Variation of rotation moment arms with hip flexion

Excessive flexion and internal rotation of the hip is a common gait abnorma lity among individuals with cerebral palsy. The purpose of this study was t o examine the influence of hip flexion on the rotational moment arms of the hip muscles. We hypothesized that flexion of the hip would increase intern al rotation moment arms and decrease external rotation moment arms of the p rimary hip rotators. To test this hypothesis we measured rotational moment arms of the gluteus maximus (six compartments), gluteus medius (four compar tments), gluteus minimus (three compartments) iliopsoas, piriformis, quadra tus femoris, obturator internus, and obturator externus. Moment arms were m easured at hip flexion angles of 0, 20, 45, 60, and 90 degrees in four cada vers. A three-dimensional computer model of the hip muscles was developed a nd compared to the experimental measurements. The experimental results and the computer model showed that the internal rotation moment arms of some mu scles increase with flexion; the external rotation moment arms of other mus cles decrease, and some muscles switch from external rotation to internal r otation as the hip is flexed. This trend toward internal rotation with hip flexion was apparent in 15 of the 18 muscle compartments we examined, sugge sting that excessive hip flexion may exacerbate internal rotation of the hi p. The gluteus maximus was found to have a large capacity for external rota tion. Enhancing the activation of the gluteus maximus, a muscle that is fre quently underactive in persons with cerebral palsy, may help correct excess ive flexion and internal rotation of the hip. (C) 1999 Elsevier Science Ltd . All rights reserved.