A controlled intervention study assessing the relation between hip abductor strength and knee valgus
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Background: Anterior cruciate ligament (ACL) injury is a common and severe lower limb injury. Knee abduction moment has been associated with risk of non-contact ACL injury, and knee valgus angle has been reported as part of the non-contact ACL injury mechanism. Fatigued and weak hip abductors have been correlated with increased knee abduction moment and knee valgus angle. Strengthening the hip abductor muscles might play an important role in ACL injury prevention. Purpose: To prospectively assess the relation between changes hip abductor strength and knee valgus and knee abduction moment. Study Design: Controlled intervention study Methods: 31 amateur female handball players with reduced knee control (mean±SD; age, 22.3±2.7 years; height 170.0±4.8 cm; weight, 72.6±8.3 kg) were divided into intervention (n=17) and control (n=14) groups. The intervention group performed a short-duration hip abductor resistance and sensorimotor control training program 2d/w for eight weeks in combination with their team training. Hip abductor strength was measured by handheld dynamometry at baseline, mid-protocol and post-test. Lower-limb kinetics and kinematics were calculated for the counter movement jump, in-jump landing and bilateral one-legged landing at baseline and post-test. Mixed design repeated measures analysis of variance (ANOVA) was performed to compare changes between groups. Results: The intervention group did not significantly increase in hip abductor strength compared to the control group in any of the four strength tests (p>0.05). The intervention group significantly reduced their knee abduction moment compared to the control group in the take-off of the CMJ in the left (F(1,21)=4.4, p=0.05, effect size (ES)=0.20) and the right leg (F(1,21)=4.9, p=0.041, ES=0.22). Increased hip abductor strength was not related to reduced knee abduction moment and knee valgus angle when comparing players increasing in strength to players not increasing in strength in the intervention group (p>0.05). Conclusion: Hip abductor strength increase was not related to reduced knee valgus. However, the intervention group did not significantly increase hip abductor strength compared to the control group. The hypothesis that increased hip abductor strength results in reduced knee valgus cannot be refuted, but this study could support the notion that solely focusing on strengthening the hip abductors is insufficient to reduce knee valgus.