Plantar flexor moment arm and muscle volume predict torque-generating capacity in young men

Abstract

Muscle volume is known to correlate with maximal joint torque in humans, but the role of muscle moment arm in determining maximal torque is less clear. Moderate correlations have been reported between maximal isometric knee extensor torque and knee extensor moment arm, but no such observations have been made for the ankle joint. It has been suggested that smaller muscle moment arms may enhance force generation at high rates of joint rotation, but this has not yet been observed for ankle muscles in vivo. The purpose of the present study was to correlate plantar flexor moment arm and plantar flexor muscle volume with maximal plantar flexor torque measured at different rates of plantar flexion. Magnetic resonance imaging was used to quantify the plantar flexor moment arm and muscle volume of the posterior compartment in 20 healthy young men. Maximal plantar flexor torque was measured isometrically and at three plantar flexion speeds using an isokinetic dynamometer. Plantar flexor torque was significantly correlated with muscle volume (0.222 < R(2) < 0.322) and with muscle moment arm at each speed (0.323 < R(2) < 0.494). While muscle volume was strongly correlated with body mass and stature, moment arm was not. The slope of the torque-moment arm regression line decreased as the rate of joint rotation increased, indicating that subjects with small moment arms experienced smaller reductions in torque at high speeds. The findings of this study suggest that plantar flexor moment arm is a determinant of joint strength that is at least as important as muscle size.

Keywords: force-velocity; joint torque; moment arm; plantar flexors.

Fig. 1.

Magnetic resonance image of a sagittal cross section of the foot and ankle showing the center of ankle rotation and plantar flexor moment arm.

Fig. 2.

Magnetic resonance image of a cross section of the lower leg showing the area of the posterior compartment filled in with a striped pattern.

Fig. 3.

Maximal plantar flexor torque measured at neutral position plotted against plantar flexor moment arm for isometric trials (top) and the 210°/s isokinetic trials (bottom). Significant correlations were found between torque and moment arm for both conditions (R² = 0.315, P = 0.01; and R² = 0.484, P = 0.001, respectively).

Fig. 4.

Maximal plantar flexor torque measured at neutral position plotted against posterior compartment muscle volume for isometric trials (top) and the 210°/s isokinetic trials (bottom). Significant correlations were found between torque and muscle volume for both conditions (R² = 0.322, P = 0.009; and R² = 0.222, P = 0.036, respectively).

Fig. 5.

Posterior compartment muscle volume plotted against plantar flexor moment arm for all subjects. A correlation was found between muscle volume and moment arm that nearly reached the level of significance (R² = 0.191, P = 0.054).