6

Design of manual handling tasks

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 handling 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 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. 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 antagonistic 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).

-158-

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