Muscle fatigue in axial rotation of the trunk

The objective of the study was to determine if different trunk muscles fatigued at a different rate in isometric axial rotation. Rotation is associated with a large number of back injuries. The fatigue behaviour may shed some light on possible reasons for such an association. Subjects were stabilized in an upright seated neutral posture in a specially designed and fabricated device-axial rotation tester such that the rotation could occur only in thoraco-lumbar region. Subjects performed isometric trunk rotation at 60% of their previously measured maximal voluntary contraction for a maximum duration of 2min or less if they could hold no longer. Surface electromyographic (EMG) was measured from seven trunk muscles bilaterally. Power spectra were calculated, the mean median frequency obtained at 10% intervals of the task cycle and compared in time within and between subjects. The EMG amplitudes were significantly different between right and left sides (p<0.01). The initial median frequencies of each muscle were significantly different from the other (p<0.01). The rate of decline of the median frequency for different trunk muscles were different (p<0.01). In isometric trunk rotation different trunk muscles demonstrated a different rate and magnitude of EMG fatigue. The latter is likely to alter the pattern of load sharing between muscles and predisposing musculoligamentous to injury either through stress concentration in deformed tissues or exceeding the strain rate tolerance in fatigue induced jerky motion (micromotion).Relevance to industryLow back injury continues to be one of the most common injuries. Rotation of the trunk is associated in over 60% of back injuries. The fatiguing behaviour of the trunk muscles provides an insight into possible control strategies such as reversing the direction of rotation periodically to allow recovery of fatigued muscles while still carrying out the task. Copyright

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