Muscle damage produced by isometric contractions in human elbow flexors

Isometric exercise is often prescribed during rehabilitation from injury in order to maintain muscle condition and prevent disuse atrophy. However such exercise can lead to muscle soreness and damage. Here we investigate which parameters of isometric contractions are responsible for the damage. Bouts of 30 repetitions of maximum voluntary contractions of elbow flexors in 38 subjects were carried out and peak force, soreness and tenderness were measured before the exercise, immediately afterwards, at 2h, and 24h post-exercise. When one arm was held near the optimum angle for force generation (90°), the force it produced was greater by 28% than by the other arm held at a longer length (155°). However, despite the smaller contraction forces of the muscle held at the longer length, after the exercise it exhibited a greater fall in force which persisted out to 24h (20% fall) and more delayed soreness than the muscle exercised at 90° (7% fall at 24h). The result indicates a length dependence of the damage process for isometric contractions at maximum effort. In four additional experiments evidence was provided that the damage occurred during the plateau of the contraction and not the relaxation phase. The damage had a prompt onset and was cumulative, continuing for the duration of the contraction. We interpret our findings in terms of the non-uniform lengthening of sarcomeres during the plateau of the contractions and conclude that muscle damage from isometric exercise is minimised if carried out at lengths below the optimum, using half-maximum or smaller contractions.

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