Temporal modulation of H-reflex in young and older people: Acute effects during Achilles tendon vibration while standing.

Abstract

Examining how timely is sensory input processed and regulated after a perturbation while standing, is a key element to understand postural control, especially in people with balance deficits, such as older adults. In this study, we investigated the age-related temporal modulations in spinal excitability, by measuring the soleus H-reflex within the first 250ms after switching on (V-ON) and off (V-OFF) a pair of vibrators, placed over the Achilles tendons. Twenty young (25.1±5.1yrs) and 16 older (71.8±6.1yrs) adults stood blindfolded, while 12-15s periods of Achilles tendon vibration (ATV) were applied with 20-24s intervals. Anterior/posterior center of pressure and electromyographic (EMG) responses [soleus (SOL) and tibialis anterior (TA)] were assessed whereas SOL H-reflex normalized to maximum M-wave (H/Mmax) and SOL EMG (H/SOL) were evaluated before ATV (Pre) and at 50, 100, 150, 200 and 250ms after the V-ON and V-OFF. Only in young adults the TA/SOL EMG ratio decreased 100 (p=0.032) and 150ms (p<0.001) after V-ON and the H/Mmax and H/SOL decreased 150-250ms after V-ON (p<0.001). At 50-250ms after V-OFF, H/Mmax was reduced compared to Pre values (p<0.001), with no differences between the age groups (p>0.05). H/SOL was decreased for the young adults (p<0.001) and remained reduced at least for the first 250ms after V-OFF. At 150ms after V-OFF, SOL/Mmax was decreased only for the older adults (p<0.001), whereas TA/SOL EMG gradually increased for both groups (p<0.001). When ATV is introduced while standing, the spinal excitability of older people is reduced later and to a lesser extent compared to young adults. Their limited capacity to down-regulate the "noisy" sensory input generated by ATV gives further evidence of a possible mechanism for their inefficient postural control.