Influence of age and posture on spinal and corticospinal excitability.

This study investigated the modulation of the excitability of spinal and corticospinal pathways from seated to upright standing across adult lifespan. The input-output relations for the Hoffmann (H) reflex and motor-evoked potential (MEP) were recorded in soleus muscle during seated and upright posture in 40 subjects assigned to either young (range: 19-39yrs, n=16), middle-aged (40-59yrs, n=12), and elderly group (60-76yrs, n=12). In seated posture, H reflex and MEP were recorded during voluntary contractions of the ankle plantar flexors inducing similar soleus electromyographic activity that during upright standing. Maximal H-reflex and MEP amplitude (Hmax, MEPmax), and the change in response amplitude from seated to standing were analyzed. The Hmax decreased with age (p=0.001) and from seated to standing (p<0.001). The change in Hmax from seated to standing was greater in elderly (-40.1%; p<0.05) compared with young (-20.7%) and middle-aged groups (-23.7%). MEPmax increased with age and from seated to standing (p<0.001). The change in MEPmax from seated to standing did not differ (p>0.05) between young (+53.6%), middle-aged (+43.2%) and elderly groups (+77.3%). These results indicate a posture-related modulation of the excitability of spinal and corticospinal pathways regardless of age, with a more pronounced modulation of spinal pathway in elderly adults. This study further documents the time course of decrease and increase in spinal and corticospinal excitability, respectively, across adult lifespan.