The height of a chair seat affects the burden on the lower limbs during the sit-to-stand (STS). To develop an objective test to evaluate muscle function of the lower limbs using floor reaction force during a STS, the relationship between chair-seat height and the burden on the lower legs must be assessed. To examine the influence of the chair-seat height, floor reaction forces during a STS performed with 5 chair heights adjusted to each subject's lower leg length were compared. The force production and quickness of movement tended to decrease in the phases of trunk flexion and knee and hip joint extension when performing a STS from a lower chair, when the chair height differed by 20% (6.2 cm) from the lower leg length, and was marked when the difference between chair height and lower leg length became larger. In 52 elderly and 50 young adults, floor reaction forces during a STS performed from a chair of the same height as subjects' lower leg lengths were compared. Elderly persons were inferior in force production (strength) and quickness of movement, which decreased as elderly stood up from a chair of a lower height.