Effects of Lower Extremity Alignment During Barbell Squat Training on Muscle Volumes

Chiu, LZF, and Fry, AC. Effects of lower extremity alignment during barbell squat training on muscle volumes. J Strength Cond Res 39(10): 1053-1061, 2025-Squat training increases monoarticular hip and knee extensor muscle size. Biomechanical studies demonstrate that hip muscle demands are different for squats performed with "knees in" vs. "knees out" tibiofemoral alignment. To: (a) determine whether these differences in hip muscle demands influence hip extensor muscle size adaptations and (b) examine the safety of these squat variations, training with "knees in" vs. "knees out" squats was compared. Eleven healthy women ("knees in": n = 6) completed 6 weeks' barbell squat training. Fat-free muscle volumes were reconstructed from magnetic resonance imaging, and self-reported knee health was assessed using the Knee Injury and Osteoarthritis Outcome Score (KOOS). Both groups increased lower-body strength ( p < 0.001) and vasti muscles' volumes ( p < 0.02). However, no muscles with a hip extensor action increased in size ( p > 0.05). Surprisingly, gluteus maximus volume decreased in the "knees in" group ( p = 0.045). Smaller gluteus maximus may indicate muscle atrophy or water loss concomitant to glycogen depletion. As the former indicates reduced and the latter increased muscle loading, "knees in" squat seems to affect gluteus maximus loading, although the direction of this change is unclear. The 5 KOOS outcomes were not different between groups, nor did they change over time ( p > 0.05). Both squat variations are tolerable for novice women trainees, and a longer training intervention may be required to establish differential hip extensor muscle adaptations between variations.