We have developed a seven segment, seven degree-of-freedom model of the human lower extremity to examine how surgical changes in musculoskeletal geometry and muscle architecture affect muscle force and its moment about the joints. This model represents a subject that is about 1.8 m tall and has the strength of a young, adult male. We defined the lines of action of forty-three muscle-tendon actuators based on their anatomical relationships to three-dimensional surface representations of the bones. A model for each actuator was formulated to compute its isometric force-length relation. The kinematics of the lower extremity were specified by modeling the hip, knee, ankle, subtalar, and metatarsophalangeal joints. Thus, the force and joint moment that each muscle-tendon actuator develops can be computed for any body position. The joint moments calculated with the model compare well with experimentally measured isometric joint moments. This model is available for download.