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tions of larger hip flexor muscles in top sprinter athletes, Ema et al7). reported that hip flexion could play an important role in track and field sprinting. Biomechanical analysis results of these flexion activ-ities have been reported in studies employing EMG of the iliopsoas and rectus femoris8) or combining EMG and hip flexion torque measurement9).Joint movements are exerted in general by multiple muscles. During knee extension, the four muscles of the quadriceps femoris play the role of the main agonist. During hip extension, the gluteus maximus and three muscles of the hamstrings play the role of main agonists, and the adductor maximus acts as a synergist. During hip flexion, the iliopsoas and rectus femoris are the main agonists, and the sartorius and adductor longus act as synergists. Dynamometers measure the total torque produced by multiple agonists and synergists. However, it cannot discriminate the muscle strength of indi-vidual muscles. The EMG estimates the change of activities in individual muscles during joint move-ments. However, it cannot predict the relative muscle strength among the agonists and syner-gists. The EMG activities of the superficially located rectus femoris have been reported in many studies7, 10-17), and those of the deeply located ilio-psoas have also been reported in some studies8, 18-23).The morphometric parameters of the skeletal muscles, such as the muscle fiber length (FL) and physiological cross-sectional area (PCSA), are well known to affect the muscle function, including the contraction speed, maximum muscle strength, and the effective contractible range24, 25). Indeed, the agonists and synergists of the hip and knee joints reportedly have different architectural parameters and functional characteristics26), suggesting that the multiple flexors and extensors of the hip and knee joints have varying contributions to the torque and speed during joint movement. However, the contribution of the individual muscles was not hitherto revealed by the previous studies employing dynamometers, EMG, and other physiological methods.The iliopsoas (psoas major and iliacus), rectus femoris, sartorius, and adductor longus are known as the hip flexor muscles. The iliopsoas and rectus femoris are considered agonist muscles among the hip flexor muscles as they have greater PCSA values26). In comparison, the sartorius and adductor longus have much smaller PCSA values, and their force vector is deviated from the sagittal plane, so they are thought to have only minor contributions compared with the iliopsoas and rectus femoris. In the present study, we determined the relative contribution of the iliopsoas and rectus femoris to the maximal hip flexion torque based on morpho-logical methods by estimating PCSA and moment arm length (MAL) values in the sagittal plane. Additionally, based on the morphometric data of the skeletal anatomical specimens, we estimated the flexion speed of individual muscles by calcu-lating the degree of flexion produced by 1% of shortening of the hip flexor muscles. The data provided by the present methods were not rele-vant to estimate the actual torque and speed of the joint movements but revealed the maximal and relative contributions of the individual muscles on a theoretical basis.MaterialsWe used 22 lower limbs of 10 male and 12 female formaldehyde-fixed adult Japanese cadavers with no apparent degeneration in the hip and knee joints that were used in the dissection course at the Nihon University School of Dentistry during the 2014 and 2016 academic years. For morphometry and estimation of the PCSA of the iliopsoas and rectus femoris, 12 specimens from 6 male and 6 female cadavers (age at death, 78.6 ± 10.6 years) were used, and for measurement of the MAL, 10 specimens from 4 male and 6 female cadavers (age at death, 79.6 ± 6.8 years) were used. The body donors gave written informed consent for the dona-tion of their tissues for research and teaching purposes. The study was carried out following all relevant guidelines and regulations and was approved by the ethical committee of the Nihon University School of Dentistry (EP14D009, EP16D017).Procedures/protocolMorphometry and calculation of PCSA in isolated muscle specimensIn the 12 lower limbs, the iliopsoas and rectus femoris were dissected out to prepare the isolated muscle specimens as stated previously27, 28).In the isolated muscle specimens, the mass (M), FL, and pennation angle (θ) were measured. The 353Materials and Methods