Redefining muscular action: human "adductor" magnus is designed to act primarily for hip "extension" rather than adduction in living young individuals.

Human leg muscles are uniquely enlarged for upright bipedalism, and the adductor magnus is one of the largest muscles. Although this muscle is recognized as a hip adductor, hip adduction torque is not greatly required during human locomotion, such as walking and running. The functional role of this giant muscle remains unclear. Here, we tested the hypothesis that the human adductor magnus acts primarily for hip extension rather than adduction in living young individuals. Utilizing diffusion tensor imaging, we reconstructed fascicles over the entire muscle in 15 young adults at the hip neutral position. We divided the muscle into three portions based on fascicle insertion and examined their three-dimensional architectures. The posterior and anterior-distal portions comprised over 80% of the whole muscle volume and physiological cross-sectional area. These portions demonstrated a longer moment arm for hip extension than adduction. Consequently, the potential torque (maximal torque-generating capacity) of the whole muscle was over twofold greater for hip extension than adduction. The hip extension potential torque was correlated with the maximal hip extension torque measured with a dynamometer. These results highlight the architectural design of the adductor magnus, favoring hip extension over adduction, providing novel insights into its functional role beyond the frontal plane in human locomotor mechanics.NEW & NOTEWORTHY The human adductor magnus, one of the largest leg muscles, is traditionally considered a hip "adductor." However, its functional role is unclear. We found that the torque-generating capacity of this muscle for hip extension was substantially greater than that for hip adduction and explained the exerted torque for hip extension. Our findings highlight the important role of the adductor magnus as a major hip "extensor," having implications for mechanisms of human locomotion and musculoskeletal simulations.