What can isometric contractions tell us about muscle injury and rehabilitation?
Allen TJ [1,2], Jones T , Tsay A. , Morgan DL  and Proske U 
1. Accident Research Centre, Monash University, Australia
2. Department of Physiology, Monash University, Australia
Most of the literature on muscle injury and weakness induced by exercise has focussed on eccentric or lengthening contractions, due to their strong association with muscle damage (1-3). While there have been reports that isometric contractions can also be accompanied by damage, much less attention has been given to these contractions. Given the advantages of isometric exercise for rehabilitation, it would be helpful to establish the conditions under which damage is minimised. Isometric contractions also offer an opportunity to improve our understanding of muscle injury mechanisms during volitional exercise, since both neural and mechanical factors are much simpler. The purpose of this study was therefore to investigate which parameters of isometric contractions were associated with muscle damage. Bouts of 30 maximum voluntary contractions (MVCs) of elbow flexors were carried out in 38 subjects over 5 separate experiments. MVC force and soreness were measured pre-exercise and at 0h, 2h, and 24h post-exercise. When one arm was held near optimum angle (90°), MVC force was ~28% greater than the other arm held at longer length (155°). However, the muscle held at longer length exhibited more damage than the muscle exercised at 90°, supporting a greater length than force dependence of the damage process. In four additional experiments results suggested that the damage occurred during the plateau of the contractions and not the activation or relaxation phases. We conclude that muscle damage from isometric exercise is minimised if carried out at lengths below the optimum, using contractions of half maximum capacity or less.
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