Intracortical inhibition in the soleus muscle is reduced during the control of upright standing in both young and old adults

Purpose In a previous study, we reported that a short-interval intracortical inhibition (SICI) decreases in old but not in young adults when standing on foam vs. a rigid surface. Here, we examined if such an age by task difficulty interaction in motor cortical excitability also occurs in easier standing tasks. Methods Fourteen young (23 +/- 2.7 years) and fourteen old (65 +/- 4.1 years) adults received transcranial magnetic brain stimulation and peripheral nerve stimulation, while they stood with or without support on a force platform. Results In the soleus, we found that SICI was lower in unsupported (35 % inhibition) vs. supported (50 %) standing (p = 0.007) but similar in young vs. old adults (p = 0.591). In the tibialis anterior, SICI was similar between conditions (p = 0.597) but lower in old (52 %) vs. young (72 %) adults (p = 0.030). Age and standing with or without support did not affect the Hoffmann reflex in the soleus. Conclusions The current data suggest that the motor cortex is involved in standing control, and that its role becomes more prominent with an increase in task difficulty