Fatigue Increases Dynamic Knee Valgus in Youth Athletes: Results From a Field-Based Drop-Jump Test.

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PURPOSE: To determine whether fatigue increases dynamic knee valgus in adolescent athletes, as measured after a standardized exercise protocol and video-based drop-jump test. A secondary aim was to determine whether individual risk factors place certain athletes at increased risk for dynamic knee valgus.

METHODS: Athletes aged 14 to 18 years were recruited for this video analysis study. Athletes were recorded performing a standard drop-jump to assess dynamic valgus. Participants then completed a standardized exercise protocol. Fatigue was quantified using a maximum vertical jump, which was compared with pre-exercise values. The drop-jump was repeated postexercise. All drop-jump recordings were randomized and scored for dynamic valgus by 11 blinded reviewers. Univariate analysis was performed to identify characteristics that predisposed athletes to increased dynamic valgus.

RESULTS: Eighty-five (47 female, 38 male) athletes with an average age of 15.4 years were included in this study. Forty-nine percent of athletes demonstrated an increase in dynamic valgus determined by drop-jump assessment after exercise. A significantly greater percentage of athletes were graded "medium or high risk" in jumps recorded after the exercise protocol (68%) as compared with before the exercise protocol (44%; P < .01). Female athletes (P < .01) and those older than 15 years of age (P < .01) were the most affected by fatigue.

CONCLUSIONS: In conclusion, our study found that exercise increases dynamic knee valgus in youth athletes. Female athletes and those older than 15 years of age were most significantly affected by exercise. Greater fatigue levels were found to correlate with an increase in dynamic knee valgus, which may place athletes at greater anterior cruciate ligament injury risk. The field-based exercise drop-jump test is a low-cost and reproducible screening tool to identify at-risk athletes who could possibly benefit from anterior cruciate ligament injury-prevention strategies.

LEVEL OF EVIDENCE: III, Comparative trial.

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