Manual Material Handling (MMH)

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Transcript Manual Material Handling (MMH)

Sketch courtesy from Riekes Material
Occupational MH tasks
• Occupational material handling (MH) can be
classified into three broad types:
– Pulling/Pushing – eg. Pushing (or pulling) medical
carts, gurneys, food carts, boxes, heavy machinery
– Holding/Carrying – Holding tools and implements,
carrying boxes, ladders, back packs, bags, parts etc.
– Lifting/Lowering – Loading/unloading pallets,
machines, order picking in warehouses etc.
• Occupational MH tasks mostly involve heavy
physical exertion.
Nature of Injury from MH
• Apart from slip, trip and fall related to
MH, a vast majority of injury occurs in
the form of pain in shoulder, upper
back, lower back and knee.
• Lower back pain is the most common
problem associated with MH tasks,
which is associated with high lower
spinal loading.
• It is believed that cumulative trauma
from MH gradually degenerates
specific body tissues over time. A
degenerated tissue subsequently
manifests in the form of chronic pain.
• The pain from MH are generally not
from an acute trauma from a single
Courtesy, US Department of Energy,
Berkeley Lab
Three types of Investigation
approaches to design MH tasks
• Biomechanical approach
• Physiological (or cardiovascular) approach
• Psychophysical approach
Biomechanical approach
• It computes torque and internal forces in critical
body joints (specially lower back) that arise from
the body posture in performing the MH task. The
joint torque and forces are then compared to
the joint strength to assess the permissible MH
load (weight to be handled).
• Limitation: It cannot take into account the effect
of duration and frequency of MH task. According
to this approach the permissible load is the
same if the MH task involves one repetition or
hundreds of repetitions a day.
• It can predict localized muscle fatigue.
Physiological (or cardiovascular)
• Considers metabolic energy requirement of the
MH task and systemic fatigue.
– Goal is to keep metabolic rate less than 5 Kcal/min for
an eight hrs task, with a view to prevent accumulation
of fatigue.
• Takes into account frequency and duration of
MH task and dynamic effect of body movement.
• However, injury may occur from localized muscle
or joint overload (the weakest link), which this
method cannot isolate.
Psychophysical approach
• This approach is based on the theory of human
sensation (pshycho), that is, we can perceive our internal
stress levels (physical).
• In this approach subjects (often skilled handlers) perform
an MH task in under controlled condition. Frequency
and other MH factors (e.g. distance, height , size of the
box etc.) are kept constant, while the MH load is varied
during their task performance. Based on the load
acceptable by the subjects, an allowable load limit is
• A virtue of this approach is that it combines both
biomechanical and physiological stresses from MH
MH Variables
• Individual
– stronger the worker, lesser the risk of injury
– Selection based on strength testing
• Technique
– Training related to posture, hand orientation, foot posture etc. It
may reduce injury risk, however human forgets.
– The industries that adopted MH training produced no significant
reduction of MH injury rate.
• 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 ergonomic design approach.
Pulling & pushing
• Various limits of forces for pushing or pulling
(Table 13.1, 13.2, 13.3 and 13.5) for many
combinations of handle height and frequencies
are available for industrial population.
• Main features:
– Force capacity goes down when pulling/pushing
frequency goes up.
– Pushing capability is higher than pulling.
– Pushing 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, choose larger wheel
size to reduce push force.
• 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.
• Carrying induces internal static muscle tension in hand,
arm, shoulder and trunk muscles.
• Attach wheels or place it on a hand truck. Pulling or
pushing is better than carrying or holding.
• If a load must be carried (rucksack, backpack, tools, etc)
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.
• Make the package so heavy, that it cannot be carried
without a mechanical aid.
• Box with a handle may induce more lower back stress
compared to a box without a handle.
Back injury from lifting increases significantly
(both frequency and severity) when lifted
objects are:
– Heavy
– Bulky
– Lifted from the floor
– Lifted frequently, and
– Lifted asymmetrically (with torso twisted)
NIOSH Modified Lifting Equation
• It provides a formula to determine the Recommended Weight Limit
(RWL) for a specific lifting task.
• RWL 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
• These factors are determined, one for the lift initiation and one for
the lift destination points, and two RWL’s are determined.
• 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
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.
Guidelines for MH
• Use machine
• Minimize spinal torque
Keep load close to the body
Bend the knees
Get a good grip, Don’t slip and jerk
Work at knuckle height, don’t put load on the floor
Don’t twist during the move
Move small weights often
• Don’t lift above the shoulder.
• Select strong people based on tests