12. Manual Material Handling

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12. Manual Material Handling
NIOSH Work Practices Guidefor Manual Lifting
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NIOSH WPG for Manual Lifting

• The NIOSH Work Practices Guide (WPG) for Manual
  Lifting was compiled by a panel of experts in
  1981 for the following purposes
• Review and summarize current knowledge with
  regard to manual material handling.
• Provide concrete guidelines to assist industry in
  the prevention of overexertion injuries during
  lifting.
• A second panel of experts (many the same as the
  first panel) was convened in the late 1980s to
  review recent developments and research in this
  area and to revise the work practices guide.

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Criteria for 1991 NIOSH WPG

• Assumptions
• Overexertion injury is the result of job demands
  that exceed a workers capacity
• These injuries result by direct trauma, a single
  exertion (overexertion), or potentially as the
  result of multiple exertions (repetitive
  trauma).

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Example
Start
End
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Lifting Equation

• A load constant is the maximum recommended
  weight for lifting at the standard lift location
  under ideal conditions.
• LOAD CONSTANT 23 kg
• Decrease the load constant to account for the
  influence of known risk factors using 6
  multipliers
• horizontal location (HM)
• vertical location (VM)
• vertical travel distance (DM)
• asymmetry (AM)
• frequency (FM)
• coupling (CM)
• All Multipliers are 1
• Recommended Weight Limit (RWL)
• 23kg ? HM ? VM ? DM ? AM ? FM ? CM

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Position Measurement
HD
VD
HO
VO
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Horizontal Multiplier

• HM (25/H)
• H horizontal distance (in cm) of the hands from
  the midpoint between the ankles.

HD
HO
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Horizontal Multiplier
25

• If H 25, HM 1
• Relatively big, non-linear effect

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Vertical Multiplier

• VM (1-(0.003V-75))
• V vertical distance (in cm) of the hands from
  the floor. Measure at the origin and destination
  of lift.

VD
VO
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Vertical Multiplier
torso flexion
overhead reach

• Moderate, non-linear effect

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Distance Multiplier

• DM (0.82 (4.5/D))
• D vertical travel distance (in cm) between the
  origin and destination of the lift.
• D VD-VO
• DM accounts for metabolic demand, task dynamics,
  but not lift vs. lower

D
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Distance Multiplier

• Relatively small, non-linear effect

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Asymmetric Multiplier

• AM (1-(0.0032A))
• A angle (deg) of asymmetry angular
  displacement of the load from the sagittal plane.
  Measure at the origin and destination of lift.

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Asymmetric Multiplier

• Moderate, linear effect

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Coupling Multiplier

• Table lookup
• Accounts for differences in capability and
  acceptability with changes in coupling

Good handles or objects that can be comfortably
grasped Fair less than optimum handles or load
contacted by fingers up to the
palm Poor bulky, shifting, sagging loads or
loads with sharp edges
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Frequency Multiplier

• Accounts for fatigue and differences in load
  height (arms vs. legs/back)

(cm)

Model may not be appropriate
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Recommended Weight Limits and Lift Index

• RWL 23 kg ? HM ? VM ? DM ? AM ? CM ? FM
• Lift Index (Actual Load)/RWL
• Interpretation increased risk of low-back
  injury if the LI exceeds 1.
• lt 1 OK
• 1 boarderline
• gt 1 may have increased risk
• gt 3 likely have increased risk
• Some believe that if workers are properly
  screened (based on the task requirements) and
  trained, that they can safely work at lift
  indexes greater than 1 but less than 3.
• What are ideal lifting conditions??
• Maximize RWL (keep load close to the body, )

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Assumptions and Limitations

• Manual work activities other than lifting are
  assumed to be minimal
• The equation does not account for unpredictable
  situations such as shifting loads
• A favorable ambient environment is assumed (19-
  26 C or 66 - 79 F)
• Risk of slips not accounted for (good floor
  surface assumed)
• Lifting and lowering tasks are assumed to pose
  the same risk of injury
• Tasks involving one-handed lifts, lifting while
  seated or kneeling, or lifting in a constrained
  work area are not appropriate for this model
• Does not account for individual anthropometric
  differences

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Example
Start
End
H 13.0 cm
H 41.5 cm
V 13.5 cm
V 89.0 cm
A 0 deg
A 0 deg
D 75.5 cm F 1/min Couplings Fair
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Criteria for 1991 NIOSH WPG

• Methodologies used
• Epidemiology Injury rates vs. task
  characteristics
• Biomechanics Infrequent lifting tasks and low
  back injury risk.
• Physiology Energy requirements during
  repetitive lifting
• Psychophysical Maximum acceptable weights in
  different tasks

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Biomechanical Criterion

• Assumptions of the 1991 NIOSH WPG
• The L5/S1 vertebral joint is the site of the
  greatest stress during lifting.
• Compressive force at that joint is the critical
  stress vector.
• The criterion (at risk) level for compressive
  force at this joint is 3400 N (760).

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Physiological Criterion

• Lifting activities can place large metabolic
  demands on workers, leading to fatigue. Fatigue
  is associated with a decrease in strength and an
  increased likelihood of injury
• Assumptions of the 1991 NIOSH WPG
• WHO The baseline maximum aerobic capacity of
  U.S. workers is 9.5 kcal/min (aerobic lifting
  capacity of an average 40-year old female worker)
• WHERE Aerobic capacity for lifts above waist
  level is 70 of that for those below waist level
• HOW LONG The criterion (at risk) level for
  energy expenditure is 50 of max. for 1 h or
  less 40 of max. for 1 to 2 h 33 of max. for 2
  to 8 h

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Psychophysical Criterion

• Psychophysical Evaluations
• Maximum Acceptable Weights of Lift (MAWLs)
• Studies of Isometric Lift Strength
• Assumptions of the 1991 NIOSH WPG
• The criterion (at risk) level for maximum
  acceptable weight of lift is the load acceptable
  to 75 of female workers.
• A criterion acceptable to 75 of female workers
  will be acceptable to approximately 99 of male
  workers and 90 of the working population
  (assuming 50 male and 50 female).