Estimates of Achilles Tendon Moment Arm Length at Different Ankle Joint Angles: Effect of Passive Moment

The length of a muscle's moment arm can be estimated noninvasively using ultrasound and the tendon excursion method. The main assumption with the tendon excursion method is that the force acting on the tendon during passive rotation is constant. However, passive force changes through the range of motion, and thus moment arm is underestimated. The authors attempted to account for passive force on the measurement of Achilles tendon moment arm using the tendon excursion method in 8 male and female runners. Tendon excursion was measured using ultrasound while the ankle was passively rotated at 0.17 rad·s^-1. Moment arm was calculated at 5° intervals as the ratio of tendon displacement to joint rotation from 70° to 115°. Passive moment (M_P) was measured using a dynamometer. The displacement attributable to M_P was calculated by monitoring tendon displacement during a ramp isometric maximum contraction. M_P was 5.7 (2.1) N·m at 70° and decreased exponentially from 70° to 90°. This resulted in M_P-corrected moment arms that were significantly larger than uncorrected moment arms at joint angles where M_P was present. Furthermore, M_P-corrected moment arms did not change with ankle angle, which was not the case for uncorrected moment arms.