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360and rectus femoris to the maximal flexion torque, the rectus femoris plays a more important role in the shallow flexion of the hip joint during walking among the activities of daily living.Conversely, it was also shown that the relative rotation speed by muscle contraction for the PM and IL was 3.6 and 1.9 times larger than that of the rectus femoris in mild hip flexions up to 50°, respec-tively. Regarding estimation of the rotation speed by contraction of the flexors, it was clarified that the relative rotation speed of flexion differs between the iliopsoas and rectus femoris muscles under the specific condition that each muscle has the same muscle contraction speed. It would be reasonable to argue that the iliopsoas is particularly important for sprinters, contributing significantly to leg swing speed. In addition, weak hip flection muscles pose a risk for falling, and the main cause of injury in people with weak hip flexion muscles is stumbling or falling while climbing stairs. Strengthening the iliopsoas makes it possible to lift the leg instantly, thereby preventing falls when climbing stairs or stepping over obstacles. This study has some limitations. We did not measure the sarcomere length in each muscle; therefore, the FL for calculating the PCSA was not normalized by the ratio of the optimal and measured sarcomere length. Further refinements of the PCSA by normalizing the FL would be beneficial for achieving a more precise estimation of the rela-tive contribution to the maximal torque and the relative rotation speed. In addition, muscle fibers include slow-twitch fibers (Type I), which have a slow contraction rate and are less likely to fatigue, and fast-twitch fibers (Type II), which have a high contraction rate and are prone to fatigue. In human muscle, slow- and fast-twitch fibers are present in different ratios. Lieber24) indicated that the differ-ences in muscle fiber types have little effect on exercise performance, although there are differ-ences in the maximum contraction rates of Type I and Type II fibers. In the present study, the effect of different muscle fiber types on relative rotation speed was not considered. Further improvements that also consider the histological factors of the muscle fibers would be beneficial for achieving a more precise estimation of the relative contribution of each fiber type to the relative rotation speed. In conclusion, the present study clarified the functional characteristics of the iliopsoas and rectus femoris based on the morphometric data from skel-etal anatomical specimens and applied the results to the simple movement of hip flexion on a theoret-ical basis to provide a better understanding of the functional contribution of muscles experimentally to the actual movements of the hip joint. The results of the present study would provide useful information for the functional training of athletes and rehabilitation programs in the clinic. Future developments of this research technique would clarify the functional contribution of the individual muscles in complex joint movements.The authors would like to thank Research Prof. Emiko Senba (Department of Physical Therapy, Osaka Yukioka College of Health Science, Osaka, Japan) for cooperating in the anatomical research and Shimpei Sato (Acuity Inc., Tokyo, Japan) for their technical assistance in the measurement of three-dimensional reference values of a singular point and calculation of the muscle moment arm length.The authors received no financial support for the research.All authors have made substantial contributions to the manuscript. The details are as follows: TK: Conceptualization, Methodology, Formal analysis, Investigation, Data curation, Visualization, Project administration, Writing - original draft, Writing - review & editingTT: Resources, Writing - review & editing, SupervisionTN: Resources, Writing - review & editing, SupervisionTS: Conceptualization, Methodology, Data cura-tion, Visualization, Writing - review & editing, SupervisionAll authors approved the final version of the manuscript to be submitted.The authors declare that there are no conflicts of interest. AcknowledgmentsFundingAuthor contributionsConflicts of interest statement