Previous research has shown that the efficient control of most voluntary movements in man is associated with the completion of appropriate preparatory processes wihin the nervous system prior to the time of movement onset. These preparatory processes can be divided temporally into two categories: 1) depression of spinal reflex pathway and facilitation of transcortical pathways in the involved muscle, and 2) activation of task-associated postural muscles before voluntarty muscles in order to compensate in advance for instability associated with the movement. It is hypothesized here that the decreased coordination of voluntary responses in the older adult is due in part to the deterioration of the ability to appropriately execute these two types of preparatory processes. Experiments are proposed to study this problem from three perspectives: 1) age-related changes in the respective depression and facilitation of spinal and transcortical reflex pathways of arm muscles during the preparatory period before a reaction time arm movement. Reflexes will be elicited by perturbations of the forearm during preparation via a hydraulic system; 2) age-related changes in the postural muscle response system, including changes in latency, variability and context dependence of muscle response synergies of the legs in response to unexpected support surface displacements, similar to slipping momentarily on a smooth surface. Postural responses eill be elicited via movements of a hydraulically activated platform; 3) age-related changes in the linkage between the postural and voluntary response systems during arm movements involving balance requirements, measured by comparing onset latencies of responses of leg and arm muscles involved in the task. These studies will contribute to our understanding of the neural mechanisms underlying deterioration of movement performance in the elderly under conditions when balance is important, and should lead to the design of guidelines for nursing homes and hospitals to aid in reducing the percentage of accidents involving falls in the aging population.
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