Age-related changes in presynaptic inhibition during gait initiation.

Age-related changes in presynaptic inhibition (PSI) have not been observed during gait initiation, which requires anticipatory postural adjustment (APA). As APA is centrally modulated and is impaired in older compared to young adults, here we aimed to study the presynaptic control and co-contraction levels in the ankle muscles during gait initiation in older compared to young adults. Fifteen older (age range 65-80 years) and 15 young adults (age range 19-30 years) performed a gait initiation task on a force platform under three conditions: 1) without electrical stimulation; 2) test Hoffman reflex (H-reflex); and 3) conditioned H-reflex. H-reflexes were evoked on the soleus muscle when the APA amplitude exceeded 10-20% of the average baseline mediolateral force. Participants also performed quiet stance as a control task. Results showed that both age groups presented similar PSI levels during quiet stance (P=0.941), while in the gait initiation older adults presented higher PSI levels, longer duration and lower amplitude of APA than young adults (P<0.05). Older adults presented higher co-contraction ratio in both tasks than young adults (P<0.05). Correlations between the PSI levels and the APA amplitude (r= -0.61, P=0.008), and between the PSI levels and the co-contraction ratio during gait initiation (r= -0.64, P=0.005) were found for older adults only. APA amplitude explained 49% of the variance of the PSI levels (P=0.003). Our findings suggest that older compared to young adults have increased presynaptic control to compensate for the decreased supraspinal modulation on impaired APAs during gait initiation.