Rear Leg-derived Moment Contributes to Resistance Against Hip Extension in Bulgarian Split Squats

The Bulgarian split squat (BSS) is a unilateral exercise that emphasizes hip extension more than knee extension, compared to other squat variations. This study aimed to (1) empirically verify the existence of the rear leg-derived moment (M_RL)-a theoretically plausible but previously untested external resistive hip moment acting against the net hip extension moment (M_HE) of the front leg-and (2) examine how stance width and forward trunk-leaning angles affect M_RL during the BSS. Nine trained male participants performed bodyweight BSS under two stance conditions (wide and narrow) and three trunk-leaning conditions (additional, natural, and reduced forward lean). A motion capture system and force platforms were used to calculate M_RL, head-arm-trunk segment-derived gravitational moment (M_HAT), and M_HE. M_RL substantially contributed to the total external resistance acting against the front hip extensors, ranging from 76 to 86 Nm in the wide stance and 49 to 71 Nm in the narrow stance, accounting for 70-97% and 62-98% of the total resistance (M_HAT + M_RL), respectively. In the narrow stance, M_RL increased significantly as the trunk became more upright. The combined M_RL and M_HAT closely matched M_HE, supporting the validity of the proposed mechanical model. These findings provide the first experimental evidence of M_RL as a key resistance factor in the BSS. Moreover, M_RL may enable practitioners to increase mechanical loading on the hip extensors while maintaining a more upright trunk posture, offering a potential advantage for strength training programs aiming to target the hip extensors with minimal forward trunk inclination.