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358at 0º remained unchanged in moderate flexions up to 40° and thereafter increased gradually in deep flexions up to 90°, exceeding that of the rectus femoris at the flexion between 80° and 90°. The relative contribution of the PM decreased gradu-ally in flexions up to 50° and thereafter increased in deep flexions up to 90° to become the same level as the rectus femoris in the hip extension position at 90° (Figure 4).Relative rotation speed by contraction of the flexorsIn the hip extension position (0°), the relative rotation speeds of the PM and IL were 2.6- and 2.1-times swifter than that of the rectus femoris, respectively. The relative rotation speed of the PM remained constant in mild hip joint flexions up to 50°, and thereafter in deep flexions, decreased steeply up to 90°. The relative rotation speed of the IL remained constant in hip joint flexions up to 10º and decreased gradually in flexions up to 90°. Conversely, the relative rotation speed of the rectus femoris decreased slightly in mild flexions up to 40° and became approximately 90% compared with that at 0°; thereafter, it increased slightly up to 90° to become the same level as that at 0° (Figure 5).Figure 4　 The relative contribution of the iliopsoas and rectus femoris to the maximal hip flexion torque.The relative contribution of the rectus femoris to the maximal hip flexion torque was greater than that of the iliopsoas in mild flexion up to 60°, and its relative contribution decreased steeply in deep flexion. Moreover, the relative contribution of the iliopsoas to the torque increased sharply in the deepest flexions at 80° and 90°.Figure 5　 The relative rotation speed is represented by the change in flexion angle due to a 1% shortening of the muscle fiber.The change of flexion angle (Δα) of the psoas major remained constant in mild hip joint flexions up to 50° and decreased steeply up to 90°. Δα of the iliacus remained constant in hip joint flexions up to 10° and decreased gradually in flexions up to 90°. That of the rectus femoris decreased slightly in mild flexions up to 40° and thereafter increased slightly up to 90° to be at the same level as 0°.In this study, we evaluated the different action of the iliopsoas and the rectus femoris on the hip flexion by estimating their relative contribution to the maximal hip flexion torque and relative rota-tion speed, and found that the iliopsoas served as a rapid flexor, whereas the rectus femoris was a powerful flexor.The joint torque or force is produced by several agonist muscles and determined by various factors including neural activities34), muscle fiber types35), and contraction speed36). For improving perfor-mance in athletic sports, it is important to under-stand the contribution of individual agonist muscles in producing torque in the hip joint. However, to the best of our knowledge, this has not been inves-tigated properly to date. Hip joint flexion is exerted mainly by the iliopsoas and rectus femoris, and other accompanying muscles including the sarto-rius and adductor longus. The importance of the iliopsoas and rectus femoris as hip joint flexors is indicated by their larger PCSA (9.9 + 7.7 + 13.5 = 31.1 cm2) than that of the sartorius and adductor longus (1.9 + 6.5 = 8.4 cm2)26). The sartorius and adductor longus have much smaller PCSA values, and their force vectors deviate from the sagittal plane. Actually, Inai et al37). considered the iliopsoas Discussion