Lecture 3 Design of Manual Handling and Biomechanics

Design of Manual Handling and Biomechanics

BMFP 3553 Industrial Ergonomics

Introduction

“Low Back Pain remains the most prevalent and costly work-related injury.”

(Liberty Mutual Research Center Research Report, 1998)

Objectives for today

• Describe what happens to the lower back in term of biomechanics;

• Understand the common pitfalls associated with lifting;

• Describe the appropriate manual handling design techniques.

Introduction

• In the USA, about 500 000 workers, suffer some type of overexertion injury per year.

• In the UK, more than 25% of accidents involve handling goods in one way or another (Health and Safety Commission, 1991).

• In Malaysia, 1,111 cases involving back injuries were reported in 2007 (SOCSO Report)

Introduction

• Hoogendoorn et al. (2000) found an increased risk of low back pain in workers who lifted a 25 kg load more than 15 times per day.

• Magora (1972) found that low back symptoms were more common in workers who regularly lifted weights of 3 kg or more than in those who sometimes lifted such weights.

Introduction

• When carrying out manual handling tasks, the weight of the load being lifted is transferred to the spinal column in the form of compression and shear forces.

• The compression and shear are greater when the load is lifted quickly because higher forces are needed to accelerate the mass from rest, according to Newton’s laws of motion.

Introduction

• Spinal compression is increased when loads are lifted and is increased even more when they are lifted quickly and when the posture is imbalanced.

• Lifting technique does influence manual handling efficiency

Introduction

• According to Grieve and Pheasant (1982), the trunk can fail in these ways when a weight is lifted:– 1. The muscles and ligaments of the back can fail under

excessive tension.– 2. The intervertebral disc may herniate as the nucleus

is extruded under excessive compression.

• These injuries are often referred to colloquially as ‘muscle strains or tears’, ‘slipped discs’ and ‘hernias’.

Introduction

• A catastrophic injury such as a disc prolapse is not simply caused by a sudden event such as lifting a heavy weight.

• It is usually the end product of years of degeneration of the disc and surrounding structures.

Prevention of manual handling injuries in the workplace


• The most common approach (as well as most

useless!) in most industries is to train workers to lift safely.

• The notion that it is safer to ‘lift with the knees and not with the back’, that people can be trained to lift safely and that injury will be prevented, is deeply ingrained.


• Despite the large number of studies that have shown no benefits of the training, training is a popular approach.

• Snook et al. (1978) compared three approaches to low back injury prevention:– preemployment/pre-placement selection– training in lifting techniques– job design


• The findings showed : no difference in the proportion of injuries in companies that did or did not train their workers in lifting techniques,

• Nor were there any effects due to selection based on medical screening.


But………significantly fewer back injuries were found in companies where the loads were acceptable to more than 75% of the workforce.


• Snook et al concluded that workers are 3 times more likely to hurt their backs when performing exertions acceptable to less than 75% of the workforce.

Strength Testing: Useful/Useless?

• It has been noted in some literature that strength testing for worker selection can reduce the risk of injury.

• The idea : Job Demand = Physical Ability

• But , in general strength testing will not prevent the occurrence of low back pain.

Safe’ lifting techniques. Dangerous assumptions about manual handling safety

Assumption # 1

• Assumption No. 1. The Techniques Being Taught Are Safer, In Practice– Although there is some evidence that lifting from

a squatting position is safer than lifting from a stooping position,

– squat lifting uses weights specially designed to be lifted from a squatting position

• Most weights in industry are not designed to be lifted from a squatting position

• In some situations, as in lifting an unstable load squat lifting techniques may actually increase the load moment or they may be completely impractical.

Assumption # 2

• Assumption No. 2. ‘Safe’ Techniques Are Usable And Have No ‘Hidden Costs’– Squat lifting techniques require greater

coordination and control than the alternatives and also place a higher load on the cardiovascular system and the knees.

– For one-off lifts, the additional demands may be acceptable, but for repetitive lifting they soon take their toll.

• The knees weaken rapidly beyond about 60 degrees of knee flexion and the knee ligaments are at increasing risk of rupture (see Grieve and Pheasant, 1982, for further discussion)

• Rabinowitz et al. (1998) found that stoop lifting was associated with greater back pain and squat lifting with greater knee pain.

• Repetitive squat lifting for 15 minutes placed an escalating cardiovascular load of an extra 26 heart beats/min compared with stoop lifting. People rated the task as ‘somewhat hard’ compared with stoop lifting, which was rated as ‘light’.

Assumption # 3

• Assumption No. 3. The Training Will Transfer To The Work Situation– The author knows of only one study that

demonstrates long-term (6-month) change in lifting technique as a result of manual handling training

– Although you can teach an old dog new tricks, the old ones persist in long-term memory, and will dominate their behavior

• Ergonomists have long understood the principle that well-learnt behaviors cannot be ‘unlearnt’.

• As soon as we cease to monitor our performance, old habits tend to return.

Assumption # 4

• Assumption No. 4. Any Reductions In Risk Are Large Enough To Protect People From Injury

– Training people to ‘make more use of the legs’ does not guarantee lower back stress.

– Although squat lifting may reduce back stress by lowering the load moment, it is less clear whether the reduction is sufficient to prevent injury.

• Spinal tissues have a compression tolerance limit or threshold (Genaidy et al., 1993)

• A lowering of the load will only bring about a reduction in injury rates if the compressive forces are bought below threshold: if the absolute level of risk is reduced to a safe level

A reduction in risk is not the same as an improvement in safety. This is clearly

recognized in the European Union manual handling guidelines, which state that manual

handling should be avoided as much as possible.

Assumption # 5

• Assumption No. 5. There Are No Perverse Outcomes Associated With The Use Of ‘Safe’ Handling Techniques– There is experimental evidence that people will lift

heavier weights when they feel safe than when they feel unsafe (McCoy et al., 1988).

– Estimates of MAWL were over 50% higher when told that squat lifting was implemented (Bridger and Friedberg (1999)

• The implication is that manual handling training could act as a barrier to change by creating the impression that ‘something has been done’.

Designing handling tasks

• 3 principles (from most effective to least)– Engineering controls– Administrative controls– Behavioral controls (PPE, training)

Designing handling tasks

Design of handling task

Design of handling task

• The characteristic of the load is also important:– 20kg of lead is considered “lighter” than 20 kg of

feathers since it can be held closer to body.– Containers for one- or two-handed handling

should be designed as small as possible so that the load is kept close to the body.

Ergonomic problem:

The worker picks up a carton from a 27 inch high conveyor system. The worker turns and stacks the carton on a pallet located at floor level.

What are the ergonomic risk factors?

Propose a solution to this problem.

Workers had to transfer boxes weighing 20 kg or more from one place to the shelves using a cart. Boxes had to be lifted from the cart to the shelves. Back pain is a significant problem for workers handling this job.

1) What are the ergonomic risk factors present in this job?

2) Propose a solution for this problem.

Abdominal Belts : Health or Hoax?

• The practice of wrapping materials around the waist with the aim of improving posture and poise is found throughout history and across cultures.

• Shah (1993), for example, reports that in Nepal most people who lift and carry heavy weights wrap a5-metre length of cloth (called a ‘Patuka’) around the waist before work

Abdominal Belts

Abdominal Belts : Health or Hoax?

• Abdominal belts are thought to protect workers by restricting undue flexion or rotation of the spine

• By increasing the Intra Abdominal Pressure (IAP) the spine is protected indirectly.

• But the scientific evidence is yet to be there about back belts…..

Studies regarding back belts

• DO ABDOMINAL BELTS INCREASE IAP WHEN WORN?– McGill et al. (1990) measured back extensor EMG and

IAP when subjects lifted weights wearing a competition weightlifter’s belt.

– IAP did increase, but no reduction in back extensor muscle activity.

– When subjects held their breath when lifting, increases in IAP were also observed and were accompanied by reductions in back extensor EMG, irrespective of whether a belt was worn

• DOES WEARING AN ABDOMINAL BELT REDUCE BACK MUSCLE FATIGUE WHEN LIFTING?– Ciriello and Snook (1995) measured fatigue of the

back extensors in 13 male industrial workers who lifted average loads of 28.1 kg, 4.3 times per minute, for 4 hours a day

– Made little to no difference at all to the back muscles fatigue

• DOES WEARING AN ABDOMINAL BELT HAVE AN OVERALL PROTECTIVE EFFECT?– Miyamoto et al. (1999) demonstrated that

abdominal belts raise the intramuscular pressure in the erector spinae muscles and stiffen the trunk, which may be beneficial during lifting and during other work where the trunk is exposed to de-stabilising forces.

• DOES WEARING AN ABDOMINAL BELT GIVE LIFTERS AN INCREASED SENSE OF STABILITY AND SECURITY?– McGill et al. (1990), Reddell et al. (1992) and

Magnusson et al. (1996) all report that wearing either competitive weightlifters’ belts or abdominal belts for industrial workers increases the sense of security.

• DO ABDOMINAL BELTS PROTECT INDUSTRIAL WORKERS IN PRACTICE?– Walsh and Schwartz (1990) divided 90 grocery

warehouse workers into three groups in a 6-month investigation. • Group 1 ( control group)• Group 2 (Back pain prevention training)• Group 3 (Training + Back belts)

• Results? – There were no statistically significant differences

in injury rates or productivity between the three groups over the study period. Lost time was significantly lower in group 3, however (2.5 days lower, on average).

• Reddell et al. (1992) evaluated an abdominal belt and back program among a group of airline baggage handlers.– Lost workdays and back injuries were not reduced,

but back injuries increased and were more severe after belt use was discontinued

• IS ABDOMINAL BELT WEARING HAZARDOUS FOR WORKERS WITH LATENT CORONARY HEART DISEASE?– Blood pressure and heart rate were higher when

the belt was worn, leading to the conclusion that cardiac-compromised individuals are probably at greater risk when exercising while wearing back supports.

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Lecture 3 Design of Manual Handling and Biomechanics