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

Purpose: 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 250 ms after switching on (V-ON) and off (V-OFF) a pair of vibrators, placed over the Achilles tendons.
Methods: Twenty young (25.1 ± 5.1 yrs) and 16 older (71.8 ± 6.1 yrs) adults stood blindfolded, while 12-15 s periods of Achilles tendon vibration (ATV) were applied with 20-24 s 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/M _max ) and SOL EMG (H/SOL) were evaluated before ATV (Pre) and at 50, 100, 150, 200 and 250 ms after the V-ON and V-OFF.
Results: Only in young adults the TA/SOL EMG ratio decreased 100 (p = 0.032) and 150 ms (p < 0.001) after V-ON and the H/M _max and H/SOL decreased 150-250 ms after V-ON (p < 0.001). At 50-250 ms after V-OFF, H/M _max 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 250 ms after V-OFF. At 150 ms after V-OFF, SOL/M _max was decreased only for the older adults (p < 0.001), whereas TA/SOL EMG gradually increased for both groups (p < 0.001).
Conclusion: 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.
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