Response of human soleus single motor units to changes in joint position and intensity of peroneal nerve stimulation

This research examined the correlation between peroneal nerve stimulation and the response of voluntarily recruited rhythmically firing soleus motor units (MUs). Two hundred and twenty-five MUs were recorded in 10 neurologically normal adult subjects, eight men and two women (mean age 32.3 yr). The investigation complemented previous research by re-examining reciprocal innervation patterns while controlling for the effects of two levels of stimulation (0.9 and 1.25 motor threshold) and two ankle positions (15 degrees and 45 degrees of plantarflexion). Motor unit responses were assessed by peristimulus time histogram (PSTH) analysis and a cumulative sum (CUSUM) procedure. The responses were grouped into five categories according to latency values and the sign of the firing rate response. Results demonstrated stimulus intensity effects for all categories except the short latency (28-48 ms) facilitatory response category (P < 0.05). No effects attributed solely to ankle position were demonstrated. Cutaneous afferents were not found to contribute to the response at the lower stimulus intensity, but significant effects were observed at 1.25 motor threshold. Latency values indicate that reciprocal inhibition, reciprocal excitation, and/or heteronymous excitation occur at the spinal cord level. The concept of reciprocal excitation is questioned. Possible physiological mechanisms associated with short latency excitations and inhibitions are discussed.