THE STATIC MUSCLE LOAD IN DIFFERENT WORK POSITIONS: AN ELECTROMYOGRAPHIC STUDY∗

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Abstract
In order to ascertain which muscles and muscle groups are engaged in various standing work positions an electromyographic study was performed.About twenty muscles or muscle groups were examined. Both coaxial needle electrodes and surface electrodes were employed. The analysis of the muscle activity for different postures was based on action potentials recorded in a normal symmetric standing rest position. Among the different postures the rest position appears to require the least muscular effort for counterbalancing the effect of gravity on the various parts of the body. This state of equilibrium seems to be maintained by certain muscle groups, referred to hero a.s the “ prime postural muscles ”. Only a part of the potential power of the postural muscles is utilized in maintaining the standing symmetric rest position. Some persons engage other muscles besides the prime postural muscles, possibly because they stand in such a way that the load distribution is not optimal, or thoy havo weak prime postural muscles or because of poor muscular co-ordination as a consequence of “ non-physiologic” postural habits. When changing from the rest position to most of the other standing positions the load on the prime postural muscles is increased, and in many cases other muscle groups arc activated as well. The sacrospiualis appears to be particularly susceptible to change in the load distribution. The muscles of the lower leg, which control the foot articulation, are also very susceptible to changes in body posture. However, a change in the position of the- trunk need not result in an appreciable increase in the load on the muscles of the lower legs if the displacement of the centre of gravity of the trunk is compensated by a postural adjustment at the ankle, such that the point of intersection of tho line of gravity of the body and the supporting area will be the same as in the symmetric standing rest position. A change in posture of the hip- and knee-joints does not result in such a wide variation in activity of the muscles rogulating the position of these joints as a postural change in the foot articulation and the vertebrae.

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