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### Protocol

Subjects walked at their self-determined, normal pace for eight minutes around a 400 m running track. All subjects wore their own shoes or sneakers. An investigator walked slightly behind each subject during the test. A recently developed technique was used to measure the stride time dynamics during this relatively long walk on level ground (9,11,12). Two force-sensitive switches were placed inside the subject's right shoe, one underneath the heel of the foot and the other underneath the ball of the foot. The output of these footswitches, which provides a measure of the force applied to the floor, was sampled at 300 Hz and stored in a small light-weight (5.5x2x9 cm; .1 Kg), ankle-worn recorder. Subsequently, the recorded signal was automatically analyzed to determine initial contact time (heel-strike) of each stride throughout the walk, and hence, the stride time (the time from one heel-strike to the next heel-strike of the same foot) time series (9). The average walking speed was determined by measuring lap time.