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 sixteen healthy adults at each of 16 isometric plantarflexion (pf) torque levels ranging from 0-100% of their maximum voluntary isometric contraction (MVC) and were elicited at two intensities of stimulation: (i) supramaximal for M-response and (ii) a submaximal stimulus that produced an H-reflex in soleus that was 50% of maximum H-reflex at rest. The H-reflex peak-to-peak amplitudes were linearly related to pf torque levels ranging from 0 to 50% MVC at both supramaximal and submaximal stimulus intensities. The slope of this relationship was higher for the submaximal stimulation. Beyond 60% of MVC, the soleus H-reflex amplitude showed no further increase with increasing pf torque for both stimulus intensities. Thus, beyond 50-60% of MVC the soleus H-reflex does not provide an accurate measure of soleus motor neuron pool excitability. Further experimental results showed that the H-reflex amplitude at a given torque level depended on whether torque was increasing or decreasing. 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-reflex 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.