Effects of Femoral Nerve Stimulation on the Electromyogram and Reflex Excitability of Tibialis Anterior and Soleus

The present study was undertaken to determine whether femoral nerve stimulation would produce heteronymous reflex responses in tibialis anterior (TA) and soleus, demonstrable by averaging the electromyogram (EMG) produced by a voluntary contraction, and whether the responsible changes in excitability were sufficient to affect the H reflexes of TA and soleus. In both muscles, femoral stimuli produced short-latency, presumably monosynaptic excitation, better defined in poststimulus averages of unrectified EMG, followed by long-lasting inhibition, better defined in averaged rectified traces. The H reflexes underwent changes at appropriate latencies. The thresholds for excitation and inhibition were, respectively, below and above threshold for the quadriceps M wave. The heteronymous responses were largely independent of stimulus rate and, within limits, scaled with the level of background contraction. The ability to define these heteronymous connections using relatively simple methodology extends their utility. Such tests may prove useful in probing pathophysiological mechanisms in individual patients.