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Asymmetry and Thigh Muscle Coactivity in Fatigued Anterior Cruciate Ligament–Reconstructed Elite Skiers

Purpose The acute effects of fatigue on functional interlimb asymmetry and quadriceps/hamstring muscle activity levels, including preparatory coactivation during squat jump takeoff and landing, were evaluated in elite alpine ski racers with/without anterior cruciate ligament reconstruction (ACLR). Methods Twenty-two elite ski racers (ACLR, n = 11; control, n = 11) performed an 80-s repeated squat jump test (jump test) on a dual force plate system with simultaneous EMG recordings in vastus lateralis, vastus medialis, semitendinosus, and biceps femoris. Asymmetry index (AI) and jump height of body center of mass (HBCM) were calculated from the ground reaction force. The normalized EMG amplitudes were obtained at takeoff, at the 25-ms interval prelanding, and at postlanding for the ACLR limb (affected limb), contralateral limb, and limbs of the control subjects (control limb). Results Jump height decreased with fatigue for both groups, and ACLR skiers demonstrated elevated AI in the late takeoff phase versus the early takeoff and landing phases (P < 0.0001). No fatigue-induced changes in AI were found. The affected limb of ACLR skiers showed lower normalized quadriceps EMG activity at takeoff, prelanding, and postlanding along with increased hamstring activity prelanding and postlanding compared with the contralateral limb and control limb (P < 0.001). The affected limb, contralateral limb, and control limb all demonstrated increased quadriceps and decreased hamstring activity with fatigue (P < 0.001). Conclusions Functional AI values were not changed with fatigue, and the affected limb of ACLR skiers who successfully returned to sport demonstrated more hamstring dominant landings compared with the contralateral limb and uninjured control limbs. Skiers with/without ACLR demonstrated more quadriceps dominant landings with fatigue.