Posture-related modulation of cortical excitability in the tibialis anterior muscle in humans.

Corticospinal excitability in the lower leg muscles is enhanced during standing as compared to other postures. In the present study, we investigated how the excitability of intracortical circuits that control the tibialis anterior muscle (TA) is modulated during standing. Short-interval intracortical inhibition (SICI) and intracortical facilitation (ICF) were assessed by the paired-pulse transcranial magnetic stimulation technique during standing (STD) and sitting (SIT) with a comparable background activity level in both the soleus and the TA muscle. The results demonstrated that SICI was less effective during STD than during SIT, whereas ICF was more effective during STD than during SIT. These findings suggest that the excitabilities of these cortical neural circuits are modulated depending on posture. A decrease in SICI and an increase in ICF may reflect subliminal enhancement of the cortical excitability in the TA muscle during standing as compared with that during sitting.