Influence of aging on leg muscle reflex responses to stance perturbation.

The effect of age on latency and amplitude of leg muscle responses to stance perturbations was studied in 75 control subjects. They stood upright on a platform and were displaced by toe-up (upward tilt) and toe-down (downward tilt) platform rotations. Perturbations were induced during free and supported stance (holding on to a stable structure). Surface electromyograms (EMG) of the soleus (Sol) and tibialis anterior (TA) were recorded and latency and area of responses were measured. Body sway variables during stance with open or closed eyes were also recorded. Upward tilt evoked a short-latency response (SLR) in Sol and a long-latency response (LLR) in TA. Downward tilt evoked a medium-latency response (MLR) in TA and a LLR in Sol. This pattern of EMG responses was similar in both young and elderly subjects, although there were some differences in latency and amplitude. There was a significant relationship between latency of all responses and age. Slope of the regression lines of TA LLR, TA MLR, and Sol LLR was steeper than that of Sol SLR. Area of Sol SLR was unrelated to age, but a positive trend was identified in the other responses, significant for TA LLR. Under supported-stance condition, amplitude of TA MLR, TA LLR, and Sol LLR was decreased to a similar extent in both young and elderly subjects. There was a weak relationship between age and most body sway variables. A significant relationship was found between most sway variables and latency of Sol SLR and LLR, chiefly with eyes closed. Neither TA MLR nor LLR were significantly correlated with sway variables, but a trend was present for TA MLR with eyes closed.(ABSTRACT TRUNCATED AT 250 WORDS)