Independent modulation of corticospinal and group I afferents pathways during upright standing

Balance control during upright standing is accompanied by an increased amplitude of motor-evoked potentials (MEP) induced by transcranial magnetic stimulation and a decreased amplitude of the Hoffmann (H) reflex in the soleus muscle. Nonetheless, whether these observations reflect reciprocal adjustments between corticospinal and group I afferents pathways during upright standing remains unknown. To further investigate this question, cathodal transcranial direct current stimulation (c-tDCS) applied over the motor cortex and vibration of Achilles tendons were used to modify the excitability of corticospinal and group I afferent pathways, respectively. MEPs and H reflexes were recorded in the soleus muscle during upright standing with or without bilateral Achilles tendon vibration, these recordings being performed before and after 20 min of c-tDCS (1.5 mA) or sham stimulation applied over the sensorimotor cortex. The results indicate that tendon vibration increased MEP amplitude (+28%) and decrease (-68%) the H-reflex amplitude (p<0.05). After c-tDCS, MEP amplitude was reduced by 13% and 26% without and with tendon vibration (p<0.05), respectively. In contrast, no significant change occurred in H-reflex amplitude after c-tDCS. Regardless of the conditions (c-tDCS and tendon vibration), no significant correlation was observed between changes in MEP and H-reflex amplitudes. The results failed to demonstrate close reciprocal changes in soleus MEP and H-reflex amplitudes during upright standing. These original findings suggest independent adjustments in corticospinal and group I afferents pathways during upright standing.