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 up right 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). S urface electromyograms (EMG) of the soleus (Sol) and tibialis anterior (TA) were recorded and latency and area of responses were measured. B ody sway variables during stance with open or closed eyes were also re corded. Upward tilt evoked a short-latency response (SLR) in Sol and a long-latency response (LLR) in TA. Downward tilt evoked a medium-late ncy response (MLR) in TA and a LLR in Sol. This pattern of EMG respons es was similar in both young and elderly subjects, although there were some differences in latency and amplitude. There was a significant re lationship between latency of all responses and age. Slope of the regr ession lines of TA LLR, TA MLR, and Sol LLR was steeper than that of S ol SLR. Area of Sol SLR was unrelated to age, but a positive trend was identified in the other responses, significant for TA LLR. Under supp orted-stance condition, amplitude of TA MLR, TA LLR, and Sol LLR was d ecreased to a similar extent in both young and elderly subjects. There was a weak relationship between age and most body sway variables. A s ignificant relationship was found between most sway variables and late ncy of Sol SLR and LLR, chiefly with eyes closed. Neither TA MLR nor L LR were significantly correlated with sway variables, but a trend was present for TA MLR with eyes closed. The slopes of the regression of M LR and LLRs with age, steeper than that of Sol SLR, suggest that chang es in latency are accounted for by decrease in nerve conduction veloci ty, or by slowing of central synaptic transmission, or both. Increase in amplitude of MLR and LLRs could be a functional compensation for in creased latency. Elderlys retain the ability to modulate responses und er supported stance, a fact which distinguishes them from parkinsonian s. The relationship between Sol responses and postural sway with eyes open or closed allows to assess the respective weight of visual and so matosensory inputs in the control of body sway.