Weak motor cortex contribution to the quadriceps activity during human walking

Cortical and sub-cortical contribution to the basic locomotor rhythm is still unclear in humans. While motor cortex is involved in the ankle muscle activity during walking, recent findings suggest lesser contribution to that of knee extensors. This was further tested during treadmill walking (3-4 km/h; end swing and early stance) using transcranial magnetic stimulation (TMS). Sub-threshold TMS successively suppressed and increased Vastus Lateralis (VL) EMG activity during tonic contraction while standing, and both responses were significantly depressed during walking. Paired pulse TMS produced weak intra-cortical inhibition during tonic VL contraction, which did not change during walking. Lastly, sub-threshold TMS did not produce any change in VL H-reflex during walking. It is shown that the excitability of pathways, mediating short intra-cortical inhibition and facilitation in VL motor area, is particularly depressed during walking compared to tonic contraction. The present study thus reveals different modulation in VL than that reported in ankle muscles, suggesting lesser cortical contribution to its activity during walking.