EFFECTS OF HIP CENTER LOCATION ON THE MOMENT-GENERATING CAPACITY OF THE MUSCLES

We have developed a three-dimensional biomechanical model of the human lower extremity to study how the location of the hip center affects t he moment-generating capacity of four muscle groups: the hip abductors , adductors, flexors, and extensors. The model computes the maximum is ometric force and the resulting joint moments that each of 25 muscle-t endon complexes develops at any body position. Abduction, adduction, f lexion, and extension moments calculated with the model correspond clo sely with isometric joint moments measured during maximum voluntary co ntractions. We used the model to determine (1) the hip center location s that maximize and minimize the moment-generating capacity of each mu scle group and (2) the effects of superior-inferior, anterior-posterio r, and medial-lateral displacement of the hip center on the moment arm s, maximum isometric muscle forces, and maximum isometric moments gene rated by each muscle group. We found that superior-inferior displaceme nt of the hip center has the greatest effect on the force- and moment- generating capacity of the muscles. A 2 cm superior displacement decre ases abduction force (44%), moment arm (12%), and moment (49%), while a 2 cm inferior displacement increases abduction force (20%), moment a rm (7%) and moment (26%). Similarly, a 2 cm superior displacement decr eases flexion force (27%), moment arm (6%), and moment (22%), while in ferior displacement increases all three variables. Anterior-posterior displacement alters the moment-generating capacity of the flexors and extensors considerably, primarily due to moment arm changes. Medial-la teral displacement has a large effect on the moment-generating capacit y of the adductors only. A 2 cm medial displacement decreases adductio n moment arm (20%), force (26%) and moment (40%). These results demons trate that the force- and moment-generating capacities of the muscles are sensitive to the location of the hip center.