VARIATIONS IN SOLEUS H-REFLEXES AS A FUNCTION OF PLANTARFLEXION TORQUE IN MAN

The purpose of this study was to evaluate the effects of the levels of voluntary isometric contraction on the Hoffman reflex in human soleus and medial gastrocnemius (MG) muscles. H-reflexes were recorded in si xteen healthy adults at each of 16 isometric plantarflexion (pf) torqu e levels ranging from 0-100% of their maximum voluntary isometric cont raction (MVC) and were elicited at two intensities of stimulation: (i) supramaximal for M-response and (ii) a submaximal stimulus that produ ced an H-reflex in soleus that was 50% of maximum H-reflex at rest. Th e H-reflex peak-to-peak amplitudes were linearly related to pf torque levels ranging from O to 50% MVC at both supramaximal and submaximal s timulus intensities. The slope of this relationship was higher for the submaximal stimulation. Beyond 60% of MVC, the soleus H-reflex amplit ude showed no further increase with increasing pf torque for both stim ulus intensities. Thus, beyond 50-60% of MVC the soleus H-reflex does not provide an accurate measure of soleus motor neuron pool excitabili ty. Further experimental results showed that the H-reflex amplitude at a given torque level depended on whether torque was increasing or dec reasing. When torque was increasing, the amplitude of the H-reflex was larger than when the same torque was maintained at a constant level. In contrast, if the torque was decreasing, amplitude of the reflex was lower than when torque was increasing. Therefore, variations in H-ref lex amplitudes at a given torque level may be more closely correlated to the direction of the ongoing contraction than to the actual muscle force being produced at the time the H-reflex is elicited.