Muscle force and moment arm contributions to torque production in frog hindlimb.

The relative contribution of maximum muscle tetanic tension (Po) and muscle moment arm to maximum knee flexion torque was investigated in the frog hindlimb. Isometric torque was measured in frog semitendinosus muscle-bone complexes throughout the range of 0-160 degrees of flexion. Optimal joint angle (the angle at which isometric torque was maximum) was observed at 140 degrees of flexion. After torque measurements, the muscle was excised and the muscle length-tension relationship measured for determination of Po and optimal muscle length. In addition, the kinematics of the knee joint and therefore, the muscle moment arm was measured as a function of joint angle using principles of rigid body kinematics. Stepwise linear regression indicated that maximum torque was most highly correlated with Po (r = +0.77, P less than 0.01) and accounted for approximately 75% of the measured torque. In addition, there was no significant correlation between maximum torque and maximum muscle moment arm (r = +0.11, P greater than 0.7) suggesting that muscle force, not musculoskeletal anatomy, represents the major determinant of maximum torque production in the frog hindlimb.