Modulation of human short latency reflexes between standing and walking.

Inhibition of the magnitude of soleus muscle homonymous (H) reflexes occurs in humans when walking, compared to standing. The current study asked, (1) was the task modulation of Ia reflexes limited to soleus muscle, (2) was there support for attributing a presynaptic source to the inhibition in humans and (3) did an oligosynaptic short latency reflex show similar task modulation? In 3 subjects, H reflexes were evoked in vastus medialis and soleus, at 4 levels of contraction in the target muscle, with constant stimulus intensity when walking and standing. The reflex magnitudes in both muscles were significantly inhibited during the contractions for walking, compared to standing. Such inhibition also occurred in H reflexes of tibialis anterior muscle. An excitatory oligosynaptic reflex was then evoked in vastus medialis, through low intensity stimulation of the common peroneal nerve during walking and standing. The mean amplitudes of this reflex were not significantly different (P less than 0.05) between the two conditions, at any contraction level. The depression of quadriceps H reflexes, compared to the oligosynaptic reflexes through the same quadriceps motoneuronal pool in the same task, strongly suggested that the inhibition of H reflexes arose at other sites besides the motoneuronal cell body and proximal dendrites. We conclude that Ia H reflexes of various leg muscles of humans are inhibited when walking but that this does not generalize to the oligosynaptic short latency reflex between the anterior shank and thigh.