Purpose of this study was to estimate the moment arm of human Achilles tendon by application of gravity. Eight male subjects were requested to perform passively plantar-and dorsiflexion movements by a dynamometer with two different leg positions, i.e. one was the horizontal position of leg and thigh with the knee joint fully extended (setA), and the other was the supine horizontal position with 60° leg up (setB). Excursion of myotendinous junction (MTJ) was determined by ultrasonography during passive ankle movement. Moment arm was calculated by dividing the MTJ excursion (mm) by the angle change (rad). At the plantarflexed position (60°), the moment arm in the setB (35.1 ± 5.6 mm) was significantly higher (p < 0.05) than that in the setA (26.0 ± 10.0 mm). The moment arms obtained in the setB were within a range of reference data reported in previous studies, suggesting that the application of gravitation is useful to estimate Achilles tendon moment arm.
