Abduction-adduction moments at the knee during stair ascent and descent.

To examine the relative magnitude of the knee abduction-adduction moments during stair climbing, ten normal subjects (average weight 660 N, leg length 0.962 m, height 1.74 m) were studied during repeated trials of stair ascent and descent. Data were collected using a four-camera video system and two forces plates incorporated within a flight of three stairs. The inverse dynamics approach was used to calculate internal moments at the knee, and these moments were normalized in magnitude (to percent body weight and leg length) and time (percent stance). The primary findings were: (1) knee joint moments were similar in shape and magnitude for the first and second steps during both stair ascent and descent; (2) the abduction knee moments, although comparable in magnitude (25-45 Nm), were statistically smaller than the extension moments (60-85 Nm) for stair ascent and descent; and (3) the moment patterns were exclusively abductor throughout stance, indicating that the ground reaction vector always passed medial to the knee joint center. Although the knee abduction-adduction moment is not in the primary plane of motion, its magnitude should not be ignored when trying to understand the stability and function of the knee during stair climbing.