Vision and proprioception do not influence the excitability of the corticomotoneuronal pathway during upright standing in young and elderly adults.

ABSTRACT

This study investigated the influence of vision and proprioception on the excitability of direct corticospinal (corticomotoneuronal) pathway to the soleus in young and elderly adults during upright standing. Ten young and 10 elderly adults stood upright on a rigid surface with eyes open or closed, and on foam mat with eyes open. The corticomotoneuronal excitability was investigated by assessing facilitation of the soleus H-reflex induced by subthreshold transcranial magnetic stimulation (TMS). The torque produced by the plantar flexor muscles during a maximal voluntary contraction was also measured. The maximal plantar flexion torque was significantly lower in elderly than in young adults (p<0.05). The activity of leg muscles, recorded by electromyography (EMG) was greater in elderly than in young adults regardless of balance conditions (p<0.05), and greater when standing on foam than in the other conditions (p<0.05), regardless of age. The H-reflex facilitation was greater for elderly [182.9 (45)%] than young adults [130.5 (33.1)%; p<0.05] but did not differ across sensory conditions (p>0.05). However, the amplitude of the H reflex conditioned by TMS relative to the amplitude of the test H reflex ratio was positively associated with EMG activity of the plantar flexor muscles during upright standing (r(2)=0.47; p<0.001). These results indicate that regardless of age the excitability of the corticomotoneuronal pathway is not modulated with changes in the sensory conditions during upright standing. Nonetheless, the corticomotoneural drive to control leg muscle during upright standing increases with the level of soleus muscle activity.