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Toshimasa KUMAZAKI1), Tomihisa TAKAHASHI2), Takashi NAKANO3), Tatsuo SAKAI4, 5)2)Department of Anatomy, Nihon University School of Dentistry, Tokyo, Japan3)Department of Anatomy, School of Medicine, Aichi Medical University, Aichi, Japan4)Department of Physical Therapy, Faculty of Health Science, Juntendo University, Tokyo, Japan5)Department of Anatomy and Life Structure, Juntendo University Graduate School of Medicine, Tokyo, Japan352Corresponding author: Toshimasa KumazakiDepartment of Health and Sports Management, Osaka University of Health and Sport Sciences1-1, Asashirodai, Kumatori-cho, Sennan-Gun, Osaka 590-0496, JapanTEL: +81-72-479-3201　E-mail: tkumaza@ouhs.ac.jpCorresponding author: Tatsuo SakaiDepartment of Physical Therapy, Faculty of Health Science, Juntendo University2-1-1 Hongo, Bunkyo-ku, Tokyo 113-8421, JapanTEL: +81-3-5802-0421　E-mail: tatsuo@juntendo.ac.jpDepartment of Anatomy and Life Structure, Juntendo University Graduate School of Medicine2-1-1 Hongo, Bunkyo-ku, Tokyo 113-8421, JapanTEL: +81-3-5802-1022/1023〔Received　Feb. 25, 2022〕〔Accepted　May. 6, 2022〕J-STAGE Advance published date: Jun. 20, 2022Copyright © 2022 The Juntendo Medical Society. This is an open access article distributed under the terms of Creative Commons Attribution License (CC BY), which permits unrestricted use, distribution, and reproduction in any medium, provided the original source is properly credited. doi: 10.14789/jmj.JMJ22-0009-OAObjectives: To evaluate the difference in action between the iliopsoas and rectus femoris muscles in hip flexion by estimating the relative contribution to the maximal hip flexion torque and relative rotation speed.Materials: We examined 22 lower limbs of 10 male and 12 female formaldehyde-fixed adult Japanese cadavers.Methods: Using morphometric data from cadaver dissections, we calculated the moment arm length and physiological cross-sectional area for each muscle. We considered moment arm length and physiological cross-sectional area as indices of the maximal torque and compared them among the muscles at various hip joint angles. To evaluate the relative rotation speed, we calculated the increase of the hip joint angle for a 1% reduction of the muscle fiber length in each muscle.Results: The rectus femoris contributed approximately 2/3 to the flexion torque in mild flexion up to 60°, whereas the iliopsoas contribution increased sharply beyond 60°. The relative iliopsoas rotation speed was 2.5- to 3-times higher than that of the rectus femoris in mild flexion up to 60° under the specific condition that each muscle had the same muscle contraction speed.Conclusions: We found that the iliopsoas served as a rapid flexor, while the rectus femoris was a powerful flexor.Key words: hip joint, iliopsoas, rectus femoris, physiological cross-sectional area, torqueJuntendo Medical Journal2022. 68(4), 352-362Original ArticlesAction and Contribution of the Iliopsoas and Rectus Femoris as IntroductionAmong the activities of daily living, standing up, as well as walking, constitute the most critical and fundamental exercises1-2). In these exercises, flexion and extension of the hip and knee joints are essen-tial and critical motor elements3). The extension of 1)Department of Health and Sports Management, Osaka University of Health and Sport Sciences, Osaka, Japanthe hip and knee joints plays an important role during chair-based standing and sitting. Various studies have analyzed the biomechanics of these extension activities using force plates1, 4), motion analysis2, 4, 5), and electromyography (EMG)1, 2, 4, 6). The walking and running exercises involve flexion of the hip and knee joints. Based on their observa-Hip Flexor Agonists Examined with Anatomical Analysis