Stretch-reflex adaptation in man : interaction between load, fatigue and muscle stiffness

Abstract

The present series of studies were designed to examine the adaptability of the neuromuscular system to different loading conditions and to prolonged stretch-shortening cycle (SSC) exercise. A special interest was in the testing of the hypothesis of central fatigue and in investigating whether there exist more direct fatigue effects on the muscle spindle itself. Reflex and voluntary functions were studied pre- and post-exercise in isolated and non-isolated conditions as well as during the experimental conditions. In the different muscle loading conditions the amount of preactivation depended of the stretch exercise mode, suggesting considerable adaptability of the neuromuscular system not only to normal gravity but even to acute modulation of the muscle loading. In every fatigue experiment, the neuromuscular response was characterized by a clear reduction in force output and by a simultaneous decrement in reflex sensitivity. The changes in reflex sensitivity were associated with changes in the active and passive stiffness properties of the muscle. This suggests that the muscle spindle may be mechanically modulated leading to a reduction in its responsiveness. In consequence, disfacilitation of the α-motoneuron pool may be operative. No recovery in the H-reflex peak-to-peak amplitude was observed as long as the fatigued muscles were kept ischemic. This may indirectly emphasize the role of the small muscle afferents as an inhibitory mechanism. It is suggested that these two central mechanisms are operative during neuromuscular fatigue. However, the relative contribution of these mechanisms depends on the fatigue task; in particular, the amount of metabolic loading, the amount of muscle damage and the amount of mechanical loading.