To discuss the mechanism of muscle strain injury from biomechanical viewpoints, the change of mechanical properties of injured skeletal muscle was evaluated experimentally. We induced strain injury to rabbit tibialis anterior muscle by applying a single tension loading/unloading process under the conditions of various stretch amplitude, stretch rate, and muscle activation in situ. The results showed that isometric contractile force decreased with increasing stretch amplitude during the injury-induction process. Multiple regression analyses were conducted for the results and they showed that the stretch amplitude played a dominant role in the decrease of the contractility. The failure stress decreased significantly with the increasing stretch amplitude and dissipation energy during the injury-induction process. Moreover, the failure stresses of the muscle injured under unstimulated conditions obviously decreased in comparison with those of the muscles in intact and injured under stimulated conditions. On the basis of these results and previous experimental studies, we concluded that muscle activity played a significant roll to prevent the decrease of the failure stress and that the damage mechanism of strain injury depended on the muscle activity during injury-induction process.
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