Modulation of reciprocal inhibition between ankle extensors and flexors during walking in man

1. The modulation of disynaptic reciprocal inhibition between antagonistic ankle muscles during walking was investigated in 17 healthy human subjects. Inhibition from ankle dorsiflexors to ankle plantar flexors was evoked by stimulation of the common peroneal nerve (CPN) and evaluated as the stimulus-induced depression of rectified soleus EMG activity (latency approx. 40 ms) or the short-latency depression of the soleus H-reflex (conditioning-test intervals around 2-3 ms). In some experiments the inhibition from ankle plantar flexors to ankle dorsiflexors was investigated. In these experiments the tibial nerve was stimulated and the amount of inhibition was evaluated from the short-latency depression of the voluntary rectified tibialis anterior (TA) EMG. 2. The short-latency inhibition of the soleus H-reflex following the CPN stimulation (1.1 x motor threshold; MT) was strongly modulated during walking, being large in the swing phase and absent in the stance phase. 3. A smaller amount of EMG depression following the CPN stimulation (1. 1-1.2 x MT) was observed in the stance phase of walking as compared to tonic or dynamic plantar flexion at a similar background EMG activity level in standing or sitting subjects. 4. In four subjects a depression of the TA EMG activity was produced by stimulation of the tibial nerve (1.1-1.2 x MT). In all subjects a smaller amount of inhibition was observed in the swing phase of walking as compared to tonic dorsiflexion at a comparable EMG activity level. 5. It is concluded that the transmission in the disynaptic Ia reciprocal pathway between ankle plantar flexors and dorsiflexors is modulated during walking. Inhibition from dorsiflexors to plantar flexors seems to be large in swing and small in stance, whereas inhibition from plantar flexors to dorsiflexors seems to be small in swing.