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and rectus femoris as the hip flexors in a computer simulation study investigating the dynamics of hip flexors during sit-to-stand movements. A study on iliopsoas function reported that the iliopsoas was activated during increased hip flexion when the subjects moved from the standing position (0°; a fully extended position of the hip and knee joints) to one-leg flexion of the hip joint (90°)18). Further-more, the contribution ratio of the four hip flexors (the iliopsoas, rectus femoris, tensor fasciae latae, and sartorius) to the flexion torque changes depending on the posture during the measurement of activities in these muscles9). However, the contri-bution ratios of the muscles to the flexion torque were not clarified in these studies. The relative contribution of the individual flexors to the joint torque could not be experimentally obtained either by measurement of joint torque or by EMG of the relevant muscles. The relative contribution of the individual flexors to the maximal joint torque could be estimated on a theoretical basis by calculating the indicator of maximal joint torque as the product of the PCSA and the MAL for the individual muscles. It is well known that the joint torque exerted by a muscle is the product of muscle tension and the MAL and that the PCSA is an index of the maximal exertion tension of the muscle38). To date, the PCSA and MAL have been measured separately in different studies. In this study, we measured both of them with cadaveric dissection for the first time, thus providing novel findings. The PCSA was calculated from morpho-metric parameters, such as the muscle volume, FL, and pennation angle, which could be accurately measured in the isolated muscle specimens26, 30-32).The PCSA of a given muscle varied between the present and previous studies, especially because of the large individual variation in the muscle mass. Psoas majorIliacus　Rectus femorisPCSA, physiological cross-sectional area; CV, standard deviation divided by average.The PCSA exhibited a significant individual variation, which is represented by the large value of the CV. However, the ratio of the PCSA was almost stable among individuals, which is indicated by the small value of the CV. Table 2　The PCSA of the psoas major, iliopsoas, and rectus femoris, and its ratio to the total amountPCSAAverage ± standard deviations(cm2)5.45±2.856.91±2.9810.88±6.00CV0.520.430.55In the present study, we calculated the ratio of the PCSA among the three muscles and its coefficient of variation (CV; standard deviation divided by the average) and found that the CV for the ratio of PCSA was quite small in contrast to the large CV for the PCSA, supporting the reliability of the esti-mation of the relative contribution of each muscle to the maximal flexion torque (Table 2).The MAL of the iliopsoas and rectus femoris and its change under different hip joint angles have been calculated39) using a method based on an interactive musculoskeletal modeling software and the lower limb model of Delp et al40). Their results based on model simulation were in good agreement with the results found for the skeletal specimens in the present study. The small and stable value for the iliopsoas in mild flexions up to 60° could be explained by the bend of the insertion tendon on the femoral head, and the steep increase in deep flexions could be the result of its distance from the head. The MAL of the rectus femoris was maximal at a hip flexion of 40°, where the two vectors from the origin (anterior inferior iliac spine) to the center of the hip joint and the insertion (upper edge of the patella) crossed vertically.The present study revealed the main agonists of hip joint flexion as two functionally important parameters for the first time: the relative contribu-tion to the maximal flexion torque and the relative rotation speed by muscle contraction. From these parameters, different functional properties of the iliopsoas and rectus femoris became apparent. We also showed that the rectus femoris was a more powerful flexor than the iliopsoas in mild hip flexions up to 60° and a less powerful flexor in deep hip flexions, which is in agreement with the larger activities of the iliopsoas in deep hip flexions18). Regarding the relative contribution of the iliopsoas Ratio of PCSAAverage ± standard deviations(%)23.58±4.4530.74±5.8845.68±8.10CV0.190.190.18359