TASK-DEPENDENT EFFECTS EVOKED BY FOOT MUSCLE AFFERENTS ON LEG MUSCLE-ACTIVITY IN HUMANS

The effect of low intensity electrical stimulation of the posterior ti bial nerve (PTN) at the ankle on the active triceps surae (TS) muscles was studied in normal subjects, both in a prone position and while st anding. PTN stimulation regularly evoked the H-reflex in the flexor di gitorum brevis and, in the prone position, a short-latency facilitator y effect in the soleus muscle. During standing, the facilitatory effec t was preceded by a clear-cut reduction in electromyograph (EMG) activ ity. The inhibition-facilitation sequence was evoked in the gastrocnem ii under both conditions, on average, though individual differences we re present. An EMG modulation similar to that observed under standing conditions was present also in the prone position when subjects presse d the sole of the foot against the wall. Stimulation of sural or digit al nerves did not evoke similar effects. It is concluded that foot mus cle afferents establish oligosynaptic connections transmitting mixed e ffects to the TS motoneuronal pool, and that contact with the sole of the foot plays an enabling role for the inhibitory pathway directed to the soleus muscle.