Multi-electrode recording of Hoffman reflexes.

A multi-electrode system has been used to simultaneously record the H-reflex and M response from various locations on the gastrocnemius and soleus. With certain positions of the stimulating electrode in the popliteal fossa, large amplitude M responses were recorded in gastroenemius at a time when only low amplitude H-reflexes were recorded from soleus. These result not only reinforce the importance of casreful selection of the stimulation site but go further and suggest a definite need for the simultaneous monitoring of gastrocnemius and soleus responses to avoid contamination of the soleus H-reflex or its excitability cycle by the unwanted and unsuspected gastrocnemius M response. Conventional practice of recording an H-relfex and its excitability cycle from a single pair of bipolar electrodes located over soleus is susceptible to confounding from the unwanted M response by alterations in the initial mechanical conditions and by the various afferent discharges evoked by a preceding contraction in gastrocnemius.