Effects of fatigue on the series elastic component of human muscle.

The effects of fatigue on the muscular series elastic component were studied in man. The compliance-force relationship (K-F) of the fatigued muscle was compared with that of the unfatigued muscle. The K-F relationships were established under electromyographic control in two cases: 1. The F variation came from the decrease in maximum voluntary force appearing in fatigue which was produced by rhythmic flexions against an elastic resistance. The compliance was measured at regular intervals as fatigue developed. 2. The compliance was measured at different predetermined levels of force (maximal and submaximal) without the appearance of fatigue. The K-F relationship is curved whether the muscle is fatigued or not: the compliance increases as the force decreases. However, for the same value of force, the fatigued muscle is more compliant than the non-fatigued muscle. These results are discussed in relation to mechanical muscular properties and to the two-component muscle model of Hill.