Internal/external rotation moment arms of muscles at the knee: moment arms for the normal knee and the ACL-deficient knee

Knowledge of the three-dimensional balance of loads at the knee joint is required to adequately assess the treatment and rehabilitation of the malfunctioning knee. This report focuses upon the moment arms for the knee in internal/external (IE) rotation motion. It augments prior work that defined flexion/extension moment arms. Muscle excursions and angular motion of the lower leg during IE rotation were measured in 17 fresh-frozen hemi-pelvis specimens. Moment arms were calculated as the derivatives of excursion with respect to the angle. Rotational motion was performed for the normal and anterior cruciate ligament (ACL)-deficient knee. Of the 13 muscles measured at the knee, seven were significant contributors to IE rotation: the biceps femoris long and short head externally rotate opposite the gracilis, sartorious, semimembranosis, semitendonosus and popliteus, functioning as internal rotators. Moment arm magnitudes were greatest with the knee in a flexed position (internal [external] rotators peaked at 70° [90°] flexion). At 30° flexion, the IE rotation moment arm minima and maxima were 10.1–11.6, 6.8–9.0, 6.0–15.7, 8.2–14.1 and 0.0–10.4 mm for the semimembranosis, semitendonosus, gracilis, sartorius and popliteus, and 14.7–27.9 and 18.5–31.5 mm for the biceps femoris short and long, respectively. Moment arms for the ACL-deficient condition were significantly changed only at extremes of flexion–extension.