Influence of stance width and foot rotation on muscle activity and ground reaction forces during squats in female lacrosse players

Female lacrosse players are highly susceptible to lower extremity injuries, particularly non-contact knee injuries. Squatting is a fundamental training and rehabilitation exercise, yet the influence of stance width and foot rotation on joint mechanics and muscle recruitment in this population remains unclear. This study aimed to examine the effects of stance width and foot rotation on lower limb range of motion (ROM), surface electromyographic (sEMG) activity, and ground reaction forces (GRFs) during bodyweight squats in female lacrosse players. Ten Japanese university-level female lacrosse players performed squats under six stance conditions (narrow, shoulder-width, wide × parallel or external rotation). sEMG was recorded from ten lower-limb muscles, synchronized with 3D kinematics and bilateral force plates. Friedman tests with false discovery rate (FDR) correction evaluated stance-related differences, with effect sizes estimated using Kendall’s W. Stance significantly influenced hip (χ² = 33.31, p < 0.001, W = 0.67), knee (χ² = 19.94, p = 0.001, W = 0.40), and ankle (χ² = 24.23, p < 0.001, W = 0.49) ROM. Wide external rotation (WidER) yielded the greatest hip ROM (116.9° ± 7.9°), whereas narrow parallel stance (NarPar) produced the smallest (98.2° ± 4.8°). During the descending phase, gluteus maximus activation was significantly higher in wide stances compared with narrow and shoulder-width conditions (q < 0.013). GRFs showed consistent vertical peaks across stances (~56–62 %BW), but mediolateral peaks were substantially higher in wide external rotation (~17 %BW) than in narrow stances (~5–7 %BW). Squat stance width and foot rotation meaningfully alter joint ROM, gluteus maximus activation, and mediolateral loading in female lacrosse players. Wide external-rotation squats appear to enhance hip ROM and gluteus maximus recruitment, potentially supporting knee stability, while narrow stances increase ankle dorsiflexion demands relevant to deceleration and defensive postures. These findings support individualized stance and foot-rotation prescriptions in lacrosse-specific training and injury-prevention programs.