Modulation of the Hoffmann reflex by rapid muscle contraction or release.

The inhibition of the H-reflex of the Soleus (Sol) muscle that takes place during and after voluntary release of Sol muscle has been attributed to presynaptic inhibition of autogenetic spindle afferences. In the present study, the time-relationship between onset of H-reflex depression and termination of Sol contraction was investigated to ascertain whether the reflex inhibition is linked to the command to release, or whether it is an accompanying phenomenon connected to changes in the neural outflow from the periphery. A parallel investigation was carried out on the temporal characteristics of the facilitation of the H-reflex that precedes onset of Sol contraction, in an attempt to point at the different functional organization of the two motor tasks. Voluntary releases from a bilateral isometric plantar torque, or bilateral plantar flexions, were performed in response to an acoustic stimulus, in a reaction time (RT) situation. The intervals from the starting signal to complete termination, or to beginning, of the Sol EMG were measured. The H-reflex was evoked at random during the tasks in one leg and its amplitude was referred in time to the end, or to the onset, of the EMG recorded from the contralateral Sol muscle. The RTs of the termination of Sol EMG had an average duration of about 100 ms, being some 20 ms shorter than those of the onset of EMG. In the release-task, the H-reflex amplitude was higher than that of the controls during the holding phase, and started to decrease about 20 ms before the cessation of the EMG.