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θ(°)6.3±2.17.9±2.66.5±2.0　Psoas majorIliacusRectus femorisMuscle mass(g)81.4±47.484.1±47.190.4±52.8FL(cm)13.76±1.19**11.10±1.74**8.04±1.23**gradually in deep flexions up to 90°. The MAL of PM-I was slightly larger than that of PM-II between 60° and 90° and exhibited a similar pattern of change during hip flexions. The MAL of IL-I and IL-II remained mostly constant up to 10° and 30°, and thereafter in deep flexions, increased steeply up to 90°, exceeding that of the rectus femoris at the flexion between 40° and 60°. The MAL of IL-Ⅲ and IL-Ⅳ remained constant up to 50° and 60°, and increased steeply up to 90°, exceeding that of the rectus femoris at the flexion between 70° and 80° (Figure 3-A).The MAL of the PM remained unchanged in moderate flexions up to 50° and thereafter increased steeply in deep flexions, exceeding that of the rectus A. MAL of the iliopsoas (PM-I・II, IL- I -Ⅳ) and rectus femoris estimated in different hip joint angles.The MAL of the rectus femoris increased gradually in mild flexions to reach a maximal value at 40° and decreased gradually in deep flexions up to 90°. The MAL of PM-I was slightly larger than that of PM-II between 60° and 90° and exhibited a similar pattern of change during hip flexions. The MAL of IL-I and IL-II almost remained constant up to 10° and 30°, and thereafter in deep flexions, increased steeply up to 90°, exceeding that of the rectus femoris at the flexion between 40° and 60°. The MAL of IL-Ⅲ and IL-Ⅳ remained constant up to 50° and 60°, increased steeply up to 90°, exceeding that of the rectus femoris at the flexion between 70° and 80°.B. MAL of the iliopsoas and rectus femoris estimated in different hip joint angles.The MAL of the rectus femoris was much larger than that of the iliopsoas in the hip extension position (0°). It increased gradually in mild flexion up to 40° and then decreased gradually. The MAL of the psoas major and iliacus remained unchanged in moderate flexions up to10° and 60°, and thereafter increased in deep flexions up to 90°, exceeding that of the rectus femoris at the flexion between 60° and 70°.θ, pennation angle; FL, muscle fiber length; PCSA, physiological cross-sectional area**P<0.01, *P<0.05Table 1　Parameters of the mechanical architecture of the iliopsoas and rectus femorisPCSA(cm2)5.45±2.85*6.91±2.9810.88±6.00*Figure 3　Moment arm length (MAL) under different hip joint angles on the sagittal plane.femoris at the flexion between 60° and 70°. Conversely, the MAL of the IL increased gradually in deep flex-ions, exceeding that of the rectus femoris at the flexion between 60° and 70° (Figure 3-B).Relative contribution of individual hip flexors to the maximal hip flexion torqueThe relative contribution of the rectus femoris in the hip extension position at 0° (57.3%) exceeded that of the iliopsoas and increased gradually in flex-ions to an almost maximal value at 20°-40° (60.4%-62.3%); thereafter, it decreased drastically in deep flexions up to 90°, falling behind that of the iliopsoas at a flexion of approximately 60°-70°. The relative contribution of the IL in the hip extension position 357