The Influence of Hamstrings Fatigue on Knee Biomechanics During a Drop Vertical Jump

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Anterior cruciate ligament (ACL) injuries are more prevalent in females vs. males, and are associated with significant morbidity as a result of prolonged symptoms. One mechanism thought to underlie this gender disparity is relative weakness of the hamstrings muscles, particularly the medial hamstrings, which are ACL protective. Neuromuscular fatigue may exacerbate hamstrings weakness, resulting in increases in injury risk factors such as valgus knee angles and moments during landing, cutting, and pivoting.

PURPOSE: To examine the influence of hamstrings fatigue on female knee biomechanics.

METHODS: Twelve female athletes performed six drop vertical jump (DVJ) tasks before and after a fatiguing protocol utilizing a glute-ham bench. Knee kinematics and kinetics were recorded during the contact phase of the DVJ, including knee abduction angle, knee abduction moment, knee flexion angle, and vertical ground reaction force (vGRF). These parameters were grouped based on leg dominance and analyzed by paired t-tests.

RESULTS: Between baseline and the fatigued state, there was a significant decrease in the dominant limb knee abduction angle (but not in the non-dominant limb) across the contact phase (initial contact: 10.5 ± 5.3° vs. 6.4 ± 3.2°; toe-off: 10.1 ± 4.8° vs. 5.9 ± 1.6°; maximum: 13.5 ± 5.0° vs. 9.1 ± 2.2°; peak vGRF: 8.3 ± 4.6° vs. 4.7 ± 2.6°; for each P ≤ 0.05). Knee flexion angle did not differ between baseline and fatigue in either limb. Knee abduction moment in the non-dominant limb was significantly lower with fatigue at toe-off (0.2 ± 0.2 vs. 0.1 ± 0.3 Nm/Kg), but at no other point during contact. Knee abduction moment was not affected in the dominant limb. Peak vGRF was significantly lower with fatigue compared to baseline in both limbs (dominant: 1.5 ± 0.3 vs. 1.2 ± 0.2 x BW; non-dominant: 1.4 ± 0.3 vs. 1.1 ± 0.2 x BW).

CONCLUSION: The results were unexpected: post-fatigue knee abduction angle and moment were (partially) reduced and in theory, less risky. This may relate to the glute-ham exercises preferentially fatiguing biceps femoris, which has a valgus moment arm, vs. the medial hamstrings which have varus moment arms. If the protocol led to greater relative medial hamstrings force production, it may have reduced knee abduction angle via a varus net hamstrings moment. Further research is required to test this explanation.