Evaluation of reciprocal inhibition of the soleus H-reflex during tonic plantar flexion in man.

Changes in reciprocal inhibition from ankle dorsiflexors to ankle plantar flexors were evaluated at increasing levels of tonic plantar flexion in 11 healthy subjects. Stimulation of the common peroneal nerve (CPN) evoked a short-latency depression of the rectified and averaged soleus electromyogram (average latency of depression: 40 ms) and a short-latency inhibition of the soleus H-reflex (conditioning-test interval: 2-3 ms). When the intensity of the CPN stimulation was below approximately 1.2 x motor threshold (x MT) the inhibition of both the soleus EMG (expressed as the amount of EMG during the inhibition as percentage of the background EMG) and the soleus H-reflex (expressed as the size of the conditioned reflex as percentage of the control H-reflex size) were seen to decrease with increasing levels of plantar flexion. At intensities of stimulation higher than approximately 1.2 x MT the inhibition of the EMG and the H-reflex was very strong and was not modulated with contraction. It is suggested that the decrease of reciprocal inhibition with increasing levels of plantar flexion is due to a decreased excitability of the Ia inhibitory interneurones which are responsible for the inhibition. It is emphasized that submaximal stimulation is necessary to demonstrate this modulation of inhibition and that the functional contribution of reciprocal inhibition to motor performance cannot be revealed from the amount of inhibition evoked by artificial electrical stimulation of a peripheral nerve.