Sketch courtesy from Riekes Material
Handling
Severity of the problem
• Manual handling (lifting) is injury prone &
expensive
– BLS 2007: 140,330 out of 1,158,870 or 12% of all non
fatal injuries and illness cases in US private industries
with days away from work occurred from exposure
from overexertion in lifting.
– Median days away from work per incident was 8
days
– 26.8% cases caused 31 or more days off.
– Details:
http://stats.bls.gov/news.release/osh2.nr0.htm
Nature of Injury
• Pain in shoulder, upper
back, lower back and knee.
• It is believed that cumulative
trauma of soft tissue over
time is the cause of injury
and not from an acute
trauma due to overload.
• Lower back pain is a major
problem associated with
MH.
Courtesy, US Department of Energy,
Berkeley Lab
Approaches to investigation of
cause of MMH injuries
• Biomechanical approach
• Physiological (or cardiovascular) approach
• Psychological approach
Biomechanical approach
• Computes torque/internal-forces due to body
posture and load handled on critical body joints
and compares those to joint strength.
• Generally applicable for one time loading
situation or worst case scenario of a task.
• Can predict localized muscle fatigue.
• Shortcoming: Does not take into account effect
of duration and frequency of MMH task.
Physiological (or cardiovascular)
approach
• Considers metabolic energy requirement of the
MMH task and systemic fatigue.
– Goal is to keep metabolic rate less than 5 Kcal/min for
an eight hrs task
• Takes into account rate and duration of MH and
dynamic effect of body movement.
• However, injury may occur due to localized
muscle or joint overload, which this method
cannot isolate.
Psychological approach
• Based on the assumption that human can
inherently perceive the stress level and can
determine his or her limits of MH.
• Skilled handlers perform MH in laboratory
settings with varying load and type of activity.
Frequency and other MH factors (distance,
height , size of the box etc.) are kept constant for
a given task. Based on the maximum load
acceptable by the handler population, allowable
load limits are determined in percentile form.
• Supposed to take care of both biomechanical
and physiological factors.
MH variables
• Individual
– Selection
– strength testing
• Technique
– training
– posture
• Task
– Most effective way to limit occupational injury is to
design the MH task such that everybody can perform
it with least risk of injury
MH task types
•
•
•
•
Pulling/pushing
holding
carrying, and
lifting
Pulling & pushing
• Limits of pull and push forces for many
combinations of handle height and frequencies
are available for industrial population. Table 13.1
and 13.2.
• Force capability goes down as it is exerted more
often.
• Pushing capability is higher than pulling.
Pushing also produces less spine compressive
force.
• Push at waist level; pull at thigh level
Pulling/pushing task design
• Use a force gage to measure the force
• Reduction of friction coefficient may
reduce the force.
• Remove obstacles, larger wheels.
• A vertical push-pull bar may allow height
adjustment for both short and tall person.
• Avoid muscle power for long distance,
ramps and high frequency moves.
Holding
• Holding causes static muscle contraction
and fatiguing.
• Often higher reach requirement causes
undue muscle tension at the lumbar spine
region.
• Reduce the holding torque and reduce the
duration of holding.
Carrying
• Carrying induces internal static muscle tension
in hand, arm, shoulder and trunk muscles.
• Reduce the load and or reduce the moment arm.
Body hugging back-pack design reduces the
moment arm. Keep the load as close as possible
to the spine.
• Box with a handle may induce more lower back
stress compared to a box without a handle.
Lifting
• NIOSH lifting equation (1994) provides a formula to determine the
Recommended Weight Limit (RWL) for a specific lifting task.
• It starts with a load constant of 51 lbs (23 kg), which is the maximum
load for an ideal lifting task situation.
• This load constant is then multiplied by various factors (all are equal
or less than 1) to obtain the RWL.
– RWL= 51 x HM x VM x DM x FM x AM x CM lbs
• If control of the load is necessary at the lift initiation and lift
destination, then two RWLs are determined, one for the lift initiation
and lift destination points.
• Lifting Index (LI) = Actual Load weight during lifting / RWL,
– if LI is >1, the task is not acceptable and design modification is needed
to make the LI = 1 or less
– LI < 1 should be acceptable to 75 percent females and 99 percent
males.
Criteria used to develop niosh
lifting equation
• NIOSH lifting equation (1994) is based on
– Biomechanical criterion of max spine compressive
force 3400 N
– Physiological (metabolic) criterion of 9.5 kcal/minute
(which is VO2 max for 50th percentile female of age
40) multiplied 70% (due to arm work), 50% for one
hour, 40% for two hours, and 33% for eight hours.
– Psychophysical criterion is based on a 34 cm wide
box for a vertical displacement of 76 cm and lifting
frequency of 4 lift/min.
Scope of NIOSH lifting equation
• Applicable for two handed lifting task in free
standing posture. Not applicable MH at seated
or kneeling posture. Load must not be unstable.
• Handling should not include too much carrying,
not more than one or two steps.
• performed in normal room ambient condition.
• Other physical tasks are 10% or less.
• For other conditions, specific biomechanical and
physiological investigation will be needed to set
the limit.
Factors in NIOSH Equation (US
system of measures inch, lb)
• Horizontal multiplier HM =10/H , where H is the
projected distance from the handle to body
centerline.
– Closest to body (H<=10 inch) is optimum HM = 1, If H
is more than 25 inch, HM = 0.
• Vertical Multiplier VM = 1- .0075|V-30|, where V
= objects vertical location (knuckle location) from
floor.
– Knuckle Height (V=30 inch) is optimum, any height
other than this is penalized.
– Penalty for both up and down from knuckle height.
Factors in NIOSH Equation
(continued)
• Distance multiplier (DM) = .82 + 1.8/D, where D
is the vertical load movement distance.
If D is less than 10 inches DM = 1,
If D = 70 inches DM = 0.
• Asymmetry multiplier AM = 1- 0.0032A, where A
is the angle in degrees from mid-sagittal plane.
– For lifting in mid-sagittal plane AM = 1.
– Ignore positive or negative angle.
– Max value of A = 135o.
Factors in NIOSH Equation
(continued)
•
Frequency multiplier (FM) is based on lift
frequency (lift/min)
–
–
–
•
•
•
(1) over short (< 1hr), moderate (< 2hr), long (< 8hr)
duration.
(2) have adequate recovery times after the lifting
task
(3) whether below or above knuckle height
Refer to table 13.9 to determine FM.
Max frequency is 15 lifts/min and beyond this
FM = 0.
If the job is short term (<1hr) and F < 0.2
lift/min, FM = 1.
Factors in NIOSH Equation
(continued)
•
•
Coupling multiplier (CM) Depends on
handle design, and vertical location (V) of
load
See table 13.10 and 13.11.