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) 𝑊𝑊𝑒𝑒𝑖𝑖𝑔𝑔ℎ𝑡𝑡 = 𝐿𝐿𝐼𝐼 𝑅𝑅𝑊𝑊𝐿𝐿 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/𝑚𝑚2 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.