Fig. 7. Three-dimensional force fields produced by the primary hip extensor muscles (A) (semimembranosus, SM, top row; gracilus, GR, middle row; adductor dorsal head, ADd, bottom row) and knee extensor muscles (B) (cruralis, CR, top; gluteus magnus, GL, middle; tensor fascia latae, TFL, bottom). Force fields were constructed by placing the model ankle at different positions in the limb's workspace and maximally activating each muscle (by simulating a fixed-end muscle contraction; see Materials and methods). The peak force produced at each of 80 positions is plotted. The force field produced by each muscle is normalized to the maximum force within each field so that force fields can be compared among muscles. The left columns of A (hip extensors) and B (knee extensor) show a top view and the right columns show a side view of the leg and the muscle force fields. One block in each view represents 10 mm², i.e. line divisions are 10 mm in length. The force vector at each ankle position has three components: rostral—caudal, medial—lateral and elevation—depression. The rostral—caudal and medial—lateral components are depicted in the left column of A and B; the rostral—caudal components are along the long axis of frog in the horizontal plane, and the medial—lateral components are along the short axis of the frog. The elevation—depression and rostral—caudal components are depicted in the right column of A and B; the elevation—depression components are forces in the plane of gravity. Each muscle produced fields that were a combination of vector components. Most importantly, the magnitude of the force vector components produced by the contraction of each muscle was configuration-dependent.

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