The purpose of this study is to evaluate the stability of the lower limbers during high speed motion. The tests included analysis of motion in 15-second stationary stepping motions and muscle strength measurements of the hip flexion in a sitting position. The motions of 80 subjects ranging from a teenager to an elderly person were videotaped with a video camera operating at 60 Hz. The stability was evaluated by the change of coefficient of variation of the knee during the stepping motion. The coefficient of variation was calculated on the ratio of the standard deviation to the mean of peak height values the knee joint reached at each step. The result showed that a sexual distinction of stability exists in the teenaged and middle-aged group, being less stable in the female teenaged group and male middle-aged group. Stability of motion and muscle strength were not influenced by aging and muscle strength from the age of 12 to 70. Stability of motion and muscle strength showed a tendency of decline from the age of 70 in both males and females. The decline of muscle strength had an influence on the stability of motion in elderly, having reduced stability from less than 17kg in males and less than 9 kg in females. The decline of stability of motion may be an important factor explaining the fall risk of elderly during fast walking or running.