Low Back Pain remains the most prevalent and costly work-related injury.
(Liberty Mutual Research Center Research Report, 1998)
A large proportion of the accidents that occur in industry involve the manual han- dling of goods. In the USA, about 500 000 workers, suffer some type of overexertion injury per year. Approximately 60% of the overexertion injury claims involve lifting and 20% pushing or pulling (NIOSH, 1981). In the UK, more than 25% of accidents involve handling goods in one way or another (Health and Safety Commission, 1991).
A 10% reduction in manual handling injuries would save the British economy some
£170 million per annum.
The relation between low-back injury and workplace ergonomics is supported by the findings of epidemiological surveys. Hoogendoorn et al. (2000) found an in- creased 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. Interestingly, low back symptoms were even more common in those who rarely lifted weights.
Anatomy and biomechanics of manual handling
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.
Thus, larger forces are required to lift an object quickly, rather than slowly, and these are transferred to the spine. Additional loads are placed on the spine owing to posture – the more ‘off-balance’ or assymmetric the posture, the greater the muscle forces needed to counteract the pull of gravity. Combining accelerations of the trunk and the load with asymmetric postures introduces a requirement for high antagon- istic contractions in the muscle groups around the trunk: one set of muscles acts to accelerate the load and another to maintain the integrity of the spinal column and control the acceleration and deceleration of the trunk itself. The result of all this contraction and co-contraction is increased compression and shear on the vertebral motion segments (see Granata and Marras, 2000).
Figure 6.1 The abdominal mechanism in lifting.
The abdominal and thoracic muscles play a major role in stabilising the spine when a weight is lifted according to Morris et al. (1961). In relaxed standing, these muscles exhibit little activity. When a person leans forwards to lift a weight, a moment of flexion is placed on the spine. The heavier the weight, the greater the flexion strain.
The back muscles contract to resist the flexion (by exerting a moment of extension about the spine) and this is accompanied by antagonistic contraction of the abdom- inal and thoracic muscles. These muscles pressurise the contents of the abdomen and thorax, converting them into approximate hydraulic and pneumatic splints that are thought to oppose the flexion moment (Figure 6.1). The precise role of intra- abdominal pressure is unclear because, when the abdominal muscles contract, they increase the compression on the spine. Aspden (1989) suggested that intra-abdominal pressure compresses the convex surface of the lumbar lordosis, causing it to stiffen (in the same way that placing a load on a masonry arch increases its stability).
Whatever the reason, the reflex increase in intra-abdominal pressure when a person lifts a load appears to be a normal response.
Spinal compression is increased when loads are lifted and is increased even more when they are lifted quickly and when the posture is assymmetric. High load mo- ments and high postural moments increase the forces even more. Lifting technique does influence manual handling efficiency and it is recommended that the hip exten- sors should play the major role in powering the lift. According to Gallagher and Hamrick (1991), the gluteal muscles can generate an extensor moment about 5–7 times greater than the lumbar erector spinae, which is why trying to ‘lift with back’ is both inefficient and hazardous.
Back injuries and lifting and carrying
According to Grieve and Pheasant (1982), the trunk can fail in three ways when a weight is lifted:
Heniation of
‘intervertebral disc’
Figure 6.2 Basic mechanism of a slipped disc. The annular fibres rupture and the nuclear material is extruded posteriorly. (Adapted from Keegan, 1953, with permission.) 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.
3. The abdominal contents may be extruded through the abdominal cavity owing to excessive intra-abdominal pressure.
These injuries are often referred to colloquially as ‘muscle strains or tears’, ‘slipped discs’ and ‘hernias’.
The term ‘slipped disc’ is particularly misleading – the correct term is ‘prolapse’ as the nucleus pulposus is extruded postero-laterally through ruptured annular fibres.
Because the posterior and lateral fibres of the annulus fibrosus are weaker than the anterior fibres (Adams and Dolan, 1995), herniation usually occurs postero-laterally.
It would seem that bending to the side to lift objects would place the discs at in- creased risk as lateral flexion loads the weaker parts of the disc. Added forward flexion would seem to increase the risk even more. As the load is grasped, the spine is subjected to compression and when the load is picked up, the spinal extension may
‘trap’ some nuclear material posteriorly, stretching the posterior ligaments and pres- surising nerve roots. This may serve as a basic description of the mechanism of disc herniation (Figure 6.2). The injury is one of prolapse of the annular fibres, rather than slippage of the body of the disc itself; to prevent it, from first principles, it would seem best to avoid complex spinal postures and motions in which forward flexion, lateral flexion and rotation occur simultaneously. There is some evidence that this view is correct. Gagnon et al. (1993) investigated a lifting task in which subjects had to bend forward and laterally and twist. They measured trunk postures and load moments when the task was carried out in this way and when a ‘pivoting’
technique was used. The pivoting technique involved minimising lateral flexion and rotation by making more use of the feet to (turning with the lower half of the body).
The trunk posture was safer when the pivoting technique was used and twisting
moments were reduced. For the technique to be usable, free space for the feet is needed in the entire lifting zone.
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 degen- eration of the disc and surrounding structures. Over time, the layers of cartilage in the annulus fibrosus develop micro-tears and weaknesses, the nucleus may lose fluid and the discs may become thinner, causing the vertebral bodies above and below the disc to move closer together. Parts of the vertebral bodies may then be subject to increased load as the degenerated discs take less of the load. According to Wolff’s Law, bone adapts to the mechanical demands placed upon it (being laid down where needed and reabsorbed where not needed) and bony spurs (or osteophytes) may grow around the intervertebral foramen. Excessive loading of the facet joints may also cause problems. This process of degeneration may ultimately result in herniation of an intervertebral disc (Vernon-Roberts, 1989).
A prolapsed intervertebral disc usually causes severe pain for a considerable time.
Owing to the visco-elastic nature of the intervertebral disc, the prolapsed material does not return to its original position as soon as the load is released. Sometimes treatments such as lumbar traction can assist in returning the prolapsed material and also relieve pain in the process. Because the intervertebral discs have no direct blood supply in the adult, recovery from injury is slow and it cannot be said that the damaged tissues ever heal completely as previously healthy tissue is replaced by scar tissue (Ring, 1981).
Manual handling injuries seem to be associated with unexpected events such as slips, trips and falls. Marras et al. (1987) found that spinal loads were significantly (up to 70%) greater when the spine was exposed to sudden, unexpected loading. The lack of opportunity for physical preparation may cause the back muscles to overcompensate or increase the required forces to prevent loss of balance under sudden, unexpected loading. In either case the implications are clear: workers should be allowed plenty of time to carry out manual handling tasks and ensure that they are not distracted or pressurised by conflicting task demands or exposed to slip, trip or fall hazards.
Prevention of manual handling injuries in the workplace
The perspective of ergonomics is to design tasks to make them safe, but the most common approach in most industries is to train workers to lift safely. Figure 6.3 depicts two lifting techniques. Safety propaganda directed at workers usually describes these techniques as ‘unsafe’ and ‘safe’, respectively.
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.
Snook et al. (1978) compared three approaches to low back injury prevention: pre- employment/pre-placement selection (medical history taking, medical examination, low back x-ray); training in lifting techniques; and job design (whether the work could be done by more than 75% of the workforce without overexertion). 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. Significantly fewer back injuries were found in companies where the loads were acceptable to more than 75% of the workforce. Snook et al.
Figure 6.3 ‘Safe’ and ‘unsafe’ lifting techniques according to much safety propaganda.
But are the ‘safe’ techniques safe enough to reduce the rate of occupational back injury?
concluded that workers are 3 times more likely to hurt their backs when performing exertions acceptable to less than 75% of the workforce. Furthermore, there was scope for a 67% reduction in injuries through job redesign. Stubbs et al. (1983b) found little relationship between the point prevalence of back pain among nurses and the time spent training in safe lifting techniques. They concluded that if the lifting task is intrinsically unsafe (and in the case of manual handling of patients this appears to be the case), no amount of training will solve the problem. A better approach would therefore be to do away with the lifting task or redesign it to make it safe.
More recently, Daltroy et al. (1997) reported the results of a study of 4000 US postal workers over 5 years. Workers and supervisors were given 3 hours of ‘back school’ training followed by 3 to 4 reinforcement sessions over the next few years.
Those subsequently suffering a back injury were randomly assigned to a control group or were given further training. Although subjects’ knowledge of safe behaviour was influenced by the training, there were no overall beneficial effects of back school training on rates of primary injury (trained workers compared to controls) or on re-injury. The training programme had no beneficial effects on the rate of low back injury, the median cost per injury, the time off work following injury, the rate of other musculoskeletal injury or the rate of repeated injury after returning to work.
The back injury rate was 22.1 per 1000 worker-years which is about average for the USA. The rate of injuries causing lost workdays was 10.4 per 1000 worker years and was higher in groups of workers who had received the training, but the difference was not statistically significant. Some groups of trained workers had lower injury rates than untrained workers (e.g. 16.4 injuries per 1000 worker years versus 19.4 injuries) but the difference was not statistically significant.
‘Safe’ lifting techniques. Dangerous assumptions about manual handling safety
ASSUMPTION NO. 1. THE WAY THAT UNTRAINED PEOPLE VOLUNTARILY HANDLE LOADS IS UNSAFE
To justify training people in special lifting techniques, we first need to demonstrate that the way untrained people lift weights is unsafe. There is a great deal of research
linking back injury to the manual handling of loads and to the postures and motions involved in carrying out lifting tasks. However, there is no evidence that people voluntarily stoop or twist and eventually injure themselves because they neglect to use the ‘correct’ technique. Too often, workers are constrained by the design of the task and have little choice about how to lift. A study by Allread et al. (2000) provides indirect support for this assertion. They investigated the variability in trunk motions with respect to individual differences and repetition. Comparing different jobs, they found that most of the variability between them was due to the design of the job rather than who was doing it or how frequently. The best way to change the lifting technique people use would seem to be to change the design of the workspace and the task.
ASSUMPTION NO. 2. 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, when performed by competitive weight- lifters, 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, so it is no surprise that the squat technique is rarely seen. In some situations, as in lifting an unstable load such as a large bag of wet laundry, squat lifting techniques may actually increase the load moment or they may be completely impractical. Rabinowitz et al. (1998) compared the squat and stoop lifts in a crate handling task and found no difference in spinal shrinkage (a measure of spinal strain) between the two techniques.
ASSUMPTION NO. 3. ‘SAFE’ TECHNIQUES ARE USABLE AND HAVE NO ‘HIDDEN COSTS’
Squat lifting techniques require greater coordination and control than the altern- atives 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 (e.g. unloading crates of beer) 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’. As early as 1961, van Wely reported that the stoop lifting technique was physiologically more efficient when lifting loads over 20 kg. The usability of the deep squat is suspect for older workers who may well decide, quite rationally in their case, to ‘lift with the back to save the knees and heart’.
ASSUMPTION NO. 4. 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. It is an assumption not only that such training will transfer but that it can transfer. Ergonomists have long understood the principle that well-learnt behaviours cannot be ‘unlearnt’. Although you can teach an old dog new tricks, the old ones persist in long-term memory
and return to dominate behaviour as soon as we cease to consciously monitor our performance. Anyone who has found themselves in a unfamiliar make of car will have experienced the embarrassment of turning the windscreen wipers on to indicate a turn when distracted by fellow passengers or young children.
The positive benefits of not injuring oneself whenever a load is lifted may be insufficient to reinforce what was taught. The hidden costs of these techniques may negatively reinforce the training, which is quickly unlearnt in the workplace.
ASSUMPTION NO. 5. ANY REDUCTIONS IN RISK ARE LARGE ENOUGH TO PROTECT PEOPLE FROM INJURY
For most people the spine is flexed to 50% of its maximum in a squatting position (Adams and Dolan, 1995). Training people to ‘make more use of the legs’ does not guarantee lower back stress. Paradoxically, ‘lifting with the back’ or ‘stoop lifting’
is accomplished mainly by bending from the hips. The lift is powered by the hip extensors, muscles with an overabundance of power for returning the trunk to an erect position.
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. The point is illustrated in a study by Marras et al. (1999) in which different patient handling techniques were compared. Using a low back disorder model that gives quantitative estimates of risk, it was found that, although the risk differed between the techniques, they were all very likely to be of high risk.
A reduction in risk is not the same as an improvement in safety. This is clearly recognised in the European Union manual handling guidelines, which state that manual handling should be avoided as much as possible.
ASSUMPTION NO. 6. 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). Bridger and Friedberg (1999) interviewed 50 managers in a range of light industries and found that their estimates of maximum acceptable loads for their workers were over 50% higher when the managers were told that the squat lift was to be used. The implication is that manual handling training could act as a barrier to change by creating the impression that
‘something has been done’.
The content of safety training programmes
One possible explanation for the dismal record of training is that the concepts of safe lifting embodied in the programmes are too simplistic and are incomplete – protec- tion is lost because the techniques do not map well onto the actual task requirements.
McGill and Norman (1992) offered some tentative lifting guidelines based on the current knowledge of lumbar spine biomechanics (Table 6.1).