Body Strength and Load Handling

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Body Strength and Load Handling

• IENG 321
• SDSMT
• Spring 2009

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Manual handling is a hot topic because

• 27 of all industrial injuries
• 670,000 injuries/yr in the United States
• 60 of all money spent on industrial injuries
• 93,000,000 lost workdays/yr

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Approaches to Recommendations

• Biomechanical
• Forces and torques
• Esp. L4/L5 and L5/S1
• Permitted load does not vary with frequency

• Psychophysical (Liberty Mutual Tables)
• Controlled setting MH tasks estimating population
  capabilities
• By virtue, combines biomechanical and
  physiological stress

• Physiological
• Energy requirements of tasks and effect to
  cardiovascular system
• Greater for occasional or frequent lifting?

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Strength Factors

• Individual
• Related to gender, age, training, fitness
• Muscle cross-sectional area
• Length of muscle
• Static or dynamic exertion
• Fatigue
• Neurophysiological excitation

• Situational
• The amount of strength one will or can apply
• Motivation, ego, will to succeed, fear of injury
• Skill/experience
• Body motion
• Ability to brace
• Posture
• Body segments used
• Coupling

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Strength Exertion

• Static (Isometric)
• Controllable, easy to measure
• Rapid onset of fatigue
• Reasonably estimate slow motion
• Dynamic
• More difficult to measure
• More common
• Isoinertial testing

• Maximal
• Structural integrity of object (safety factor)
• gt 95th percentils
• Minimal
• Ability to perform task
• 5th percentile

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Strength Exertion
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Hand Strength

• Grasping, holding, pressing, pushing, pulling,

• Extrinsic Muscles
• Muscles in forearms, tendons cross carpal
  ligament
• Mostly gross motor control

• Intrinsic Muscles
• Muscles and tendons strictly in hand
• Mostly fine motor control

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Hand Strength
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Hand Strength
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Grips and Grasps
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Hand Tool Design

• Provide friction
• Gloves may help, but generally increase required
  grip strength
• Mechanical interlocking
• Keep wrist straight
• Natural grasp centerline 60o-70o
• May need to supply left hand specific tooling
• 10 population left handed

http//cdc.gov/niosh/docs/2004-164/default.html ht
tp//cdc.gov/niosh/pdfs/95-114.pdf
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Whole Body Strength

• Pushing
• Pulling
• Lifting
• Lowering

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Foot Strength

• Standing
• Little force
• Infrequent operation
• Support body weight on one foot
• Alternating feet can help maintain muscle pump

• Sitting
• Exert large force with sufficient support

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Pushing/Pulling Factors

• Handles
• Poor hand/handle surface is the weakest link
• One hand vs. two hands
• Postural freedom vs. fatigue
• Body posture
• Standing vs. sitting vs. kneeling
• Application height
• Where to push or pull from?
• Direction
• Generally perpendicular to shoulders

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Push/Pull Summary

• Two hands are usually better than one.
• Force capability goes down as it is exerted more
  often.
• Initial force capability is higher than sustained
  capability.
• Pushing capability is higher than pulling.
• Push at waist level pull at thigh level.

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Task Modifications

• Measure the force required to move all wheeled
  equipment periodically check the forces.
• Force gauges
• Install vertical push/pull bars on carts.
• Push rather than pull loads.
• Spinal compressive loads
• Other safety concerns?
• Avoid muscle-powered pushing and pulling for
  ramps, long distances, and frequent moves.
• Use mechanical aids and momentum.
• Reduce horizontal transfer force by reducing
  friction (rolling contact).

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Holding

• Problems
• Holding gives a static load combining body weight
  and object weight.
• Low-back pain arises from spine biomechanics.
• Solutions
• Reduce the magnitude and duration of the torque.
• Use balancers.
• Limit high loads to short durations.

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Carrying

• Replace carrying with pushing or pulling.
• Minimize the moment arm of the load relative to
  the spine.
• Consider carrying large loads occasionally rather
  than light loads often.
• Use teamwork (common in medical facilities).
• Consider using balancers, manipulators,
  conveyors, or robots.
• Reduce lifting by raising the initial location.
• Avoid carrying objects up and down stairs.
• See Liberty Mutual Tables

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NIOSH Lifting Guidelines

• 51 lbs is the maximum that can be lifted or
  lowered (load constant).
• Recommended weight limit (RWL) is load constant
  multiplied by various factors.
• Weight 90 of U.S. industrial workers can safely
  lift
• Lifting index load weight/RWL
• Applicable when conditions are met!!

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Basic NIOSH Lifting Formula

• RWL LC HM VM DM FM AM CM
• LC Load constant (51 lbs)
• HM Horizontal multiplier
• VM Vertical multiplier
• DM Distance multiplier
• FM Frequency multiplier
• AM Asymmetry multiplier
• CM Coupling multiplier

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Multiplier Formulas

• Horizontal multiplier
• HM BIL H
• BIL Body interference limit (10 in.)
• H Horizontal location
• Maximum 25 in. (functional reach)
• Measured midpoint of hand to midpoint of line
  connecting ankles
• Estimated Consider vertical location from floor
  and width of container
• lt 10 in. 10 W/2
• gt 10 in. 8 W/2

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Multiplier Formulas

• Vertical multiplier
• Significant control at origin only?
• Determines if destination of hands required.
• VM 1 VC V KH
• VC Vertical constant (.0075 in.)
• V Initial vertical location of knuckles
• Maximum 70 in.
• KH Knuckle height of typical lifter (30 in.)
• lt30 in. whole body lift
• gt30 in. upper body lift

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Multiplier Formulas

• Distance multiplier
• DM .82 DC D
• DC Distance constant (1.8 in.)
• D Absolute value of vertical travel distance
• Asymmetry multiplier
• AM 1 .0032 A
• A Angle of symmetry

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Multiplier Formulas

• Frequency multiplier
• See Table 13.9.
• Lifting frequency mean number of lifts in a
  15-minute period
• Lifting duration /session in hours may be
• Short .001 h to 1 h with recovery time of
  1.2 duration
• Moderate gt1 h 2 h with recovery time of .3
  duration
• Long gt2 h but 8 h

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Multiplier Formulas

• Coupling multiplier
• Depends on
• Height of initial/final hand location and
  container coupling
• Whether coupling is good, fair, or poor (see
  Table 13.11 or NIOSH guide for classification)

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Example

• Load Weight 15 lb
• Ho 12 in
• Vo 33 in
• Hf 12 in
• Vf 22 in
• Ao 5o
• Af 6o
• Frequency .5 lifts/min
• Duration 3 h
• Recovery Time 6 hours

• Questions
• What does this assume about control of the object
• Is the recovery time a necessary piece of
  information to determine the FM?
• What is the RWL(s) and LI(s)?
• How does this relate to the Liberty Mutual Tables?

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Back Problems

• Wear and tear worse than muscle strain
• Low Back Pain (LBP)
• Reduces mobility and vitality
• Long absences from work
• Current main cause of early disability

• Major At Risk Jobs
• Nurses
• Laborers
• Farmers
• Baggage handlers
• Warehouse workers

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Intervertebral Disc Strain
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Basic Biomechanics
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Intra-abdominal Pressure (IAP)

• Increase in pressure of abdominal cavity with
  contraction of abdominal muscles
• Helps stabilize trunk
• Reduces spinal loading
• Back Belts
• Thought to increase IAP and minimize risk of
  injury

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Safe Manual Handling

• Material handling aids
• Proper techniques
• We will visit this again in Workstation Design

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Guidelines for Manual Handling

• Bend the knees.
• Dont slip or jerk.
• Dont twist during the move.
• Use machines.
• Move small weights often.
• Get a good grip.
• Put a compact load in a convenient container.
• Keep the load close to the body.
• Work at knuckle height.