belts…..
Studies regarding back belts
 DO ABDOMINAL BELTS INCREASE IAP WHEN
WORN?
 McGill et al. (1990) measured back extensor
EMG and IAP when subjects lifted weights
wearing a competition weightlifter’s belt.
 IAP did increase, but no reduction in back
extensor muscle activity.
 When subjects held their breath when lifting,
increases in IAP were also observed and were
accompanied by reductions in back extensor
EMG, irrespective of whether a belt was worn

DOES WEARING AN ABDOMINAL BELT REDUCE
BACK MUSCLE FATIGUEWHEN LIFTING?

Ciriello and Snook (1995) measured fatigue of the
back extensors in 13 male industrial workers who
lifted average loads of 28.1 kg, 4.3 times per
minute, for 4 hours a day

Made little to no difference at all to the back
muscles fatigue

DOES WEARING AN ABDOMINAL BELT HAVE
AN OVERALL PROTECTIVE EFFECT?

Miyamoto et al. (1999) demonstrated that
abdominal belts raise the intramuscular pressure
in the erector spinae muscles and stiffen the
trunk, which may be beneficial during lifting
and during other work where the trunk is
exposed to de-stabilising forces.

DOES WEARING AN ABDOMINAL BELT
GIVE LIFTERS AN INCREASED SENSE OF
STABILITY AND SECURITY?

McGill et al. (1990), Reddell et al. (1992) and
Magnusson et al. (1996) all report that
wearing either competitive weightlifters’ belts
or abdominal belts for industrial
workers
increases the sense of security.
 DO
ABDOMINAL BELTS PROTECT
INDUSTRIAL WORKERS IN PRACTICE?

Walsh and Schwartz (1990) divided 90 grocery
warehouse workers into three groups in a 6month investigation.
 Group
1 ( control group)
 Group 2 (Back pain prevention training)
 Group 3 (Training + Back belts)
 Results?

There were no statistically significant
differences in injury rates or productivity
between the three groups over the study
period. Lost time was significantly lower in
group 3, however (2.5 days lower, on
average).

Reddell et al. (1992) evaluated an
abdominal belt and back program among
a group of airline baggage handlers.

Lost workdays and back injuries were not
reduced, but back injuries increased and were
more severe after belt use was discontinued

IS
ABDOMINAL BELT WEARING
HAZARDOUS
FOR
WORKERS
WITH
LATENT CORONARY HEART DISEASE?

Blood pressure and heart rate were higher
when the belt was worn, leading to the
conclusion
that
cardiac-compromised
individuals are probably at greater risk when
exercising while wearing back supports.
Ergonomic problem:
The worker picks up a
carton from a 27 inch
high conveyor system. The
worker turns and stacks
the carton on a pallet
located at floor level.
What are the ergonomic
risk factors?
Propose a solution to this
problem.
Workers had to transfer boxes weighing 20 kg or more
from one place to the shelves using a cart. Boxes had
to be lifted from the cart to the shelves. Back pain is a
significant problem for workers handling this job.
1) What are the ergonomic risk factors present in this
job?
2) Propose a solution for this problem.
The NIOSH approach tothe design
and evaluation of lifting tasks
 Recommended
Weight Limit (RWL),
maximum acceptable weight (load) that
nearly all healthy employees could lift over
the course of an 8 hour shift without
increasing the risk of musculoskeletal
disorders (MSD) to the lower back.
 Lifting
Index (LI) is calculated to provide
a relative estimate of the level of physical
stress and MSD risk associated with the
manual lifting tasks evaluated.
The NIOSH approach tothe
design and evaluation of lifting
tasks

Recommended WeightLimit
(RWL): Answers the question… “Is this
weight too heavy for the task?”

LiftingIndex (LI): Answers the question…
“How significant is the risk?”
The NIOSH Lifting Equation
The NIOSH Lifting Equation
Values of coupling multiplier CM for
use in the 1991 NIOSH equation for
determining RWL
Lifting Index (LI)
A
Lifting Index value of less than 1.0 indicates a
nominal risk to healthy employees. A Lifting
Index of 1.0 or more denotes that the task is
high risk for some fraction of the population.

As the LI increases, the level of low back injury
risk increases correspondingly. Therefore, the
goal is to design all lifting jobs to accomplish a
LI of less than 1.0.
Lifting Index (LI)
ℎ
= 

EXAMPLE
EXAMPLE
EXAMPLE
BMMD 3553 Ergonomics Design
Anthropometric application
and workspace design
4.1 INTRODUCTION
What is anthropometry?
∗ Measurement of the human body.
∗ This term is derived from Greek words
“Anthropos” (human) and
“metrein” (measure)
Anthropometry is the study of
human sizing - the dimensions of the
different parts of the body
∗ Anthropometric
information
describes
the
dimensions of the human body, usually through the
use of bony landmarks to which height, breadths
(width), depths, distances, circumferences (linear
distance around the outside of a closed curve or
circular object) and curvatures are measured.
Anthropometry and its uses
∗ Body size and proportion vary greatly between different
population and racial groups-a fact which designers must
never lose sight of when designing for international market.
Importance of anthropometric
considerations in design
If a piece of equipment…
Would fit
roughly
- 25% of Thais
- 10% of Vietnamese
∗ It is usually impracticable and expensive to
design products individually to suit the
requirements of every user.
∗ Mass-produced and designed to fit a wide
range of users -the custom tailor, dressmaker,
and cobbler are perhaps the only remaining
examples of truly user-oriented designers in
western industrial societies.
Availability of anthropometric
data
∗ Anthropometry of military populations is usually
well documented and is used in the design of
everything from cockpits to ranges and sizes of
boots and clothing.
∗ Data are available for U.S., British, and other
European groups, as well as Japanese citizens.
Availability of anthropometric
data
∗ Pheasant (1986) provides a useful and wellillustrated collection of anthropometric data
and a method of estimating unknown
anthropometric dimensions from data on
stature.
∗ Problems with much of the anthropometric
data from the United States and Europe are
the age of the data and the lack of
standardization across surveys.
4.2 designing for
population of users
INTRODUCTION
Good ergonomic design makes provision
for the range of variability to be expected
in the user population.
Variation in user population can also affect
design for safety.
Thus, ergonomic design is important for
human use and safety based on population
of user.
What is Population ?
A group of people
sharing common
ancestors,
common
occupations,
common
geographical
locations or age
groups.
A user population
may consist of
people from
different races
(i.e. groups
differing in their
ancestry) or
different ethnic
groups (different
cultures, customs,
language, and so
on).
Data Sampling
Surveys can only measure a sample of the people they are
interested in. Samples sizes range from 10's to 1000's,
depending on the scope and purpose.
In order to have a good match between the sample and the
'population', generally a mix of random and targeted selection
is used, to make sure for example that a minority group has
enough representation.
The larger the sample, the less likely it is to have an unexpected
bias
Sampling the Population
It's a characteristic of human variation that most people are
near to the average, then there are proportionately fewer
and fewer people towards the extremes.
In ergonomics it is normally the extremes that we are
interested in, because that is where any given aspect of a
design will start to "not fit". The percentage of people who
are smaller than a given size is called a "percentile", and
typically designs are specified to fit from 1st/2nd/5th
percentile to 95th/98th/99th
Percentile Humans
Anthropometric dimensions for each population are ranked by size and described
as percentiles.
Engineering Anthropometry for Design
∗ Design Idea
∗ Accommodate the body characteristics of the
population
∗ Universal operability is 90-95% of the
population
∗ Build in adjustment to meet objectives
∗ Some dimensions only require one set of
dimensions
∗ Example: 95% reach
Human Variability
∗ Is there a Average Human?
∗ Humans vary in dimensions based on
∗ Gender
∗ Ethnic groups
∗ Nationalities
∗ Over 300 anthropometric measurements on the
body
∗ It is hard to say that any one person is 50%-tile on all
measurement.
Design and Use of Anthropometric Data
∗ Design for the Extreme -- An attempt to accommodate
all (or nearly all) of the population
∗ Design for Adjustable Range – design to accommodate
all (e.g., office chairs, desk height, key board height)
∗ Range typically is 5th percentile of females to the 95th
percentile of males in relevant characteristics
∗ Design for the Average – there is no average human
∗ There are times when the average may be acceptable
(e.g., counter height at grocery store)
Design and Use of Anthropometric Data
∗ Design Principles Discussion
∗ Setting limits to 5th and 95th percentiles can
eliminate a fairly high percentage of population
∗ Bittner (1974) – looked at 5th and 95th percentiles
on 13 dimensions
∗ Would have excluded 52% of population
instead of 10% implied by percentiles
Design and Use of Anthropometric Data
∗ Bittner (1974) – looked at 5th and 95th percentiles
on 13 dimensions
∗ Why? – body measurements are not perfectly
correlated
∗ Short arms ≠ short legs
∗ To derive composite measures taking into
account imperfect correlations requires
regression analysis
Percentile Covered
∗ Herman Miller found that chairs theoretically designed
to fit the 5th-percentile female to the 95th-percentile
male actually fit far fewer people (Dowell, 1995a).
Design and Use of Anthropometric Data
∗ General approach
1. Determine body dimensions important in the design
Example: chair
popliteal height (lower leg length), seat depth
(buttock to popliteal length)
hip breadth, midshoulder sitting height (back height),
elbow height, lumbar height
lumbar depth
2. Define population (e.g., adult - male, adult - female,
children)
3. Determine what principle should be applied
4. Select % of population to be accommodated
Design and Use of Anthropometric Data
∗ General approach
5. Locate anthropometric tables appropriate for the
population
6. If special clothing worn – add allowances
7. Build prototype and test using representative tasks
∗ Anthropometric data
∗ Structural dimensions – taken in standard & still
positions
∗ Functional dimensions – obtained in various work
postures
Examples
If you were choosing a door
height, you would choose the
dimension of people's height
(often
called
'stature'
in
anthropometry tables) and pick
the 95th percentile value – in
other words, you would design
for the taller people. You
wouldn't need to worry about
the average height people, or
the 5th percentile ones – they
would be able to fit through the
door anyway.
Examples
At the other end of the scale, if you were designing an aeroplane
cockpit, and needed to make sure everyone could reach a particular
control, you would choose 5th percentile arm length – because the
people with the short arms are the ones who are most challenging to
design for. If they could reach the control, everyone else (with longer
arms) would be able to.
POSSIBLE CONSEQUENCES
Considerable inconvenience, accidents, injuries
and low productivity have been shown to be the
result of misfits between people and equipment.
Workplaces, equipment, tools and protective
clothing must fit the physical characteristics of
the intended user population.
Cause of misuse of Anthropometric
data
4.3 Types of anthropometric
data
Anthropometric Data Explained
Of course not all people are the same size. There will be huge differences
between the heights, weights, and other dimensions due to: gender, age, diet,
growth rate, genetic make up and other factors. Therefore the Anthropometric
data needs to be organized in a specific way.
Types of anthropometric data
∗ Structural anthropometric data
∗ measurements of body parts in a static position
∗ Functional anthropometric data
∗ Related to range of movements of the body part
involved
example, data are available concerning the
maximum forward reach of standing subjects
Limitations on the use of structural data
∗ Structural data may be used for design in situations
where people are adopting static postures
∗ Caution should be used when applying these data to
design problems that involve movement, particularly
skilled movement.
∗ Functional anthropometric data are useful for designing
workspaces and positioning objects within them,
particularly in the design of aircraft cockpits, crane cabs,
vehicle interiors and complex control panels in the
process industries
4.4 PRINCIPLES OF APPLIED
ANTHROPOMETRY
Applying statistics to design
∗ the designer has to analyze in what ways (if any)
anthropometric mismatches might occur
∗ decide which anthropometric data might be
appropriate to the problem
∗ In many design applications, mismatches occur only
at one extreme (only very tall or very short people
are affected) and the solution is to select either a
maximum or a minimum dimension
∗ If the design accommodates people at the
appropriate extreme of the anthropometric range,
less-extreme people will be accommodated
SD: Standard Deviation is the square root of variance
Percentile?
∗A percentile (or a centile) is a
measure used in statistics
indicating the value below which a
given percentage of observations
in a group of observations fall.
Example
∗ If my test score is at the 66th percentile, it means that
66% test takers scored below me.
∗ 66% is NOT necessarily EQUAL to 66th percentile!!!
Percentile?
∗ An Example
A class of 20 students had the following scores on their most recent test:
75, 77, 78, 78, 80, 81, 81, 82, 83, 84, 84, 84, 85, 87, 87, 88, 88, 88, 89, 90.
-
For 20th percentile of the class:
rank = (20/100)(n+1)
rank = (20/100)(20+1)
rank = (20/100)21 = 4.2 (whole #)
= 4 (whole #)
The score of 80 marks has 4 scores below it.
Since 4/20 = 20, 80 marks is the 20th percentile of the class.
75, 77, 78, 78, 80, 81, 81, 82, 83, 84, 84, 84, 85, 87, 87, 88, 88, 88, 89, 90.
Percentile?
∗ …continue
A class of 20 students had the following scores on their most recent test:
75, 77, 78, 78, 80, 81, 81, 82, 83, 84, 84, 84, 85, 87, 87, 88, 88, 88, 89, 90.
-
For 90th percentile of the class:
rank = (90/100)(n+1)
rank = (90/100)(20+1)
rank = (90/100)21 = 18.9 (whole #)
= 19 (whole #)
-The score of 90 marks has 19 scores below it.
Since 19/20 = 95, 90 marks corresponds to the 95 percentile of the class.
75, 77, 78, 78, 80, 81, 81, 82, 83, 84, 84, 84, 85, 87, 87, 88, 88, 88, 89, 90.
Using Percentiles
∗ Since most body dimensions are normally distributed,
follows a symmetric bell curve

Percentiles tell you how well/bad you are doing,
compared to the rest of the population.
Using Percentiles
It bears repeating that percentiles are a comparison score. The
number of a percentile represents how well or how poorly you did
as compared to other students. It does not represent the number of
questions you answered correctly. If you score in the 70th
percentile, you scored better than 70 out of 100 people who took
the test. If you score in the 50th, read this as better than 50 people
who took the test.*
p = m + ks
p = Measured value or value looking to solve for
m = Mean (Average)
k = Factor related to normal distribution (Z tables)
s = Standard Deviation
∗ Z table
∗ Z table
Using Percentiles
∗ Determine Single Point
∗ Select desired percentile
∗ Determine k
∗ Calculate P
∗ Determine Range
∗ Select upper and lower
percentile
∗ Determine kmax and kmin
∗ Calculate Pmax and Pmin
∗ Range = Pmax - Pmin
Anthropometry Problem #1.0
∗ The instructor’s height is 170 cm. What percentile is his
stature among US Adult males? US stature mean = 175.6 cm,
SD = 6.7 cm.
∗ Use the formula p= m + k (SD)
170 = 175.6 + k(6.7)
∗ Solve for k,
k = -0.8358
∗ Find the corresponding value of k in the z table. Check row -0.8
with column 0.04 (based on k value gained)
∗ That would be your answer : 0.2005 = 20th percentile.
∗ This means, 20% of the U.S adult males are shorter than me.
∗ Z table
k = -0.8358
20 percentile
Anthropometry Problem #2.0
∗ What is the stature of a 85th percentile female? Mean
stature : 1629 mm, SD = 64 mm.
∗ Use the formula p= m + k (SD)
p= 1629 + 1.04(64)
∗ The corresponding value of k=1.04 in the z table. Will show
row 1.0 with column 0.04 (based on k value gained)
∗ Solve for p,
p= 1695.56
∗ Answers ≈ 1696mm
∗ Z table
k = 1.04
Anthropometry Problem #2.1
∗ My popliteal height is 38.5cm. What percentile is my
popliteal height among the US population? Mean
popliteal height : 434 mm, SD = 25 mm.
∗ Use the formula p= m + k (SD)
∗
∗ Solve for k,
385= 434 + k (25)
k = -1.96
∗ The corresponding value of k=-1.96 in the z table. Check
row -1.9 with column 0.06 (based on k value gained)
∗ Answers ≈ 2.5 percentile
∗ Z table
2.5 percentile
k = -1.96
Designing for reach and clearance
∗ You either design for reach or clearance (low or high
percentiles)
∗ Design for reach
∗ Low percentiles are used
∗ Either 1st percentile or 5th percentile are used
∗ Design for clearance
∗ High percentile values are used
∗ Either 95th or 99th percentile
∗ A door handle height?
∗ Uses either 95th or 99th percentile standing knuckle
height.
∗ The width of a chair?
∗ Uses either 95th or 99th percentile of hip breadth of a
female
∗ The height of a doorway?
∗ Use 95th /99th percentile of a person’s height
∗ A door lock height?
∗ 1st/5th percentile vertical reach
∗ A door lock height? Use 1st / 5th percentile of vertical
reach.
∗ Seat heights ?
∗ Uses 1st / 5th percentile popliteal height
∗ Seat depth?
∗ 1st/ 5th percentile of buttock knee length
Examples
∗ Which percentile (high/low) will you use for these
situations:
- Reach distance from the driver to the car dashboard
- Escape hatch in aircrafts
- Grip force required to open bottles
Steps to Apply Anthropometric Data
1. Select those anthropometric measures that directly
relate to defined design dimensions.
Examples : hand length related to handle size.
2. For each of these pairings, determine whether the design must
fit only one given percentile (minimal or maximal) of the body
dimension, or a range along that body dimension.
Examples : the escape hatch must be big enough to
accommodate the largest extreme value of shoulder
breadth and hip breadth, considering clothing and
equipment worn;
Steps to Apply Anthropometric Data
3. Combine all selected design values in a careful
drawing, mock-up, or computer model to ascertain
that they are compatible.
∗ For example: the required leg-room clearance height,
needed for sitting persons with long lower legs, may
be very close to the height of the working surface
determined from elbow height.
Steps to Apply Anthropometric Data
4. Determine whether one design will fit all users. If not,
several sizes or adjustment must be provided to fit all
users.
∗ Examples are: one extra large bed size fits all
sleepers; gloves and shoes must come in different
sizes; seat heights of office chairs are adjustable.
Designing to fit the body
∗ Normal distribution often used to describe certain
measures (depends on sample size)
∗ Central Limit Theorem
∗ There is no true average human
∗ Use the following steps
∗ Select measurements that relate to the design
∗ Determine if design is to fit a certain percentile or a
range
∗ Combine values to ascertain compatibility
∗ Determine if one design will fit all users
4.5 designing for everyone
Designing for everyone
Make different
sizes
Design
adjustable
products
1. Make different sizes
∗ Design same product with several different sizes.
∗ Use anthropometry data to determine a minimum
number of different sizes and the dimensions of
each size that accommodate all users.
∗ Example: hand tool--- screw driver or chopsticks
∗ Research: to evaluate the effects of the length of the chopsticks on the foodserving performance of adults and children
The results showed that the food-pinching performance was affected by the
length of the chopsticks, and that chopsticks of about 240 and 180 mm long
were optimal for adults and pupils, respectively. Longer chopsticks require
greater effort to exert same pinch force at tip than shorter chopsticks.
2.Design adjustable products
∗ Alternative approach to manufacture product
whose critical dimensions can be adjusted by users.
∗ Steps:
Determine what the critical dimensions for user
are
Design mechanism of adjustability with the
emphasis on ease of operation
Instruction or training program; explain to users
the need to use the product and how to adjust it
correctly.
Seat work
Desk
∗ The seat height should not higher than popliteal
height of user so that both feet can rested firmly on
the floor to support the weight of the lower legs.
∗ Desk height should coincide with the user’s sitting
elbow height.
EXAMPLE:
Anthropometry Step-by-step
Decide who you
are designing for
Decide which body
measurement are
relevant
Decide whether
you are designing
for the “average”
or “extremes”
Consider other
human factors
Chapter 5
Work Capacity, Stress
and Fatigue
BMMD 3553 Ergonomics Design
1
Objectives
• Understand the concept of stress and
fatigue related to humans.
• Be able to describe the muscle
structure, function and capacity
• Be able to explain muscle contraction
2
Introduction
• Stress is our body’s response to the things
that happen to us.
• Stressors are those things that cause you
stress (e.g. your lecturers, your boss, your
work)
• Stress is described by different ways by
different researchers.
3
Introduction
• Stress is viewed by researchers from 2 viewpoints:
• Hooke’s Law (stress is viewed from
mechanical point of view)
• Selye’s model (Physiological description)
4
Mechanical viewpoint of stress
• Humans react to stress. Stress is appropriately
termed as “applied loading” (this is the classical
view on stress)
• Hooke’s Law : a model for stress, once loading is
applied to a spring, the spring lengthens.
• But what happens if the spring is loaded beyond
capacity? Permanent deformation occurs.
5
Selye’s Model (Physiological)
6
Selye’s Model of Stress
• According to Selye (1956), stress is “Non specific
response of the organism to any demand made upon it“
• H. Selye (1956) was interested in the endocrinological
responses to life events and his key insight was that
many, very different, noxious stimuli produce the same
effects.
• General Adaptation Syndrome : Alarm, Resistance, and
Exhaustion.
7
What happens when a person is
stressed (Selye)?
• Fight or flight reaction
• Three stages
• Alarm
• Resistance
• Exhaustion
8
9
Alarm
• Intrusion of noxious stimuli (job stressor)
leads to strong hormonal responses to get
the person ready to respond to the stressor
• Heart rate and blood pressure increase
• Blood vessels dilate
A noxious stimulus is "an actually or potentially tissue damaging event."
10
Resistance
• If the stressor persists, the body tries to
adapt to the continued exposure.
• This requires further physiological resources
• Energy required to maintain this adaptation is
limited
11
Exhaustion
• Body can no longer adapt – runs out of
resources
• Ulcers
• Immune disorders
• Cardiovascular disease
12
Measures of Stress
• Urinary catecholamine concentration =
used for level of stress and resulting
physiological arousal,
• Salivary cortisol levels = degree of
emotional response to the situation
• These endocrine markers are indicators of
stress.
13
Fatigue
• Fatigue is usually inferred from its effects:
most directly, decline in physical or mental
task performance.
• When the task becomes more difficult to
perform, then fatigue has likely occured.
• The interest lies in localized muscle
fatigue, which is the focus of this chapter.
14
Fatigue
• In essence, fatigue manifests itself as an
increasing resistance to continuing with a
task. As long as this resistance can be
overcome, performance continues, but
with subjectively greater effort.
15
Muscles, structure and function
and capacity
Muscles  Muscle Fibers 
Myofibrils  Sarcomeres 
Actin and Myosins
16
Muscles, structure and
function and capacity
•3 types of muscles in the human
body:
• Skeletal muscle
• Smooth muscle
• Cardiac muscle
17
Muscles, structure and function
and capacity
• Where do muscles get their energy from?
• The energy required for muscle contraction
is obtained from phosphate compounds in
the muscle tissue.
• These compounds are formed from the
breakdown of food
18
Muscles, structure and function
and capacity
ADP = Adenosine di-phosphate
ATP = Adenosine tri-phosphate
ATP has one more phosphate group than ADP, and
because ATP has one more phosphate group than
ADP, it contains more potential energy because
more bonds could be broken
ATP forms ADP when it breaks a phosphate group
to utilize its bond energy to do work, and ADP
forms AMP (adenosine mono-phosphate)
19
Muscles, structure and
function and capacity
• Energy for action comes from breaking down
ATP
• By the breaking of one of the phosphate
bonds, ATP is converted to ADP (adenosine
diphosphate) and energy is made available
inside the cell.
20
Muscles, structure and function
and capacity
• In order for the cell to continue functioning,
the ADP must be reconverted back to ATP so
that energy can continue to be made
available when required.
• A second phosphate compound known as
creatine phosphate acts like a ‘back-up’
energy store to ‘recharge’ the ADP to ATP.
21
Muscles, structure and function
and capacity
• ATP is also required to break the actin and
myosin attraction. Remember, the msucle
works by sliding the actin and myosin
filaments.
• Sooner or later, the ATP supply will finish
and more ATP is needed
22
Muscles, structure and
function and capacity
• Mitochondria is reponsible for converting
our food into a form of energy in the form
of ATP.
23
Muscle contraction
• According to the sliding filament theory:
• The mechanism of muscle contraction
consists of the actin filaments sliding
over the myosin filaments
• Since the actin and myosin filaments are
arranged in overlapping, alternating
bands like a multilayered sandwich,
sliding of the former over the latter
causes the sarcomeres to shorten
24
Muscle contractions
Eccentric contractions. The muscle
lengthens while contracting.
Isometric contractions. The muscle length
remains constant during contraction.
Concentric contractions. The muscle
shortens while contracting.
25
In what ways muscle can
fatigue ?
1. Energy demand > Energy supply
2. Mechanical capacity of muscles are
exceeded
3. Accumulation of waste products
such as lactic acid
# 1 and 3 are often talked about when discussing muscle
fatigue
26
Muscle Fatigue
• Muscles convert glucose and oxygen into
CO2 and H20, thus creating energy.
• Regular blood supply is required to
remove waste products .
• During exercise, blood flow is facilitated by
muscle action.
• Fatigue can be attributed to the depletion
of nutrients (glucose)
27
Static vs Dynamic Work
• Static Work
• Sustained
muscular
contraction
• Reduced blood
flow
• No increase in
muscle
oxygenation
• Anaerobic exercise
(oxygen
independent)
• Dynamic Work
• Repeated muscle
contractionrelaxation cycle
• Increase in blood
flow
• Increase in muscle
oxygenation
• Aerobic exercise
(oxygen
dependent)
28
Muscle Fatigue
• Usually, the oxygen requirements >
capacity of blood circulation system, thus,
the burning of glucose is done without
oxygen.
• This is called “anaerobic” process.
• Lactic acid is produced in this process, and
it hinders the ability for the muscles to
work.*
29
Muscle Fatigue
• When muscles contract, they occlude the blood
vessels within them and thus diminish their own
blood supply.
• Repeated or sustained activities, rapid
movements and large forces can stimulate pain
receptors in a muscle.
• Since skeletal muscle makes up 40% of the
tissues of the body, it should come as no surprise
that many of the aches and pains we experience
in our daily lives are of muscular origin
30
Methods to measure muscle
fatigue
• Electromyography
(EMG). Electrical
activity in muscles can
be detected either
using surface
electrodes placed on
the skin overlying the
muscle or by needle
electrodes inserted
into the muscle body.
31
Rhomet’s Equation for Muscle Fatigue
32
33
Physical Work Capacity(PWC)
• Physical work capacity refers to a worker’s
capacity for energy output.
• Energy is provided from by
-oxygen-dependent and
-oxygen-independent processes.
34
Physical Work Capacity
• Aerobic = Oxygen dependent process in
creating energy. The oxygen is needed
to breakdown glucose.
• Anaerobic = Energy creation without
oxygen. Body’s demand for energy >
ability to provide oxygen
35
Physical Work Capacity
• Exercise physiologists and sports scientists
have used the term ‘VO2 max’ to describe
an individual’s capacity to utilise oxygen
(aerobic capacity).
• VO2 max is the maximum rate of oxygen
consumption (or maximal aerobic
capacity) during an incremental exercise.
It reflects the physical fitness of a person.
• Measured in litres of oxygen/minute
36
VO2 max has traditionally
been estimated by having
subjects run on a treadmill or
pedal a bicycle ergometer
while their oxygen uptake is
measured.
The running or cycling speed
is
increased
in
an
incremental manner and
oxygen uptake is measured
approximately every 3–5
minutes after the subject has
adapted to each new work
rate.
As might be expected, it is
observed that oxygen uptake
increases as the work rate is
increased.
37
PWC : VO2 Max.
• Clearly, oxygen consumption and heart
rate cannot continue to increase
indefinitely.
• In any work situation, a point is reached at
which a person cannot increase the work
rate any more.
38
The Importance of VO2Max
• VO2 max is the gold standard for determining aerobic
fitness and cardiovascular endurance.
• Since muscles need oxygenated blood for intense
and/or prolonged exercise, the heart must pump
enough blood through the circulatory system to meet
the demands of intense exercise.
• As a rule of thumb, the more oxygen you can use
during intense exercise, the more energy your body
can produce. It, in fact, reflects the aerobic physical
fitness of the athlete.
• Not only that, research also shows that people with
high VO2max scores, are healthier, live longer, and
enjoy a better quality of life.
39
PWC: VO2 Max
• NIOSH (1981) has published data
concerning the maximum aerobic capacity
of US workers.
• 50th percentile = male : 63 kj/min female:
44.0 kj/min
• 5th percentile = male : 52.3 kj/min female:
33.5 kj/min
40
Physical Work Capacity
• For continuous work, NIOSH states that
energy expenditure
<
33% of an
individual’s maximum capacity
• Translated into : 21 kJ/min for men
14.6 kJ/min for women over an 8-hour
shift.
41
Factors affecting PWC
• Weight (more energy needed to move, it is
possible to increase one’s relative Vo2 max
by shedding excess kilograms of fat.)
• Age (Vo2 max declines gradually after 20
years of age)
• Gender (Women have a lower Vo2 max
than men)
42
Factors affecting PWC
• Smokers (Smoking reduces work capacity
by reducing the oxygen carrying capacity
of the blood)
• Training (can increase Vo2 max)
• Motivation (important determinant of
work capacity)
43
44
45
References
• Bridger R.S., Introduction to Ergonomics, Mc Graw Hill
Companies, 1995.
46
BMMD 3553
Objectives
 Describe the anatomy of standing and sitting
 Describe principles of standing and sitting work
 Understand why static work is bad
Introduction
 In everyday life, people rarely stand still for any length
of time – if not walking or moving, they adopt a variety
of resting positions.
 Short periods of walking and gross body movements
are vital :
 to activate the venous pump and assist the return of
blood from the lower limbs
Introduction
 Many jobs require static positions, thus inviting a lot
of problems for the workers.
 Prolonged daily standing is known to be associated
with low back pain.
Some advantages of the standing work position:
 1. Reach is greater in standing than in sitting.
 2. Body weight can be used to exert forces.
 3. Standing workers require less leg room than seated
workers.
 4. Lumbar disc pressures are lower
 5. It can be maintained with little muscular activity and
requires no attention.
 6. Trunk muscle power is twice as large in standing than
in semi-standing or sitting.
Standing work
What happens when we stand?
For the back muscles (erector spinae):
 During relaxed standing there is very little activity.
 During forward leaning/holding weights, some muscle
activity will be present.
For the leg muscles:
 Always on when standing. leaning forward causes muscle
activity to increase.
Standing work
 The increase in energy expenditure when a person
changes from a supine to a standing position is only
about 8%.
 Fidgeting or walking activates circulation again for the
legs.
Standing work
 Prolonged standing causes physiological changes
including:
 peripheral pooling of blood,
 an increases in heart rate,
 diastolic and mean arterial pressure.
 Constrained standing is particularly troublesome for
older workers or for those with peripheral vascular
disease because the ‘venous muscle pump that returns
blood to the heart ceases to function.
Standing work
 Varicose veins are veins that have become enlarged and
twisted.
 Often happens in the legs;
 In which the valves function ineffectively, resulting in
pooling of blood and painful swelling.
Sitting work
 If standing is so bad, so there is nothing wrong with
sitting right?
 In sitting position, several changes happens to your
body.
 Disc pressures are lower in standing than in sitting and
lower still when lying down.
Spinal problems in standing and
sitting
 Low back pain can be caused by having the trunk
inclined forwards.(refer to previous chapters as to
why)
 Excessive lumbar lordosis = excessive loading of facet
joints = should be avoided when standing.
Issues with sitting
 When sitting, blood pooling occurs as circulation is
fighting against gravity.
 In sitting, the discs bear more of the load (Adams and
Dolan, 1995), whereas when lying the absolute load on
all structures is lowered.
 Rohlmann et al. (2001) found that disc pressure was
lower in relaxed sitting than in standing, but higher
when subjects attempted to extend the spine to sit
erect.
 Both Nachemson (1966) and Rohlmann et al. report
lower disc pressures when subjects recline against a
backrest.
 This implies that seated workers should be able to
adopt relaxed postures.
Standing or sitting?
What’s the verdict? Should we:
a) stand all the time while we work or,
b) should we sit all the time while we work?
Another question is, how long can we stand or sit while
working?
An ergonomic approach to
workstation design
Workstation design
 Typically designer tries to fit 90% of the users
 Desk heights are approximately 73 cm AND assumes the
chair is adjustable.
 If the chair is too high, the thigh bears the weight of the
feet and this can impede circulation.
 Tall users = may find desk too low
 Short users can be given footrest.
Design for standing workers
 As a rule of thumb, all objects that are to be used by
standing workers should be placed between hip and
shoulder height to minimize postural stress
Workspace design faults
1.
Working with the hands too high and/or too far away
2. Work surface too low: trunk flexion and back muscle
strain.
Workspace design faults
3. Constrained foot position due to lack of clearance:
worker stands too far away.
4. Working at the corner of the bench: constrained foot
position, toes turned out too much.
5. Standing with a twisted spine having to work at the
side rather than directly ahead.
Evaluation of standing aids
 Footrest = Rys and Konz (1994) have reviewed the
ergonomics of standing. A 100 mm foot platform used by
subjects was perceived as more comfortable than normal
standing in 9 of 12 body regions, including the neck
 ‘Anti-fatigue mats’= Footrests seem to relieve some of the
load on the resting leg. Mats do not seem to reduce lower
leg fatigue although they do reduce discomfort in the lower
leg, feet and back (Rys and Konz, 1994)
 Toespace = Panels or obstructions in front of benches cause
users to stand farther away from the worksurface. The
postural adaptation is for people to bend forwards.
Anti Fatigue
Mats
Footrest
Design for seated office work
Design for seated workers
Characteristics of an ergonomic
chair
Key features of chair design
1. Seats should swivel and have heights adjustable
between 38 and 54 cm.
2. Free space for the legs must be provided both
underneath the seat to allow the user to flex the knees
3. A 5-point base is recommended for stability if the
chair has castors.
Characteristics of an ergonomic
chair
4. The function of the backrest is to stabilise the trunk. A
backrest height of approximately 50 cm above the seat
is required to provide both lumbar and partial thoracic
support.
5. If the backrest reclines, it should do so independently
of the seat to provide trunk–thigh angle variation
Characteristics of an ergonomic
chair
6. Lumbar support can be achieved either by using extra
cushioning to form a lumbar pad, or by contouring the
backrest.
7. The seat pan must have a slight hollow in the buttock
area to prevent the user’s pelvis from sliding forwards.
This keeps the lower back in contact with the backrest
when reclining.
Characteristics of an ergonomic
chair
8. Arm rests should be high enough to support the
forearms when the user is sitting erect.
They should also end well short of the leading edge of
the seat so as not to contact the front edge of the desk.
If the armrests support the weight of the arms, less load
is placed on the lumbar spine.
Characteristics of an ergonomic
chair
Guidelines for the design of static
work
 ISO 1226 gives time limits for how long can we lean
forward.
 inclinations greater than 60 degrees are not permitted at
all under ISO guidelines,
 neither are negative inclinations (leaning backwards)
permitted without back support.
 Angles from 0 to 20 degrees are acceptable for 5 minutes
and angles from 20 degrees to 60 degrees can be held
from 4 to 1 minutes (sloping line in Figure 4.16).
Relationship between pain and
static work
 The higher the angle of flexion for the neck and the
trunk = the higher the occurrence of pain.
 Other body parts = angles of ulnar deviation, Shoulder
stiffness was related to increased elbow angle.
Conclusion : Work postures should be as close to neutral
as possible
Static Work and Upper Limb
Disorders Part 2
BMMD 3553
Ergonomics Design
1
How to Implement Ergonomics Design
in Workplace
• Force (Magnitude)
• Posture
• Repetition (Frequency)
• Duration ( How long?)
Task Demands
Intervention
Ergonomics
Design
MSD
Intervention
Assessment
Design
2
Objectives
• Understand the relationship between task
demand, ergonomics and musculoskeletal
disorders (MSD).
• Be able to describe the nature of upper limb
disorders
3
Introduction
• Most repetitive tasks require a combination of
both static and rhythmic muscle activity
• If task demands are excessive, pain may be
experienced in the muscles providing the
stabilisation.
4
Introduction
• The relationship between task demands,
ergonomics and musculoskeletal disorders is:
– of a probabilistic nature
– and is confounded by the fact the disorders can
arise,
– As a result of many activities of daily life, both at
work and elsewhere.
5
Introduction
• In the past, MSD have been documented, but
people tend to offer different theories of
causation;
• However, WMSD are multifactorial in nature
– Interplay between many variables
– Workplace ergonomics, work organisation, social
aspects and health of workers
6
• The current state of knowledge is of a web of
factors that are associated with
musculoskeletal outcomes
• The outcomes themselves are often defined
subjectively, inferred from questionnaire
responses,
– little precise information is captured on the
magnitude of the ergonomic exposures,
– making it difficult to estimate dose–response
relationships.
7
What is dose response
relationship?
• Dose response relationship describes:
–Changes that occur;
–In an organism
–After the exposure of “doses”
8
What is dose response
relationship?
• Doses can be anything (chemicals, biological
agents, physical )
• In our case,, we are interested more in the
“physical doses” (force and repetition)
• Ask questions like: How much force/work that
a person can do before he suffers from
WMSD?
9
Nature of Work Related
Musculoskeletal
Disorders
10
How do MSD’s develop?
• Force (Magnitude)
• Posture
• Repetition (Frequency)
• Duration ( How long?)
There are plenty of scientific evidence that most
of MSD’s are associated with one or more of
the above factors.
11
How do MSD’s develop?
• A prolonged exposure to ergonomic risk
factors will lead to the development of MSD
“………demand for force exertion, repetition of
activities or assuming postures for prolonged
periods places stress on human physical
systems, which is inherently unnatural….”
(Kumar, 2001)
12
How do MSD’s develop?
• The mechanism of WMSDs is thought to be
repeated micro- trauma at the cellular level.
• When repair capacity < exposure, injury starts
to develop.
Note : These are only educated conjectures! A
solid empirical evidence is yet to be
established for a dose response relationship
13
How do MSD’s develop?
• Muscles have excellent endurance for loads <
15% of max. contraction
• Above this, rest is needed otherwise problems
happens.
14
How do MSD’s develop?
First, you must understand these:
• Structure of muscles
• Structure of tendons
• Structure of ligaments
15
Models of the development of
WMSDs
• Armstrong et al. (1993) have developed a
model of musculoskeletal disorders that
emphasizes :
– Exposure ( external factors)
– Dose (Internal factors- caused by external)
– Capacity ( ability to resist)
– Response ( changes that occur)
16
Carpal Tunnel Syndrome
17
Carpal Tunnel Syndrome (CTS)
• Carpal tunnel syndrome happens due to the
compression of the median nerve.
• The tissue surrounding the median nerve gets
inflamed, thus putting pressure on the median
nerve.
18
Carpal tunnel syndrome
19
Possible causal mechanism of CTS
• The muscles that flex the fingers lie in the forearm
and have long tendons that pass through a narrow
opening in the wrist before inserting into the fingers.
• This opening, known as the carpal tunnel, is also
traversed by the nerves and blood vessels of the
hand
• If for some reason, the tendons get irritated or some
inflammation occurs, the median nerve gets
20
squeezed . Hence CTS develops.
Carpal Tunnel Syndrome (CTS)
• An increase in the pressure in the carpal
tunnel can cause carpal tunnel syndrome if it
affects (‘entraps’) the median nerve or
reduces the blood supply to the nerve by:
– compressing the capillaries,
– resulting in nerve damage and reduced
conduction velocity of neural signals.
• Characterized by numbness and tingling
21
CTS Patient operation
22
• CAUTION!
What then is the connection of repetitive work, wrist
posture and force with carpal tunnel?
• Some studies indicate the connection, and some say
they dont, (i.e non work activities are more
important)
• but, as a good measure, reducing repetitive work,
wrist posture and force could be beneficial in
reducing CTS.
23
How to avoid CTS: Using mouse
24
How to avoid CTS: Keyboard
design
• Ulnar deviation with regular keyboard
25
How to avoid CTS: Arm Position
26
Exercise to avoid CTS
27
Tennis Elbow (lateral epicondylosis)
• Overexertion of the extensor muscles of the
wrist can lead to a condition known as ‘tennis
elbow’.
• There was strong evidence for an association
between combined stressors (e.g. force and
posture) and tennis elbow.
28
Tennis Elbow
29
Possible causal pathways
• The act of grasping and holding objects is only
possible if the wrist is stabilised by the
muscles of the forearm, many of which
originate at the elbow.
30
Tennis Elbow
• Nirschl and co-authors have proposed a
theory of lateral epicondylitis that emphasizes
the role of eccentric tensile loading.
• They postulate that tendon tearing occur
when tensile forces arising from eccentric
movements exceed the tolerable rate of strain
(elongation) of the tendon fibers.
31
• What’s the benefit of knowing all this
crap about tennis elbow, CTS and
pathomechanics ?
• Who gives a damn anyway right?
32
Hand tool design principles
–Bend the handle, not the wrist
–Maintain neutral wrist posture
–Reduce the required grip forces
–Damp the vibration from powered
tools
33
34
Hand tool design
35
36
Chapter7Work
Physiology
BMMD 3553
1
2
Introduction
 Be
able to describe the physiological
cost of doing work for humans
 Be
able to explain the various ways of
measuring physiological cost of work
(VO2 Max and heart rate)
 Describe
subjective measures of effort
3
Why do I have to study this?
o
o
If we don’t understand how the human
body functions, how can we set the
appropriate work rate for them in the
workplace?
In this chapter, we are going to focus on
the physiological cost of work
4
What is physiology?
“The branch of biology that
deals with the normal
functions
of
living
organisms and their parts”
(Google Definition)
5
Introduction
There are several ways to measure
human work capacity:
1.The physiological approach
2.The biomechanical approach
3.The psychophysical approach
6
The physiological approach
o Looks
at how much
energy is consumed the
our body in order to
perform tasks.
o Often measured in oxygen
uptake or heart rate.
7
Biomechanical approach
Looks at how much forces is
being exerted or acting
upon the body in order to
determine the actual work
done or stress imposed on
the body.
8
The psychophysical approach
The psychophysical approach
looks
at
studying
the
sensations
perceived
by
humans at the exposure of a
physical stimuli.
9
Work physiology
In this chapter, we are going to focus
on the physiological measures of
measuring work.
(measuring
PWC - Physical Work Capacity)
10
Measurement of the
Physiological Cost of Work
o How
do you measure the
physiological cost of work?
o You
can do it by measuring
the oxygen uptake or heart
rate
11
Work Physiology (heart rate)
o
o
o
Apart from measuring oxygen uptake,
heart rate is also measured to indicate
work capacity.
Heart rate increases as a function of
workload and oxygen uptake.
Because it is more easily measured than
oxygen uptake, heart rate is often used as
an indirect measurement of energy
expenditure.
12
Work physiology (heart rate)
o
o
Heart rate can be likened to a
signal that integrates the total stress
on the body.
Heart rate measurement can
therefore be used as an index of
the physiological workload.
13
•
Work physiology – heart rate
Heart rate increase as work load and
energy demands are increased.
•
It reflects the increased demand for
the cardiovascular system to transport
more oxygen to the working muscles
and remove more waste products from
them.
•
Heart rate is linearly related to oxygen
consumption.
14
Work physiology – heart rate
o
o
In general, the change of heart rate before,
during, and after physical work follow the
same
pattern
as
that
of
oxygen
consumption or energy expenditure.
Maximum heart rate directly determines the
maximum work capacity or maximum
energy expenditure of an individual.
15
Work physiology – heart rate
•
The maximum heart rate for each
individual depends on age, gender,
health and fitness level.
Max. heart rate = 206 – (0.62 x age)
@
Max. heart rate = 220 – age (beats/minute)
16
Work Physiology
o
o
In principle, any increase in
oxygen uptake over and
above that required for basal
metabolism can be used as
an index of physiological cost
to an individual.
Note: remember in previous
lecturer, VO2 max is the
maximal oxygen uptake
during an incremental
exercise.
17
Work Physiology
o The increase of metabolism from resting to
working is called working metabolism or
metabolic cost of work.
o The metabolic or energy expenditure is the
sum of the basal metabolism rate and
working metabolism rate.
18
Work physiology
Energy Expenditure
=
Basal metabolism
+
Working metabolism
--- unit is (kcal/min) ---
19
Work physiology
What is basal metabolism?
“The minimum amount of
energy that you need in
order to sustain your bodily
functions.”
20
Energy Expenditure Rates
for Various Activities
21
Work physiology (measuring PWC)
o
PWC is typically measured by the
VO2max.
o
VO2max is a measure of our maximum
capacity for oxygen uptake.
o
VO2max is usually measured in
liters/min.
22
Work physiology (measuring PWC)
 VO2
max has traditionally been estimated
by having subjects run on a treadmill or
pedal a bicycle ergometer while their
oxygen uptake is measured.
 The
running or cycling speed is increased
in an incremental manner and oxygen
uptake is measured approximately every
3–5 minutes after the subject has adapted
to each new work rate.
23
Work Physiology
Energy requirements must
be less than 1/3 of worker’s
PWC in an 8 hour shift
24
Work physiology
o Rate
for energy expenditure of a work
is linearly related to the amount of
oxygen consumed by the body and
heart rate.
o Therefore,
oxygen consumption rate
and heart rate are often used to
quantify the workload of physical
work.
25
Work physiology
o
o
o
There is a linear relationship between
oxygen consumption and energy
expenditure.
1 liter of oxygen consumed = 4.8 kcal of
energy is released.
Energy expenditure of work can be
determined by multiplying the oxygen
consumption (liter/min) by 4.8 kcal/liter.
26
Heart rate
Oxygen consumption
27
Calculation of Rest Period in
Manual Work
Where
w = length of the working period
b = oxygen uptake
s = ‘standard’ uptake for continuous work
Example
o
28
If a worker spends 0.5 hour felling a tree at an
oxygen uptake of 2.64 litres/min and the standard is
taken to be 1 litre/min. Calculate the rest
allowance.
Answer:
Rest allowance = 0.5 (2.64 – 1) / (2.64 – 0.03)
= 0.31 hour
29
Heart rate and VO2 max
o VO2max
is the max amount of
oxygen that can be consumed by
a person.
o It
was found that VO2max is related
to heart rate.
o So
if heart rate can be measured,
then VO2max can be found as well.
30
Heart rate and VO2 max
o
o
o
o
David Swain (1994) and his US based research
team using statistical procedures examined the
relationship between %MHR and %VO2max.
Their results led to the following regression
equation:
%MHR = 0.64 × %VO2max + 37
The relationship has been shown to hold true
across sex, age and activity.
31
Evaluation of non- physical stress
o Physiological
method can be
applied to the investigation of light
work
o Under
mental stress heart rate can
increase.
32
o
o
Physiological methods have long been used in the
aviation industry and in military applications as
indices of mental stress.
When subjected to high workload, these changes
take place:
o
o
o
o
Level of cortical arousal - is the activation of the recticular
section of the brain (middle brain, cerebelum, etc) and increases heart
rate, breathing rate, vigilance, muscle tone, etc. it basically
supercharges you like adrenaline.
Increased heart rate
Changes in the electrical skin resistance
Changes in the certain concentration of hormones
33
o
o
o
Although many jobs require great concentration it is
not necessarily the case that they impose negative
emotional stress on the worker.
Example : Among surgeons, oxygen consumptions
during surgery was found to be low.
Heart rate and heart rate variability are also a
measures of workload.
34
o Subjective measures of physical effort
o The most common method of obtaining
subjective estimates of physical effort is by use of
the Borg Rating of Perceived Exertion (RPE) scale
(Borg, 1982).
o Workers rate their perceived level of exertion
during or after performing the task on a scale
from 6 to 20, corresponding to heart rates of 60
to 200 beats per minute.
o The Borg scale is normally used with other
measures, typically heart rate and oxygen
consumption.
35
36
o In general, subjective measures are best used with
objective measures because, on their own, they
are susceptible to experimenter effects (source
experimenter or subject’s perception of the
experimental ‘demands’).
o Mital et al. (1993) found that experienced subjects
tended to underestimate the workload in
demanding experimental tasks.
o The Borg scale has the advantage of being a
global measure – the ratings are influenced by all
the demands in the work situation, whereas
physiological and subjective measures vary
according to a subset of these demands.
37
Summary
The goal of ergonomics in
the design of the physical
component of jobs is to
minimize unnecessary and
possibly harmful stress.
38
References
 R.S
Bridger “ Introduction to Ergonomics”.
 www.ufv.ca/faculty/kpe/.../physiology%2
03r/workphysio3.ppt
Chapter 8:
Heat, cold and the design of the
physical environment
BMMD 3553
1
Objectives
• Understand the principles behind thermal
comfort
• Understand how the environment affects our
body
• Be able to describe how our body deals with
the environment
2
Introduction
• Humans have a
remarkably well-adapted
ability to tolerate heat.
• Skin temperatures may
vary compared to the
core temperature.
3
Introduction
• Goal is to maintain 37 Celcius.
• 39.5 C can be disabling and 42 C can be fatal.
• Sources of heat are the liver, brain and the
heart, and the muscles.
• Muscular work efficiency is 20%. The rest is all
heat.
4
Introduction
• Thermoregulation is achieved by creating a
balance of :
- the metabolic heat produced and
- the rate of heat loss.
5
Introduction
• How does the environment influence our body
temperature?
• There must be a state of heat balance. (given
an equation).
• Heat may be gained or lost.
• Our body produces heat and loses it to the
environment.
6
Introduction
• Convection =
transfer of heat
through the
movement of
air/fluids
7
Introduction
• Radiation = heat transfer by electromagnetic waves
or photons.
8
Introduction
• Sweat production and
evaporation (E) is a
mechanism by which
heat is lost to the
environment.
• Surrounding temp >
Body temp, no heat
loss occurs.
9
Introduction
• In a cold environment, metabolic heat
production takes place by shivering, or some
physical activity.
• Heat loss can be reduced by wearing heavy
clothes (convection and radiation is reduced).
How does this work?
10
How about work in foundry? How
to solve the problem?
11
Measuring the thermal
environment
• Dry-bulb temperature (DBT)
– It is the temperature measured by a regular
thermometer exposed to the airstream
– Does not indicate moisture in the air
• Wet-bulb temperature (WBT)
– It is the temperature you feel when your skin is
wet and is exposed to moving air.
– Gives you an indication of moisture.
12
Measuring the thermal
environment
• Globe temperature (GT)
– Measured by a thermometer placed in a black
sphere
– Also sometimes referred to Mean Radiant
Temperature.
– Radiant heat (from the sun or from hot objects) is
absorbed by the sphere and heats up the
thermometer.
13
Measuring the thermal
environment
• Mean Radiant
Temperature (MRT)= is
simply the area
weighted mean
temperature of all the
objects surrounding the
body.
• Almost equivalent to
Globe Temperature.
14
Measuring the thermal
environment
• WBGT incorporates the following :
– Dry bulb temp
– Wet Bulb temp
– Globe temp
15
Air movement and wind chill
• Air movement moderates the effects of high
temperatures and exacerbates the problems
of low temperatures (causing ‘wind chill’).
16
Thermoregulatory mechanisms
Our body deals with temperature
fluctuations in several ways:
• Peripheral vasomotor tone
• Sweating
• Shivering
17
Thermoregulatory mechanisms
• Peripheral vasomotor tone
– When it is hot, arteries and blood vessels dilate
and heat is conducted to the skin ()
– In the cold, vasoconstriction occurs, thus reducing
blood flow.
– Insulation capacity of a person is measured by CLO
values . A person wearing a business suit has a
CLO value of 1.
Clo-value:
This is the amount of insulation that allows a person
at rest to maintain thermal equilibrium in an
environment at 21 °C (70 °F ) in a normally
ventilated room (0.1 m/s air movement)
18
Thermoregulatory mechanisms
Sweating
Shivering
• Sweat cools the body
when it evaporates.
• A special behavior
exhibited by the motor
units of muscles where
the end result is heat
production
• In very humid
environments sweating is
ineffective. Also, “reverse
sweating” can occur
where water vapor
accumulates on the skin.
19
In places where thermal environments
are extreme, plenty of water must be
made available for workers to
prevent dehydration.
20
Work in hot climates
• Peripheral vasodilation
increases the blood flow to the
skin.
• Working muscles also demands
blood supply
• As a result, the cardiovascular
system is under strain.
21
Heat illnesses
• Heat stroke
• Heat exhaustion
Venous return
Venous return is the rate of
blood flow back to the heart.
• Heat syncope = inadequate venous return
• Heat hyperventilation = can occur while
wearing protective clothing
22
Relative humidity
Relative humidity (R.H) is the ratio of the partial pressure of water vapor to the equilibrium
vapor pressure of water at the same temperature. Relative humidity depends on temperature
and the pressure of the system of interest.
• If the DBT > 38 C, but the R.H < 20%, then
sweating is effective.
• But if R.H is 90% and DBT = 32 C, with no air
movement, so low level of work activity can
be performed.
The dry-bulb temperature (DBT) is the temperature of air measured by a thermometer freely
exposed to the air but shielded from radiation and moisture. DBT is the temperature that is
usually thought of as air temperature, and it is the true thermodynamic temperature. It
indicates the amount of heat in the air and is directly proportional to the mean kinetic energy
of the air molecules
23
Heat tolerance
• Work in hot environments can be made more
tolerable by introducing job aids or rest
pauses. (metabolic heat is reduced)
• Workers differ in their ability to tolerate stress.
– Heat intolerant, heat tolerant
24
Heat acclimatisation
Become accustomed to a new climate or condition
• Heat acclimatisation is a physiological process
of adaptation rather than a psychological
adjustment to life in a hot environment.
• It involves an increase in the capacity to
produce sweat and a decrease in the core
temperature threshold value for the initiation
of sweating.
25
Heat acclimatisation
• A state of acclimatisation is best achieved by
exercising in the heat and drinking plenty of
fluid.
• Heat acclimatisation occurs naturally but it
may also be induced artificially.
– Surface acclimatisation chambers are used where
workers exercise and their temperatures are
monitored.
26
Factors influencing worker ability
• Age = children have less sweating capacity,
older person unable to tolerate high heat
stress.
• Physical fitness = Physically fit workers are less
stressed by hot conditions even if they are
accustomed to a temperate climate.
• Body fat = Excess body fat degrades heat
tolerance . Same heat load will cause a greater
increase in temperature.
27
How do you manage heat stress?
•
•
•
•
•
•
•
Reduce humidity by using…….
Increase air circulation by using…….
Reduce the work activity
Frequently enforced rest breaks
Job rotation
Provide adequate water for drinking
Cool spots and refuges to lower the heat
stress.
28
Spot cooling systems*
29
Work in cold climates
• Core temperature can be maintained in the
cold if the person is working and suitable
protective clothing is provided.
• If the core temp < 33 C, CNS is disrupted. At 29
C, hypothalamic core temperature control
breaks down completely.
30
Central Nervous System CNS
•
•
The central nervous system is the part of the nervous system consisting of the
brain and spinal cord. The central nervous system is so named because it
integrates information it receives from, and coordinates and influences the activity
of, all parts of the bodies of bilaterally symmetric animals—that is, all multicellular
animals except sponges and radially symmetric animals such as jellyfish—and it
contains the majority of the nervous system.
Many consider the retina and the optic nerve, as well as the olfactory nerves and
olfactory epithelium as parts of the CNS, synapsing directly on brain tissue without
intermediate ganglia. As such, the olfactory epithelium is the only central nervous
tissue in direct contact with the environment, which opens up for therapeutic
treatments. The CNS is contained within the dorsal body cavity, with the brain
housed in the cranial cavity and the spinal cord in the spinal canal. In vertebrates,
the brain is protected by the skull, while the spinal cord is protected by the
vertebrae. The brain and spinal cord are both enclosed in the meninges. In central
nervous systems, the interneuronal space is filled with a large amount of
supporting non-nervous cells called neuroglial cells
31
Work in cold climates
• Peripheral temperatures and repetitive work
• Cooling of the peripheral tissues, particularly
in the hands and feet, causes
– reductions in strength
– neuromuscular control,
– resulting in a loss of dexterity.
32
Acclimatisation to cold?
• Local acclimatisation to cold may occur in the
extremities as a reduction in the peripheral
vasoconstrictor response.
• Increased blood flow through the hands can
occur after repeated exposure to cold
conditions.
33
Acclimatisation to cold
• Up to 25% of heat loss takes place at the head.
• During cold temperatures, peripheral
vasoconstriction takes place.
• Behavioral adaptation to the cold, through
experience, is of great importance; wearing
correct clothing and keeping ‘on the move’ are
examples.
34
• Perception of cold
– The perception of cold seems to depend
on experience.
– Accustomed people = feel comfortable
with layers of clothing, despite local
cooling at the extremeties
– Unaccustomed = may confuse being cold
(low core temperature) and feeling cold (
low temp on the extremeties)
35
Protection against extreme
climates
• Specify work rest cycles
• Design cool spots
• Issue protective clothing
– Cooling jackets
– If temp > 37 C, more clothing needed to protect
from heat gain.
36
37
What is Thermal Comfort?
• - That condition of
mind which
expresses
satisfaction with the
thermal
environment.
• ISO 7730
38
Factors affecting thermal comfort
ENVIRONMENTAL FACTORS
•
•
•
•
Air temperature
Relative humidity
Air speed
Radiant conditions
PERSONAL FACTORS
• Clothing
• Activity level
• Mental state
– MRT or
– Solar intensity
39
Factors affecting thermal comfort
• Other factors affecting comfort:
• age
• sensation of old people and younger people
• adaptation
• people in warm climates may adapt to hot environment
• sex
• women: lower skin temp., evap loss and lower met. rate
• clothing and perferrence of temp.
40
What should be Estimated?
•Parameters to estimate and calculate are:
Met
Clo
Estimation of Metabolic rate
Calculation of Clo-value
Clo-value:
This is the amount of insulation that allows a person at
rest to maintain thermal equilibrium in an environment at
21 °C (70 °F ) in a normally ventilated room (0.1 m/s air
movement)
41
Prediction of Thermal Comfort
• Fanger’s comfort criteria
• developed by Prof. P. O. Fanger (Denmark)
• Fanger’s comfort equation:
f (M, Icl, V, tr, tdb, Ps) = 0
where
M = metabolic rate (met)
Icl = cloth index (clo)
V = air velocity (m/s)
tr = mean radiant temp. (oC)
tdb = dry-bulb temp. (oC)
Ps = water vapour pressure (kPa)
42
Prediction of Thermal
Comfort
• Fanger’s equation is complex
–but it may be transformed to comfort
diagrams
–it can also be used to yield these
indices:
• predicted mean vote (PMV)
• predicted percentage of dissatisfied (PPD)
43
Prediction of Thermal
Comfort
–PMV
• predict mean value of the subjective
ratings of a group of people in a given
environment
–PPD
• determined from PMV as a quantitative
measure of thermal comfort
• Tells you the % of people dissatisfied with
the thermal environment.
44
Predicted Mean Vote scale
+3 Hot
+2 Warm
+1 Slightly warm
+0 Neutral
- 1 Slightly cool
-2 Cool
-3 Cold
The predicted
mean vote (PMV)
is the mean
response of group
of people
regarding thermal
sensation using
the 7 point scale.
45
PMV and PPD
• PMV-index (Predicted Mean Vote) predicts the subjective
ratings of the environment in a group of people.
• PPD-index predicts the number of dissatisfied people.
46
47
The Comfort Equation
48
Thermal comfort in buildings
• The thermal comfort of a factory or office
worker depends on there being an average
skin temperature of approximately 33°C
• Draughts, sunlight falling on an arm or the
face and sitting next to a cold wall are all
causes of thermal discomfort due to uneven
skin temperature distribution.
49
Thermal comfort in buildings
• ISO 9241 recommends for indoor climates:
– Winter = 20-24 C
– Summer = 23-26 C
• RH values:
– 60-80 % at 20 C
– 50-70 % at 22 C
– 45-65 % at 24 C
50
References
• http://www.energy.kth.se/courses/4a1607/fil
es/Lecture2_2008.pdf
• R.S Bridger (1995) Introduction to Ergonomics
51
BMMD 3553
ERGONOMICS
DESIGN
1
About 450millionpeople–65% of theEuropean
population–areexposedtonoiseintensitiesabove55dB,
alevel highenoughtocauseannoyance, aggressive
behavior andsleepdisturbance.
(EuropeanEnvironment Agency Report, 1995)
2
Objectives
 Understand the principles of sound and noise
 Recognize human auditory limitations
 Understand the principles of sound measurements.
3
Introduction
 Sound is created by vibrations from a source and is
transmitted through a media (such as the atmosphere)
to the ear.
 When sound is generated, it causes vibration and
makes the air molecules to be moved back and forth.
This alternation creates corresponding increase and
decreases in the air pressure.
4
Introduction
 Acoustic waves can be defined as pressure fluctuations
in an elastic medium.
 The amplitude of acoustic waves is expressed in 1 Pa
(Pascals)
 The threshold of hearing (lowest amplitude of
pressure oscillations in air detectable by the ear) is
0.00002 N/2 at a frequency of 1000 Hz.
5
Frequency (Hz) vs Loudness (dB)
6
Introduction
 Pitch = Pitches are compared as "higher" and "lower" in the
sense. (measured in Hz)
7
 Loudness = associated with amplitude of sound (dB pressure)
High vs Low Frequency waves
(Pitch)
8
Low vs High pressure waves
(Loudness)
Low Amplitude
High Amplitude
9
Loudness
10
Introduction
 The number of cycles per second is called the
frequency of the sound. Frequency is expressed in
hertz (Hz) and is equivalent to cycles per second.
 We have a hearing range of 20 to 20,000 Hz (highest
sensitivity between 1000 to 3000 Hz).
11
Introduction
 Our hearing is not equally sensitive to all frequencies.
In addition, people differ in their relative sensitivities
to various frequencies.
12
Sensitivity of the ear
 Auditory sensitivity is greatest between 1000 and 4000 Hz
 The loudness of a noise depends on its frequency as well as
its sound pressure level
13
Sensitivity of the ear
 Hearing sensitivities decreases with age as shown in
the table below.
14
15
SPL dB
DeciBel (dB)
•
The term dB (deciBel) and the dB scale are used world-wide for the
measurement of sound levels. The deciBel scale is a logarithmic
scale where a doubling of sound pressure corresponds to a 6 dB
increase in level.
• It is important to realize that the term 'dB' can have different
meanings and is not a fixed value like the volt or the meter etc. The
value of a dB depends on the context in which it is used.
• Here are some examples of different sound intensities as
expressed in dB(HL):
180 dB: Rocket at take-off
140 dB: Jet engine at take-off
120 dB: Rock band
110 dB: Loud thunder
90 dB: City traffic
80 dB: Loud radio
60 dB: Ordinary conversation
30 dB: Soft whisper
0 dB: Softest sound a person can hear
16
17
SPL dB
Frequency
•
•
•
•
•
The frequency of a sound is the number of cycles of a sound
wave in one second.
The unit of measurement is hertz (Hz).
The frequency of a sound increases as the number of cycles
per second increase.
Vibrations between 20 and 20,000 cycles per second are
interpreted as sound by a normal healthy person.
A high-pitched sound could be a piccolo flute or a bird singing.
Low-pitched sounds could be thunder heard from far away or
tones from a bass guitar.
18
Measurement of sound
 The amplitude of sound is evaluated by measuring the
sound pressure level (SPL).
 A decibel is the measure of sound pressure level. The
formula for sound pressure level is sound intensity (in
decibels) = 20 log(P1/P2)
-- where P1 is the sound pressure amplitude we want to
express in dB and
-- P2 is the standard reference level. P2 is fixed at the
threshold of hearing under optimal conditions (a pure tone
of 1000 Hz at 20 micro Newtons/square meter).
19
20
Measurement of sound
 Loudness doubles with each 10 dB increase in sound
intensity.
 Frequency influences the experience of loudness.
 Sound sustained at a level of 85 – 90 dB has the
potential to damage hearing
21
Measurement of sound
 Old age is marked by compression of the audible
frequency range from 16– 20 000 Hz to 50–8000 Hz, a
condition known as presbycusis.
(Presbycusis is the most common type of Sensorineural Hearing Loss caused by the
natural aging of the auditory system. It occurs gradually and initially affects the
ability to hear higher pitched (higher frequency) sounds)
 The higher frequencies are usually lost first and most
aged people cannot hear sounds above 10 000 Hz.
 which is why it is better to lower the voice when
attempting to communicate with older people
22
Measurement of sound
 Audiometric testing determines the minimum
intensity (the threshold) at which a person can detect
sound at a particular frequency.
 As sensitivity to particular frequencies is lost owing to
age or damage, the threshold increases.
23
Measurement of sound
 A person is determined to have hearing damage if
their threshold shift increases.
 A standard threshold shift (STS) is defined by OSHA
(USA) as the threshold shift of 10dB at 2000, 3000 or
4000 Hz in either ear.
24
Measurement of sound
 Temporary threshold shifts can occur after exposure to
loud noise.
 Repeated exposure leads to permanent threshold shifts
(noise induced deafness)
 Exposure to certain chemicals (toluene) can hasten
hearing damage.
25
Psychosocial aspects of noiseinduced hearing loss
 Noise-induced hearing loss can have serious
implications for quality of working life and
employability.
 Noise-induced hearing loss can have serious
implications for quality of working life and
employability.
26
27
Measurement of sound
 Sound level meters provide several different measures
of sound intensity.
 The dB (linear) scale is used to give the Sound
Pressure Level (SPL).
 The sum of the pressure levels of the various
frequencies is called the Sound Level (LS).
28
Measurement of sound
 In practice, there are usually several sources of sound
and it is the combined effects of these on the worker
that is of interest.
 If three adjacent machines have noise levels of 90, 95
and 101 dB, them combined noise will be 102 dB.*
 Removal of the 90 dB machine will still leave a noise
level of 102 dB whereas removal of the 101 dB source
leaves a noise level of 96.5 dB.
29
Measurement of sound
 This has many practical implications for noise control.
 In particular, it emphasizes that the best approach to
noise control in a room with several noise sources is to
begin with the noisiest source.
30
Measuring noise exposure
 An exposure level of 85 dB(A) is regarded as the first
action level at which workers must be informed and
offered ear protection.
 In the USA, OSHA has specified 90 dB(A) as the
maximum permissible exposure to continuous noise
for an 8-hour shift.
31
Measuring noise exposure
 A noise dosimeter
integrates the noise
measured at the
microphone over a
period of time and
expresses it as a % of the
daily allowable noise
dose (e.g. as a percentage
of 90 dB(A) for 8 hours).
32
Measuring noise exposure
 Dosimeters can be used to decide whether workers are
being exposed to excessive noise and;
 Whether they require ear protection and also to
specify for how long per day they may be exposed to a
particular noisy work situation.
33
Measuring noise exposure
 If the worker is exposed to different noise levels at with
different lengths, then the Time Weighted Average for
8 hours can be calculated.
 According to OSHA, the Action Level is 85 dB
Refer to :
http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=
9736
34
TWA - Time Weighted Average
Noise Levels - and Noise Dose
o A time weighted average (TWA) is the maximum average
exposure to such hazardous contaminants to which
workers may be exposed within the workplace without
experiencing significant adverse health effects over the
standardized work period (8 hours per day).
35
TWA - Time Weighted Average
Noise Levels - and Noise Dose
o This is the parameter that is used by the OSHA
Regulations and is essential in assessing a workers
exposure and what action should be take
36
Working Out the Noise Dose and
TWA
 Before working out the worker's TWA you have to
measure the different high noise levels that the worker
is subjected throughout a normal working day.
 The Time Weighted Average is calculated using these
noise levels together with the amount of time that the
worker is exposed to them.
37
 First calculate the noise dose as:
38
Measuring noise exposure
Dose = 100 × [ (6 /
2
8
(86−90)/5 ) + (3 /
8
2(92−90)/5
)]
39
L = refers to sound level (dB)
T = Reference duration (hour)
Please refer to:
http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p
40
_id=9736
 Once you have the Dose% figure, you can calculate the
TWA using the following equation:
41
42
Permissible Noise Exposures
Exposure per day (hours)
8
6
4
3
2
1.5
1
0.5
<=0.25
dBA
90
92
95
97
100
102
105
110
115
43
44
45
Noise surveys
 Personal sampling = If people move around at work or
if noise levels are closely tied to a worker’s activity,
personal sampling is necessary.
 Area sampling = the workplace is evaluated;
this works well if the noise is fairly constant and
workplaces are fixed
46
Ear protection
47
Characteristics of effective ear
protectors
 Impervious to air
 Adaptable to the shape of the user’s head or external
auditory canal to:
• Form an airtight seal
• Avoid pressure ‘hot-spots’ to ensure comfort
 Remain firmly in place without causing pressure
ischaemia
48
Ear protection
 Acceptance of ear protection is often a major problem
in industry.
 Workers exposed to excessive noise frequently refuse
to wear any ear protection, complaining
 that it causes discomfort, interferes with speech
communication and, in jobs where
 the machine is the source of noise, degrades task
feedback.
49
50
Industrial noise control
 Several approaches to noise control can be identified:
 Eliminate the threat to hearing by redesigning the
source of noise using a less noisy sound source.
 Remove personnel from the noisy environment.
 Protect personnel by issuing ear plugs or muffs or build
an acoustic refuge.
51
Industrial noise control
 Fans or blowers = are much noisier when running at
high rather than low speed.
 Muffling Pneumatic tools such as paving breakers,
screwdrivers, and dentists’ drills = produce noise due to
the exhaustion of compressed air to the atmosphere.
This can be reduced by piping the air away from the
operator
52
Industrial noise control techniques:








Sound curtain
Sound enclosure
Sound barrier wall
Silencers
Baffles
Composite foam
Sound blanket
Duct lagging
53
Sound curtains or sound blankets are an effective means of noise
reduction and sound proofing of process machinery, pumps,
compressors and anything in a facility that requires noise control and
access to the equipment. Sound curtains are a proven solution for
industrial noise control.
54
Sound Enclosures offer the highest level of sound control and noise
reduction utilizing modular steel panels with various constructions
to meet your equipment access, ventilation, and usable floor space
needs.
55
Sound barrier walls are used indoors and outdoors for the control of
noise where a roofed sound enclosure system is impractical or
impossible. Constructed of modular steel panels and typically
structural steel columns. A sound barrier wall is an effective solution
for many industrial noise control equipment applications.
56
Silencers also known as mufflers, are the most effective means of
solving airflow generated noise. Typically mounted inline with
ductwork or directly to the sound generating equipment itself,
acoustical silencers offer the highest level of sound reduction for air
generated noise to exceed your industrial noise control goals.
57
Baffles are an economical and unobtrusive method for adding sound
absorption to a noisy area where equipment access is at a premium
and enclosures are not a practical solution for sound control. Typically
ceiling mounted and available in many standard or custom sizes to
meet your industrial noise control application needs
58
Composite foam is an easy and effective means of noise control for
lining existing machine cabinets or steel housings to reduce
unwanted noise. Available with or without protective facings,
additional sound barriers, and PSA adhesive backing. A simple
solution for industrial noise control equipment.
59
Sound blankets are removable sound barrier/sound absorber
composites custom fabricated to fit snugly to the noise source and
reduce sound emissions. Effective application where space and
accessibility is at a premium.
60
Acoustic Duct Lagging is a lagging, composite material that is typically
used to wrap noisy pipes and ducts to block the noise that transmits
through the walls of the pipe or duct a air or other contents move
through
61
Industrial noise control
 Part ejection Pneumatic ejectors are sometimes used to
remove parts from presses = Mechanical ejectors are
usually quieter.
 Pneumatic tools Hydraulic or electric equivalents are
usually quieter.
 Vibration can be exacerbated by imbalance or
eccentricity in rotating members, by inadequate
mountings and by wear.
62
Noise insulation
 An acoustic enclosure can be built around the source
or an acoustic refuge built for the operator.
 If the source is enclosed, noise is inter reflected by the
walls of the enclosure, losing energy as it does so.
63
Noise Insulation
 Can be installed to absorb noise and block its
transmission from one place to another.
Noise Reduction Coefficient(NCR)


0 = no ambient sound is absorbed
1 = all the sound is absorbed
 Kleeman (1991) recommended a minimum NCR of 0.85 for
upholstered screen used in office.
 Thick pile carpets (NRC up to 0.7) very effective at
absorbing vibration.
 Pleated curtain can be places over bare walls or windows
to reduce noise transmission and can be very effective.
 It may also be less expensive to cover bare walls with
curtain rather than with accoustic tiles.
64
Active noise control
 An anti-phase version of the source, of equal
amplitude, is produced.
 Active control is only really effective at lower
frequencies but has many applications because much
machine noise is of a periodic/cyclic nature.
65
 Modern active noise control is achieved through the use of a computer,
which analyzes the waveform of the background aural or nonaural
noise, then generates a signal reversed waveform to cancel it out by
interference.
 This waveform has identical or directly proportional amplitude to the
waveform of the original noise, but its signal is inverted. This creates
the destructive interference that reduces the amplitude of the
perceived noise.
 The active methods differ from passive noise control methods
(soundproofing) in that a powered system is involved, rather than
unpowered methods such as insulation, sound-absorbing ceiling tiles
or muffler.
 The advantages of active noise control methods compared to passive
ones are that they are generally:
1.
2.
3.
More effective at low frequencies.
Less bulky.
Able to block noise selectively.
66
Effects of noise on task
performance
 Performance of auditory task is compromised.
 Poulton (1976b, 1977) has suggested that continuous
noise can interfere with work by masking auditory
feedback and inner speech.
 Haslegrave (1990) concluded her review by stating that
effects are difficult to determine and are task-specific.
67
Effects of noise on task
performance
 Music while working in factories with the belief that
music improves task performance.
 No evidence exists to prove that.
 It seems likely that ‘music while you work’ would be
appropriate for those engaged in non-verbal tasks at
risk of falling asleep through boredom.
68
Effect of noise on task performance
• Most of us may think of loud assembly lines or construction sites
•
•
•
•
•
when we think of noise pollution in the workplace.
Regular offices are not immune. With more people packed into busy
office spaces, office noise is a common complaint.
Co-workers who talk, drum their fingers on the desk, or offer other
distracting noises can decrease the productivity of those around them
without realizing it.
Research proved noise can be distracting.
One study examined children exposed to airport noise and found that
their reading ability and long-term memory was impaired.
Those working in noisy office environments have also been found to
be less cognitively motivated, and to have higher stress levels,
according to a Cornell University study.
69
VIBRATION
 Vibration is defined according to Wikipedia as: “…..
mechanical oscillations about an equilibrium point.”
 The unit of vibration is the root mean square (rms) or
peak acceleration of the oscillation.
Oscillation is the repetitive variation, typically in time,
of some measure about a central value (often a point of equilibrium) or between two or more different states
70
VIBRATION
o Vibration measurement instrument = accelerometer
o Accelerometer are placed such that vibration can be
measured in three translational axes ( backwards and
forwards, up and down, and side to side) and three
rotational axes (pitch, yaw and roll).
71
VIBRATION
 WBV (whole body vibration) is characterized as
mechanical oscillations that affects the entire body.
Usually happens as a result of sitting or standing on a
vibrating platform.
 Examples are trucks, buses and forklifts.
72
VIBRATION
 Exposure to high levels of WBV can result in adverse
health outcomes.
 It can also worsen the symptoms of low back pain.
73
VIBRATION
 The ‘Vibration Directive’ (Directive 2002/44/EC ) sets
minimum standards for controlling the risks from
whole-body vibration.
 It has values for vibration limits for the whole body
and the hand arms.
74
EU Directive 2002/44/EC
Exposure limit values and action values
For hand-arm vibration:
ELV (Exposure Limit Value) = 5 m/s2, standardized
for 8 hr period
ALV (Action Limit Value) = 2.5 m/s2, standardized for
8 hr period
75
EU Directive 2002/44/EC
Exposure limit values and action values
For whole body vibration:
ELV (Exposure Limit Value) = 1.15 m/s2, standardized
for 8 hr period
ALV (Action Limit Value) = 0.5 m/s2, standardized for 8
hr period
76
References
 RS Bridger, Introduction to Ergonomics
 EU Directive 2002/44/EC
77
78
BMMD 3553
1
“The eyes lead the body.”
(Dr J. Sheedy, School of Optometry, University of
California at Berkely)
2
Objectives
 Understand how human vision works
 Understand the principles behind vision and lighting
3
Introduction
 Light is electromagnetic radiation that is visible.
 The electromagnetic spectrum is extremely wide but
the visible part is extremely narrow
The electromagnetic spectrum is the range of
frequencies (the spectrum) of electromagnetic radiation
and their respective wavelengths and photon energies.
4
5
Vision and the eye
 The eye is a fluid-filled membranous sphere that
converts electromagnetic radiation into nerve
impulses that it transmits to the brain along the optic
nerve.
 Blinking is a reflex action that occurs every 2–10
seconds.
6
Vision and the eye
 The function of blinking is to stimulate tear
production and flush out foreign objects (such as dust
particles) from the surface of the eye.
 Tasks requiring concentration can reduce the blink
rate.
7
Vision and the eye
 This can cause particles of dust to accumulate on and
lead to drying and irritation of the surface of the eye.
 VDU (Visual Display Unit) related tasks are the
common culprit.
VDU is a device with a screen that
displays characters
or graphics representing data in a
computer memory.
8
9
Vision and the eye
• The human eye is like a camera.
• It has an adjustable lens through which light rays are
transmitted and focused.
• The light falls on a sensitive area called the retina.
10
Vision and the eye
 In normal or corrected vision persons, the light rays
are exactly focused on the retina.
 The retina consists of about 6 to 7 million cones
concentrated near the center and about 130 million
rods distributed in the outer areas of the retina
around the sides of the eyeball.
11
12
13
Vision and the eye
• The cones receive daytime vision and the rods are
important in dim light and at night.
• Greatest sensitivity is in the fovea (the dead center of
the retina).
• For clear vision, the eye must be directed so that the
image of the object is focused on the fovea.
• The image at the retina will be inverted.
14
Vision and the eye
 The cones and rods are connected to the optic nerve
which transmits neural impulses to the brain which
integrates them to give the visual impression of the
object.
 This process also corrects the inverted image on the
retina.
15
Vision and the eye
• Accommodation = adjusting the lens for proper
focusing of the light rays in the retina.
• In normal accommodation, the lens flattens to see far
objects and bulges to see near objects.
16
Vision and the eye
• If accommodation of the eyes is inadequate:
• Nearsightedness = the lens remains in a bulged
condition preventing proper focusing to see far
objects.
• Farsightedness = the lens remains in a flat
condition preventing proper focusing to see near
objects.
17
Vision and the eye
 Dark adaptation:
 Pupil increases in size when entering dark rooms
 Decreases in size when entering bright places
 Complete dark adaptation usually needs 30 min or more
while reverse adaptation (from dark to light) takes place
in 30 seconds to two minutes.
18
19
Vision and the eye
Colour vision:
•The cones of the retina are the basis for colour
discrimination.
• Some people find it difficult to discriminate between
red and green, blue and yellow. Few people are colour
blinded.
20
Color vision theory
 The colour opponent process is a color theory that
states :
 the human visual system interprets information about
colour
 by processing signals from cones and rods
 in an antagonistic manner.
21
Try reading this paragraph
until to the end and see how
you feel about this color
scheme. Do you think that it
is appropriate to have this
type of color scheme for
lecture slides?
22
23
24
25
26
 Try comparing the opponent color channels to tubes
that can only carry one kind of marble at a time.
 For example, one tube carries both red marbles (red
light) and green marbles (green light), but only one
type can travel through the tube at a time.
27
 When red marbles come out the end of the tube, they
hit a switch in the brain and turn it on, signaling red,
while the green marbles signal green.
 Because only red or green can travel at one time, we
cannot see colors that are combinations of red and
green ("red-green").
28
Color vision implications
• The notion that red and green and blue and yellow are
related neurologically has interesting design
implications.
• Generally, these opponent colour combinations should
be avoided in display design because of the afterimage
problem, particularly in active displays such as VDU
screens.
29
30
Guidelines for color selection
• Choose compatible colour combinations. Avoid
red/green, blue/yellow, green/blue, red /blue pairs.
• Use high colour contrast for
character/background pairs.
• Use redundant coding (shape or typeface as well as
colour); 6–10% of males have defective colour
vision.
31
Measurement of light
 Measurement of light is essential in the design and
evaluation of workplaces.
 The measurement of light is known as photometry.
32
Measurement of light
 The main photometric units are luminous intensity,
luminous flux, luminance and illuminance.
 Luminous intensity
 Luminous flux
 Luminance
 Illuminance
33
• Luminous intensity = The power of a source or
illuminated surface to emit light. Units = candela (cd)
• Luminous flux = The ‘rate of flow’ of luminous
energy. Units = lumen (lm) . Used to measure bulb
capacity.
• Luminance = The light emitted by a surface (cd/m2)
• Illuminance = The amount of light falling on a
surface. Units = lux (lx)a
34
The amount of light falling on a surface
35
Lighting standards
 Standards differ.
 Investigations by various companies in the USA and by
the Industrial Fatigue Research Board in Britain
demonstrated that the performance of visually
demanding tasks could be improved by increasing the
level of illumination. (more is better)
36
Lighting standards
 A more recent trend has been to reduce the levels of
illumination in workplaces, particularly offices.
 This has occurred partly because of the desire to
conserve energy and also as a response to the
introduction of VDUs into the workplace.
37
Lighting standards
 Ergonomists have recommended that illumination
levels be lower in VDU offices to avoid glare and
reflection problems.
 In practice, the choice of an appropriate level of
illumination depends not only on the task but also on
the distribution of objects in the visual field and their
luminances.
38
Contrast and glare
 The direction of gaze is involuntarily drawn to bright
objects in the visual field.
 This is known as phototropism. (Ex: jewellery shops).
 Glare occurs when there is an imbalance of surface or
object luminances in the visual field – the brighter
sources exceeding the level to which the eye is
adapted.
39
Contrast and glare
 For example, if the ambient luminance is high
compared to the task luminance,
 The retina will adapt to the former rather than the
latter and the task will appear dim and will be more
visually demanding.
40
Contrast and glare
• Glare is a visual phenomenon that is caused by a
difference in luminous intensity, or a bright spot.
Brightness is relative and so luminous intensity is a more
scientific measurement. But basically a bright spot throws
off your eyes’ auto-brightness meter and the resulting glare
can cause eye strain , discomfort, fatigue and temporary
vision loss.
41
Contrast and glare
• There are two types of glare:
Direct glare is caused by intensely bright light sources
shining directly into the eyes, such as the sun coming in the
window and bright light fixtures shining down from the
ceiling.
Reflected glare includes bright spots that are caused when
light reflects on a computer monitor or work surface and
into the eyes
42
How to Reduce and Eliminate Glare
 Glare is caused by the reflection of light off of surfaces and is a
primary cause of eye strain. You can get rid of glare by
controlling the light source, the surface reflecting it or by
filtering it before it reaches your eyes.
 The Light Source :
Direct light causes the most glare. Use reflected light
instead.
Diffuse your light. Translucent filters (like lamp shades
or globes) soften the light.
Use curtains or translucent plastic blinds on windows.
Closing these will diffuse the incoming light instead of
reflecting them like solid metal or wood blinds.
43
How to Reduce and Eliminate Glare
 The Surface :
Shininess is measured by reflection and glare. That
means the duller the surface the less glare there will
be. Use work surfaces that have matte finishes.
Some items, like computer screens, are inherently
smooth and therefore glossy. Use a glare filter over
them
44
45
Lighting design considerations
 For visual comfort and to meet visual demands the
following should be considered (Grandjean, 1980):
 A suitable level of illumination
 A balance of surface luminances
 Avoidance of glare
 Temporal uniformity of lighting
46
Illumination levels
• More is not better
• High levels of illumination may increase glare and may
wash out important visual details.
• Illumination levels are inadequate, increasing them
may improve performance.
• Illumination levels can only improve up to a point,
then non-visual limits to performance (such as
motivation, fatigue or manual dexterity)
47
48
 In practice, a balance of surface luminances is achieved by
specifying appropriate illuminances and corresponding
reflectances of the surfaces in a room.
49
Avoidance of glare
• Glare can be reduced by choosing a suitable combination
of direct and indirect lighting.
• With direct lighting, most of the light is directed towards
the target in the form of a cone (Figure 10.8). This produces
hard shadows and sharp contrasts between illuminated
and non-illuminated areas.
• Indirect lighting is reflected off other surfaces in a room
and produces a smoother transition between surface
luminances and reduces shadows.
50
 Correct positioning of workstations with respect to windows,
lamps and bright surfaces is therefore very important.
51
52
Temporal uniformity of lighting
 Fluctuating luminances can be more disturbing than
static contrasts.
 Incandescent bulbs radiate light fairly uniformly over
time, whereas fluorescent lamps are known to flicker.
53
• However, malfunctioning or old lamps can produce
visible flicker that may cause visual discomfort.
• In factories, flicker is a hazard since it can have a
stroboscopic effect – rotating or oscillating machine
parts may appear stationary or to move more slowly,
• If their frequency is similar to that of the flickering
source that illuminates them.
54
55
Visual fatigue, eyestrain and near
work
• Near work is thought to cause visual fatigue and
occular symptoms
• It has been reported in microscope operators (e.g.
Soderberg et al., 1983) and in VDU workers
• Worsened by organisational factors such as the
rigidity of work routines and the duration of work
periods (Gunnarson and Soderberg, 1983).
56
57
Psychological aspects of indoor
lighting
• Sundstrom (1986) has reviewed the research on
lighting and satisfaction.
• 400 lux is generally acceptable.
• Further increases in illuminance bring only modest
increases in satisfaction.
• Glare is associated with dissatisfaction.
58
 The contribution of daylight from windows to the
illuminance on indoor surfaces is usually much less
than it appears to be
 Although direct sunshine can cause severe glare on
surfaces such as VDU screens, chromed surfaces, etc.).
59
Trivia
 Does reading in dim light really hurt your eyes?
60
THE END
61
BETD 3553
Objective
 Understand the display principles by Wickens et al
 Apply the display principles in real life
Introduction
 Christopher Wickens et al. defined 13 principles of
display design in their book An Introduction to Human
Factors Engineering.
 These principles of human perception and
information processing can be utilized to create an
effective display design.
 A reduction in errors, a reduction in required training
time, an increase in efficiency, and an increase in user
satisfaction are a few of the many potential benefits
that can be achieved through utilization of these
principles.
• Certain principles may not be applicable to different
displays or situations.
• Some principles may seem to be conflicting, and there
is no simple solution to say that one principle is more
important than another.
• The principles may be tailored to a specific design or
situation.
• Striking a functional balance among the principles is
critical for an effective design
Perceptual Principles of
Display Design

Displays should be
legible (is that legible?)

If the characters or objects
being displayed cannot be
discernible, then the
operator cannot effectively
make use of them.
Perceptual Principles of
Display Design

Absolute Judgment Limits – avoid making the operator
judge the represented variable level on the basis of a
single sensory dimension (color, size, pitch, etc.)

Top-Down Processing – signals are perceived and
interpreted based on operator’s past experience
Click Here for
Concentrate!
Card Trick Example
I have
removed
the card
you were thinking
about
Think
about
one card
and remember
it, then click
Perceptual Principles of
Display Design
Redundancy
Gain –
presenting a signal in more
than one way increases the
likelihood it will be interpreted
correctly
 ex: the traffic light: stop
if RED, move if GREEN
Discriminability – similar
appearing signals are likely
to be confused
 ex: Speed or RPM?
Mental Model Principles of
Display Design

Principle of Pictorial Realism – Display looks like the
variable it represents

Principle of Configural Displays – elements are configured in
same manner as environment it represents
Some “Door Ajar” indicators not only tell you that
the door is open, but show you which one

Principle of the Moving Part – Moving elements should
move consistently with the user’s mental model
The tape indicator moves in the
same direction the tape is playing to
make it easier to know whether to FF
or REW
Attention Principles of
Display Design

Minimize Information Access Cost – frequently accessed
sources of info should be readily available


Proximity Compatible Principle – info that needs to be
integrated or compared should be presented close together
(allows for patterns to emerge)


Ex: right mouse button (PC) brings up menu of common commands
Close spatial proximity increases the likelihood of parallel
processing
Principle of Multiple Resources – facilitate processing of info
by presenting via more than one medium

Click for example
Describe at least 2 display principles from this
picture.
Memory Principles of Display
Design

Knowledge in the World – Showing something that
directly resembles what’s happening on the real
world. Eg: Visual Echo of a phone number

Principle of Consistency – Displays should present
info in a consistent manner


Ex: All Microsoft programs have same main menu (File Edit
View)
Principle of predictive aiding - Anticipates what
information people will need to remember in order to
execute tasks they intend.
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