Fig. 5. Moment arms about a single axis of the hip joint depend not only on the rotation angle about that axis but also on rotation angles about the other two hip axes. The left column of each panel (A—C) shows data for the model frog, and the right column shows data measured in experimental frogs. The top row of each panel shows data for semimembranosus (SM) and the bottom row shows data for sartorius (SA). For each plot (four per panel), the right and left horizontal axes represent the hip angles (in degrees) and the vertical axis represents the moment arm (in mm) about the flexion—extension (FLEX/EXT) (A), abduction—adduction (ABD/ADD) (B) and external—internal rotation (EX/IN) (C) axes of the hip. (A) Extensor moment arms for SM were dramatically reduced when the femur was adducted or abducted away from the test position. The peak flexor moment arm for SA was reduced when the femur was adducted or abducted away from the test position. (B) The abduction moment arms for SM varied little across the range of abduction—adduction when the femur was extended, but varied to a much greater extent (by 30-40 %) when the femur was flexed. The opposite effect was observed for SA adduction moment arms. (C) Internal rotation moment arms for SM were largest at extended hip positions and smallest at flexed hip positions. External rotation moment arms for SA were largest at flexed positions and smallest at extended positions.

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