TI 2111 Work System Design and Ergonomics 12. Manual Material Handling NIOSH Work Practices Guide for Manual Lifting

TI 2111 Work System Design and Ergonomics

12. Manual Material Handling

NIOSH Work Practices Guidefor Manual Lifting


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.


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’).


Example

Start End


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


Position Measurement

VO

VD

HO

HD


Horizontal Multiplier

HM = (25/H) H = horizontal

distance (in cm) of the hands from the midpoint between the ankles.

HD

HO


Horizontal Multiplier

20 30 40 50 60 70 800

0.2

0.4

0.6

0.8

1

1.2

Horizontal Distance (cm)

Multiplier

25

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


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.

VO

VD


Vertical Multiplier

0 20 40 60 80 100 120 140 160 1800

0.2

0.4

0.6

0.8

1

1.2

Vertical Distance (cm)

Multiplier

torso flexion overhead reach

Moderate, non-linear effect


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


Distance Multiplier

0 20 40 60 80 100 120 140 160 1800

0.2

0.4

0.6

0.8

1

1.2

Distance Moved (cm)

Multiplier

Relatively small, non-linear effect


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.

A

sagittal plane


Asymmetric Multiplier

0 20 40 60 80 100 120 140 160 1800

0.2

0.4

0.6

0.8

1

1.2

Asymmetry Angle (deg)

Multiplier

Moderate, linear effect


Coupling Multiplier Table lookup Accounts for differences in capability and acceptability

with changes in coupling

V<75 cm V75 cm

1.0

.95

.90

1.0

1.0

.90

Good

Fair

Poor

Coupling

Initial load height

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


Frequency Multiplier

V<75 V75

0.85 0.85

Frequency lifts/min

0.950.951.001.000.2

V75V<75V75V<75

Š 1 hour Š 2 hour Š 8 hour

0.81 0.810.920.920.970.970.50.75 0.750.880.880.940.94 10.65 0.650.840.840.910.91 20.55 0.550.790.790.880.88 30.45 0.450.720.720.840.84 40.35 0.350.600.600.800.80 50.27 0.270.500.500.750.75 60.22 0.220.420.420.700.70 70.18 0.180.350.350.600.60 80.00 0.150.300.300.520.52 90.00 0.130.260.260.450.45 100.00 0.000.230.000.410.41 110.00 0.000.210.000.370.37 120.00 0.000.000.000.340.00 130.00 0.000.000.000.310.00 140.00 0.000.000.000.280.00 150.00 0.000.000.000.000.00>15

initial load height

≤

Model may not be appropriate

(cm)

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


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, …)


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


Example

Start End

H = 13.0 cmV = 13.5 cmA = 0 deg

H = 41.5 cmV = 89.0 cmA = 0 deg

D = 75.5 cm; F = 1/min; Couplings = Fair


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


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#).

cervical vertebrae

thoracic vertebrae

lumbar vertebrae

sacral vertebrae

coccygeal vertebrae


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


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).

Documents

TI 2111 Work System Design and Ergonomics 12. Manual Material Handling NIOSH Work Practices Guide for Manual Lifting