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Manual handling is associated with

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 emphasizes forces and torques load limit does not vary with frequency

Physiological emphasizes energy requirements and cardiovascular

demands important for repetitive lifting

Psychophysical combination of the above predicts “real” capability based on performance under

controlled conditions

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Manual handling variables

Individual

Technique

Task

Goals:

increase the strength of the worker

decrease the stress due to technique and task

both

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Pushing and pulling strength factors

Handles One hand vs. two hands Body posture Application height Direction

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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.

Install vertical push/pull bars on carts. Push rather than pull loads. Avoid muscle-powered pushing and pulling for

ramps, long distances, and frequent moves. Use mechanical aids and momentum. Reduce force by reducing friction.

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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 guidelines

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. Consider using balancers, manipulators,

conveyors, or robots. Reduce lifting by raising the initial location. Avoid carrying objects up and down stairs.

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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.

Lifting index =

load weight / RWL

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NIOSH lifting example

At initiation,

Horizontal location, H1 = 10 in.

Vertical location, V1 = 40 in.

Vertical location, V2 = 51 in.

Angle of asymmetry, A = 0

Frequency, F = 12 /min.

Load = 14 lbs.

Duration = 2 hr.

What is the RWL?

What is the Lifting Index, LI?

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Basic NIOSH lifting formula (pg. 253)

RWL = LC × HM × VM × DM × FM × AM × CM RWL = recommended weight limit LC = load constant (51 lbs.) HM = horizontal multiplier VM = vertical multiplier DM = distance multiplier FM = frequency multiplier AM = asymmetry multiplier CM = coupling multiplier

Note: review the conditions under which this will not apply – turned around, they make a good set of criteria for lifting task design!

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Multiplier formulas

Horizontal multiplier

HM = BIL / H• BIL = Body interference limit

• H = Horizontal location

Vertical multiplier

VM = 1 – VC × | V – KH |• VC = Vertical constant

= 0.0075 for inches, 0.003 for cm.

• V = Vertical location

• KH = Knuckle height (assume 30 in.)

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Multiplier formulas (cont.)

Distance multiplier

DM = .82 + DC/D• DC = Distance constant

= 1.8 for inches, 4.5 for cm.

• D = Vertical travel distance

Asymmetry multiplier

AM = 1 – .0032A• A = Angle of symmetry

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Multiplier formulas (cont.)

Frequency multiplier

See Table 13.9, pg. 255. 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 = >1 h ≤ 2 h with recovery time of ≥ .3 × duration

Long = >2 h but ≤ 8 h

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Multiplier formulas (cont.)

Coupling multiplier

See Table 13.10, 13.11, Fig. 13.13 Depends on:

Height of initial and final hand–container coupling Whether coupling is good, fair, or poor

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Solution …

HM =

VM =

DM =

FM =

AM =

CM =

RWL =

LI =

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Force limits

FL = A × F × DIST A = Age factor F = Frequency factor DIST = Distance factor

Differences between force limits and NIOSH guidelines. Different factors Different criteria FL permissible load ~1.8 times that of NIOSH

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Resources

Manual Handing Guide (Mital et al.) Biomechanical software

ErgoIntelligence MMH 2D Static Strength Prediction Program 3D Static Strength Prediction Program

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Guidelines for manual handling Three categories:

Select individual1. Select strong people based on tests.

Teach technique2. Bend the knees.3. Don’t slip or jerk.4. Don’t twist during the move.

Design the job5. Use machines.6. Move small weights often.7. Get a good grip.8. Put a compact load in a convenient container.9. Keep the load close to the body.10. Work at knuckle height.

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Examples to discuss