THE H-REFLEX IN THE PASSIVE HUMAN SOLEUS MUSCLE IS MODULATED FASTER THAN PREDICTED FROM POST-ACTIVATION DEPRESSION

The purpose of the present study was to investigate the influence of a fferent activity (mainly homonymous Ia-afferent activity) on the modul ation (post-activation depression) of the soleus H-reflex during isola ted and passive sinusoidal ankle joint rotations at a speed and amplit ude comparable to slow walking. The H-reflex modulation was measured i n the relaxed soleus muscle on human subjects during different imposed patterns of 20 degrees haversine ankle joint rotations (0.5-0.6 Hz) w hile they were sitting comfortably in a chair. Eighteen healthy males and four male patients with clinically complete spinal cord lesion abo ve the soleus motoneuron pool participated in the study. During a sing le dorsi-plantar flexion rotation the H-reflex was depressed to 27 +/- 7% (mean +/- S.E.M.) of the initial level within 600 ms. The course o f this depression was reversed when the dorsi-flexion velocity started to decrease. At the end of the dorsi-flexion movement the depression was already relieved to a level of 73 +/- 6% of the initial level. The K-reflex returned more slowly to the initial level within 2 s after t he end of the movement cycle. During two consecutive ankle joint rotat ions and continuous ankle joint rotations both at 0.5 Hz the H-reflex was modulated but also generally depressed while the movement was impo sed. The reflex only returned to the reference level after the movemen ts were stopped. These observations indicate the action of a fast and a slow mechanism in the post-activation depression of the soleus H-ref lex. The H-reflex modulations observed in the spinal cord injured pati ents were comparable to the reflex modulations observed in the healthy subjects, except the depressions were smaller. This suggests that a m ajor part of the amplitude of the H-reflex modulation observed in heal thy subjects was caused by peripheral and spinal influences. The fast 500 ms recovery of the H-reflex had a time course comparable to presyn aptic inhibition. The slow 2 s recovery after the end of a given impos ed movement may be explained by a change in the probability of transmi tter release from the homonymous soleus Ia-afferent synaptic terminals after repeated activations. (C) 1998 Elsevier Science B.V.