The effect of transcranial magnetic stimulation on the soleus H reflex during human walking

1. The effect of transcranial magnetic stimulation (TMS) on the soleus H re flex was investigated in the stance phase of walking in seventeen human sub jects. For comparison, measurements were also made during quiet standing, m atched tonic plantar flexion and matched dynamic plantar flexion. 2. During walking and dynamic plantar flexion subliminal (0.95 times thresh old for a motor response in the soleus muscle) TMS evoked a large short-lat ency facilitation (onset at conditioning-test interval: -5 to -1 ms) of the H reflex followed by a later (onset at conditioning-test interval: 3-16 ms ) long-lasting inhibition. In contrast, during standing and tonic plantar f lexion the short-latency facilitation was either absent or small and the la te inhibition was replaced by a long-lasting facilitation. 3. When grading the intensity of TMS it was found that the short-latency fa cilitation had a lower threshold during walking than during standing and to nic plantar flexion. Regardless of the stimulus intensity the late facilita tion was never seen during walking and dynamic plantar flexion and the late inhibition was not seen, except for one subject, during standing and tonic plantar flexion. 4. A similar difference in the threshold of the short-latency facilitation between walking and standing was not observed when the magnetic stimulation was replaced by transcranial electrical stimulation. 5. The lower threshold of the short-latency facilitation evoked by magnetic but not electrical transcranial stimulation during walking compared with s tanding suggests that cortical cells with direct motoneuronal connections i ncrease their excitability in relation to human walking. The significance o f the differences in the late facilitatory and inhibitory effects during th e different tasks is unclear.