12. Manual Material Handling

1. 12. Manual Material Handling NIOSH Work Practices Guidefor Manual Lifting

2. 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 1980’s to review recent developments and research in this area and to revise the work practices guide.

3. 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’).

4. Example Start End

5. 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

6. Position Measurement HD VD HO VO

7. Horizontal Multiplier • HM = (25/H) • H = horizontal distance (in cm) of the hands from the midpoint between the ankles. HD HO

8. If H ≤ 25, HM = 1 Relatively big, non-linear effect Horizontal Multiplier 25

9. Vertical Multiplier • VM = (1-(0.003|V-75|)) • V = vertical distance (in cm) of the hands from the floor. Measure at the origin and destination of lift. VD VO

10. Moderate, non-linear effect Vertical Multiplier overhead reach torso flexion

11. 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

12. Relatively small, non-linear effect Distance Multiplier

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

14. Moderate, linear effect Asymmetric Multiplier

15. 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

16. Accounts for fatigue and differences in load height (arms vs. legs/back) Frequency Multiplier (cm) ≤ Model may not be appropriate

17. 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. • < 1 OK • = 1 boarderline • > 1 may have increased risk • > 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, …)

18. 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

19. 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

20. 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

21. 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#).

22. 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

23. 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).