Integrated Biomechanical Analysis of Pelvis–Hip Coordination During Single-Leg Extension: Effect of Foot Contact and Upper-Limb Strategies

Hip Lock, a coaching construct for single-leg actions, comprises stance-side hip co-contraction for pelvic stabilization and free-side pelvic elevation, but how task strategies influence these components remains unclear. This study examined which combination of foot-contact and upper-limb strategies strengthens both components. Thirty healthy young men performed a single-leg extension task under four conditions combining foot-contact strategy (flat-foot vs. heel-raised contact) and upper-limb strategy (arm restriction vs. free arm use). Three-dimensional kinematics, surface electromyography, and ground reaction force were recorded, and 25 variables were analyzed using two-way repeated-measures analysis of variance with false discovery rate correction. Frontal-plane hip co-contraction was higher with flat-foot contact (η2p = 0.31) and extensor–flexor hip co-contraction was higher with free arm use (η2p = 0.31), whereas free-side pelvic elevation was greater with heel-raised contact (η2p = 0.52) and was not reduced by arm use. No interaction remained significant after correction. Pelvic elevation was greatest under heel-raised contact with free arm use (17.4 ± 4.9°). Thus, foot-contact strategy shifted the emphasis between stabilization and elevation, whereas arm use increased co-contraction without compromising elevation. These preliminary findings suggest that heel-raised contact with free arm use may serve as a candidate condition for future Hip Lock training studies.