Journal of Biomechanics 32 (1999) 493—501

Excessive ßexion and internal rotation of the hip is a common gait abnormality among individuals with cerebral palsy. The purpose of this study was to examine the inßuence of hip ßexion on the rotational moment arms of the hip muscles. We hypothesized that ßexion of the hip would increase internal rotation moment arms and decrease external rotation moment arms of the primary hip rotators. To test this hypothesis we measured rotational moment arms of the gluteus maximus (six compartments), gluteus medius (four compartments), gluteus minimus (three compartments) iliopsoas, piriformis, quadratus femoris, obturator internus, and obturator externus. Moment arms were measured at hip ßexion angles of 0, 20, 45, 60, and 90 ¡ in four cadavers. A three-dimensional computer model of the hip muscles was developed and compared to the experimental measurements. The experimental results and the computer model showed that the internal rotation moment arms of some muscles increase with ßexion; the external rotation moment arms of other muscles decrease, and some muscles switch from external rotation to internal rotation as the hip is ßexed. This trend toward internal rotation with hip ßexion was apparent in 15 of the 18 muscle compartments we examined, suggesting that excessive hip ßexion may exacerbate internal rotation of the hip. The gluteus maximus was found to have a large capacity for external rotation. Enhancing the activation of the gluteus maximus, a muscle that is frequently underactive in persons with cerebral palsy, may help correct excessive ßexion and internal rotation of the hip. ( 1999 Elsevier Science Ltd. All rights reserved