Muscle, joint, and tendon contributions to the torque profile of frog hip joint.

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The relative contributions of muscle force, moment arm, and tendon compliance were determined as a function of joint angle in the frog semitendinosus-hip joint system. Muscle, joint, and tendon properties were individually measured and then combined to predict the torque generated at the hip joint as a function of joint angle (i.e., the hip torque profile). Predicted torques were then compared to experimentally measured torques using a stepwise regression model to quantify the relative importance of muscle, joint, and tendon contributions to the hip torque profile. Variation in moment arm accounted for 74% of the variability observed in the hip torque profile, while addition of the muscle's intrinsic sarcomere length-tension property accounted for an additional 19% of the torque profile variability. Tendon compliance, which permitted a small amount of sarcomere shortening, accounted for only about 4% of the torque profile variability. We conclude that in this muscle-joint system, the relative fiber length-to-moment arm ratio is the major determinant of the shape of the isometric joint profile. The fiber length-to-moment arm ratio in other mammalian systems is also discussed.