Trunk rotation: Ergonomic and evolutionary perspective

Trunk rotation is a unique kinesiological property of primates and is most developed amongst humans. The evolutionary pressures acting on ancestors of humans for over 200 million years have resulted in this functional ability. However, the physical demands on the human frame subsequent to the industrial revolution have taken a heavy toll, resulting in a high incidence rate for back injuries; over 60% of those are associated with trunk rotation. In this paper, the broad functional evolutionary paths have been traced, outlining the morphological characteristics which enabled trunk rotation in the animal kingdom for the first time. The mechanical limitations of the morphological characteristics and their interplay in humans are described. This clearly highlights the nature and degree of incompatibility between industrial demands on workers and their morphological and mechanical limitations contributing to vulnerability. The experimental results show a thoracolumbar rotation of 70-74° on either side of the neutral, and it is executed by the contraction of contralateral external obliques and ipsilateral latissimus dorsi and internal obliques. The ipsilateral external oblique is assigned the stabilization function. In axial rotation, the experimental subjects could produce only 25% of their extensor effort. The power spectral analysis revealed that, whereas the axial rotation was performed by the motor units supplying the oblique fibres, the combined motion rotation was achieved by the activity of motor units in addition to those responsible for axial rotation. In rotational activities there is a differential fatigue ( p < 0.01) and rate of fatigue of the trunk muscles contributing to muscular imbalance. A theoretical mechanical analogue model has been developed to indicate the pattern of activities and possibly of rotational injuries.

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