The Effect of Fatigue on Hip, Knee, and Ankle Kinetics During Rear Foot-Elevated Split Squats

Rear foot-elevated split squats (RFESS) are a widely used movement to increase lower body strength, but little is known about how fatigue alters joint torque demands across the ankle, knee, and hip. Previous research has shown that fatigue changes kinematics during RFESS, increasing hip and knee motion while reducing ankle movement, but the effect on joint torques remains unclear. PURPOSE: To examine the effects of fatigue on ankle, knee, and hip torque in the sagittal and frontal planes during the eccentric and concentric phases of the RFESS in active females aged 18–25. METHODS: Participants performed repeated body weight RFESS until reaching a fatigue threshold defined by cadence and squat depth. Body landmark data was collected with an 8-camera Vicon motion capture system and ground reaction forces were collected with a Kistler force plate. Joint torques were computed with Nexus software. The effect of fatigue was determined by repeated-measures ANOVA comparing pre-, mid- and post-fatigue repetitions (p < 0.05). RESULTS: During the eccentric phase, in the sagittal plane, no significant differences were observed at the ankle, while knee torque decreased significantly post-fatigue (p < 0.001) and hip torque increased significantly across all fatigue levels (p < 0.001). In the frontal plane, ankle torque showed no significant differences, whereas knee torque decreased significantly post-fatigue (p = 0.002) and hip torque decreased significantly across all fatigue levels (p < 0.001). During the concentric phase, in the sagittal plane, ankle torque increased significantly post-fatigue compared to pre-fatigue (p = 0.020), knee torque progressively decreased across all fatigue levels (p < 0.001), and hip torque increased significantly from both pre- and mid-fatigue to post-fatigue (p < 0.001). In the frontal plane during the concentric phase, ankle torque increased significantly post-fatigue (p = 0.006), knee torque decreased significantly post-fatigue (p < 0.001), and hip torque decreased significantly across all fatigue levels (p < 0.001). CONCLUSIONS: These findings suggest that fatigue created a redistribution of torque demands across joints. The knee appears to be most vulnerable to fatigue, while the hip compensated by increasing sagittal plane torque but decreased frontal plane contributions.