Strength Setbacks: The Impact of Youth Sport-Related Knee Joint Injuries on Thigh Muscle Strength. A 24-Month Prospective Cohort Study

<b>OBJECTIVE:</b> To compare injured and uninjured limb knee extensor and flexor peak torque between youth who experienced a sport-related, traumatic knee joint injury and comparable uninjured youth, at baseline (≤4 months of injury) and semiannually for 2 years. Differences by injury type and sex were also explored. <b>DESIGN:</b> Prospective cohort study. <b>METHODS:</b> Bilateral knee extensor and flexor concentric isokinetic peak torque at 90° per second was assessed semiannually in 186 youth (106 injured, 80 controls) for 2 years. Between-group differences in strength over time were estimated with generalized estimating equations (95% confidence interval [CI]). Confounding was controlled using inverse probability weighting. Strength differences between those with anterior cruciate ligament (ACL) tears and those with non-ACL tear injuries as well as between male and female participants were explored. <b>RESULTS:</b> Compared to uninjured controls, injured limb knee extensor strength was lowest at baseline (-37.1 Nm; 95% CI, -45.3 to -28.9) and 6-month follow-up (-13.3 Nm; 95% CI, -20.4 to -6.2), with minimal strength gain beyond the 12-month follow-up (1.7 Nm; 95% CI, -14.3 to 17.6). Knee flexor strength of the injured limb was lowest at baseline (-24.6 Nm; 95% CI, -31.5 to -17.8), and there was minimal strength gain beyond 6 months (2.3 Nm; 95% CI, -7.7 to 12.3). The average residual deficit was similar to the knee extensors (10% to 11%) at 24 months. Exploratory analysis suggested no difference based on injury type or sex. <b>CONCLUSION:</b> Injured limb knee extensor and flexor weakness was present after different youth sport-related knee joint injuries. Strength deficits peaked early after injury, improved over time, and plateaued after 12 months, with lingering deficits at 24 months. Thigh muscle strength trajectory was similar across injury types and sex. <i>J Orthop Sports Phys Ther 2025;55(2):1-11. Epub 20 December 2024. doi:10.2519/jospt.2024.12663</i>.