Effect of aging on postural reflex induced by sudden drop of support surface during upright standing in humans

Abstract

The present study investigated the effect of aging on the postural reflex induced by a sudden drop of the supportive surface during upright standing posture. Ten neurologically healthy elderly (68.2 ± 4.8 years) and ten young subjects (27.0 ± 4.1 years) participated. The experiment consisted of 80 consecutive perturbations that included 20 perturbations for examining the habituation process and three security conditions (eyes closed, with warning signal, and with bar touch) placed among four control conditions (eyes open). The magnitudes of electromyographic (EMG) activity in the lower limb muscles (Sol: soleus, TA: tibialis anterior, RF: rectus femoris, BF: biceps femoris) in response to the perturbation was examined. The results demonstrated that EMG responses in the TA, RF, and BF muscles showed significantly larger in the elderly group than those in younger group, whereas Sol showed no such difference between the two groups. For the security conditions, a larger postural response modulation, especially for TA, was observed in the younger group compared to the older group. Moreover, the TA response showed a remarkably different time course of adaptation to consecutive postural perturbations between groups, that is, a significantly slower adaptation of the postural response in elderly persons. These results suggest that (1) the aging effect on the automatic postural reflex is different from muscle to muscle, (2) aging affects the central set of the postural reflex, and (3) TA is highly modulated with respect to the security level and also largely affected by aging.