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.

Factors influencing quantified surface EMGs.

Isometric flexions of the elbow are studied in man. For the three joint angles studied, the torque, the surface EMG of the biceps brachii muscle, and the quantified EMG are recorded. The EMGs are picked up by means of bipolar electrodes located in such a way as to vary the interelectrode orientation, the interelectrode distance and the position on the muscle. Longitudinal placements collect stronger signals that transverse ones. Under certain conditions, the magnitude of the quantified EMG depends on the distance between the electrodes, and on their location on the muscle. The results are discussed in terms of conduction volume and detection volume.

Muscular fatigue and rate of tension development.

The effects of long-term fatigue upon maximal force and peak rate of tension development (PRTD) (dF/dt max) are studied in man (elbow flexors), in the rat (pseudo-isolated gastrocnemius muscle) and in the frog (isolated sartorius muscle). The muscles are fatigued by voluntary anisometric anisotonic contractions against an elastic resistance in man, and by maximal tetanic contractions in the frog and the rat. In man, the excitation level of the muscle is controlled by the integrated surface EMG of the biceps brachii. In the animals, the muscles are stimulated by a neurostimulator. The PRTD and the maximal isometric force are measured during fatigue tests. In man, frog and rat, the maximal voluntary isometric torque or the maximal force and the PRTD decrease initially more or less rapidly according to the power developed during the fatigue process, and then less rapidly. The relationship between PRTD and maximal force is linear in the animals and curvilinear in man. The variations of maximal force and PRTD are discussed in relation to the level of excitation of the muscles and of the composition in different motor units types and their spatio-temporal recruitment. From a biomechanical point of view, it seems necessary to study the behavior of the series elastic component during the evolution of long term fatigue.