We have created models of three lower extremity cadaveric specimens and four subjects with cerebral palsy from MR images. The models characterize the geometry of the pelvis, femur, and proximal tibia, the kinematics of the hip and tibiofemoral joints, and the paths of the hamstrings, iliopsoas, and adductor muscles. We have quantified the accuracy of muscle-tendon lengths and moment arms estimated using these models by comparing hip and knee flexion moment arms estimated from the models of the specimens to the moment arms determined experimentally on the same specimens. Errors in the hip and knee flexion moment arms of the muscles, averaged over functional ranges of motion, were less than 4 mm (within 10%). Based on these analyses, we believe that the models provide accurate estimates of muscle-tendon lengths and moment arms in vivo.


The four subjects with cerebral palsy who were imaged ranged in age from 7 to 27 years and are shown here. The subjects exhibited different levels of walking ability and degrees of femoral deformity. None of the subjects had undergone previous orthopaedic surgery, and all were able to ambulate without orthoses or other assistance. We have used these MR-based models, in conjuction with kinematic data obtained from gait analysis, to examine causes of the excessive knee flexion and hip internal rotation in persons with crouch gait. We have also used the MR-based models to help evaluate and improve a generic model of the lower extremity with a deformable femur.

Associated Publications

Arnold, Blemker, and Delp. Evaluation of a deformable musculoskeletal model for estimating muscle-tendon lengths during crouch gait. Annals of Biomedical Engineering, 2001. (Download PDF)

Arnold and Delp. Rotational moment arms of the medial hamstrings and adductors vary with femoral geometry and limb position: implications for the treatment of internally-rotated gait. Journal of Biomechanics, 2001. (Download PDF)

Arnold, Asakawa, and Delp. Do the hamstrings and adductors contribute to excessive internal rotation of the hip in persons with cerebral palsy? Gait and Posture, 2000. (Download PDF)

Arnold, Salinas, Asakawa, and Delp. Accuracy of muscle moment arms estimated form MRI-based musculoskeletal models of the lower extremity. Computer Assisted Surgery, 2000. (Download PDF)