Sural nerve evoked responses in human hip and ankle muscles while standing

The objective of this study was to investigate the influence of background EMG, as manipulated by changes in limb loading, and electrical stimulus intensity on sural nerve evoked EMG responses in proximal hip (ipsilateral gluteus medius and contralateral adductor longus) and distal ankle (ipsilateral soleus) muscles during quiet standing. We studied 14 healthy subjects who were instructed to stand on a force platform and load the right leg to 20, 30, 60, and 80% body weight. Trains of stimuli were delivered to the right sural nerve at three different intensities (1.5, 2, and 3 perception threshold). Average evoked responses from each of the three muscles were obtained for each combination of body weight and stimulus intensity. Reflex responses were consistently seen in gluteus medius and soleus but not adductor longus. The responses in gluteus medius and soleus consisted of complex inhibitory and excitatory responses with the primary effect in gluteus medius being a short latency excitation (mean latency of 57 ms) and in soleus, a short latency inhibition-excitation (latencies of 47 and 58 ms, respectively). The amplitudes of the responses were influenced primarily by background EMG. The results demonstrate consistent sural nerve evoked EMG responses in both a hip (gluteus medius) and ankle (soleus) muscle. While the findings for soleus generally corroborate and extend previous studies, the responses observed for the lateral hip muscle have not been previously reported.