Effects of foot placement on the lower extremity in the swing phase during stair ascending: Implications for transfemoral prostheses

Stair ascending, especially step-over-step gait, is a difficult motor task for people with transfemoral amputation. Although many types prosthetic knees have been developed to attempt such locomotion, finding alternative methods is also important. The purposes of the present study were to (a) elucidate the effects of foot placement, as part of stair ascending strategies, on the lower extremity motion of able-bodied subjects and (b) investigate the effectiveness of these strategies on transfemoral prosthetic leg swinging during stair ascending in a computer simulation. In the experiment, 2 different foot placement strategies on a step were conducted with able-bodied subjects: the full-foot contact (FFC) and half-foot contact (HFC) strategies. HFC made the toe trajectory straighter, mainly owing to the knee joint motion. The joint moments at the hip and knee were smaller in the HFC condition than the FFC condition. These results suggest that HFC causes the leg swing to have more inertial motion than FFC and demonstrate the applicability of the HFC condition for transfemoral prostheses without any actuators. In the computer simulation based on the results of the first experiment, various inertial properties of a transfemoral prosthesis were examined to accomplish leg swing without tripping. Only in the HFC condition specific inertial properties allowed the prosthesis to avoid colliding with the stairs. These simulation results suggest that with natural motion, transfemoral prostheses without assistive devices could allow successful leg swing for step-over-step gait with the HFC condition and appropriate inertial properties.