Reduced reflex sensitivity persists several days after long-lasting stretch-shortening cycle exercise

The mechanisms related to the acute and delayed secondary impairment of the stretch reflex function were investigated after long-lasting stretch-short ening cycle exercise. The results demonstrated a clear deterioration in mus cle function immediately after fatigue, which was accompanied by a clear re duction in active and passive reflex sensitivity. For active and passive st retch reflexes, this reduction was biphasic (P < 0.05 to P < 0.001). Howeve r, for the ratio of the electrically induced maximal Hoffmann reflex to the maximal mass compound action potential, only one significant reduction was seen immediately after fatigue (71.2%, P < 0.01). A similar significant (P < 0.01) decrease in the stretch-resisting force of the muscle was also det ected. Clear increases were found in the indirect markers of muscle damage (serum creatine kinese activity and skeletal troponin I), which could imply the occurrence of ultrastructural muscle damage. It is suggested that the acute reduction in reflex sensitivity is of reflex origin and due to two ac tive mechanisms, disfacilitation and presynaptic inhibition. However, the d elayed second decline in the sensitivity of some reflex parameters may be a ttributable to the secondary injury, because of some inflammatory response to the muscle damage. This might emphasize the role of presynaptic inhibiti on via group III and IV muscle afferents.