Poor knee strength is associated with higher incidence of knee injury in adolescent female football players: The Karolinska football injury cohort.

Purpose: To investigate the association between common measures of trunk and lower extremity range of motion (ROM), strength, the results of one-leg jump tests at baseline and the incidence of subsequent substantial knee injuries in adolescent female football players.

Methods: Players were assessed at baseline regarding (1) ROM of trunk, hip, and ankle; (2) trunk, hip, and knee strength; and (3) one-leg jump tests. Players were prospectively monitored weekly for 1 year regarding knee injuries and the volume of matches and training. Hazard rate ratios (HRRs) and 95% confidence intervals (CIs) were calculated with Cox regression for the association between the baseline tests and the incidence of substantial knee injury (moderate/severe reduction in training volume or performance, or complete inability to participate in football). Exposures were categorized in tertiles (high, medium and low values). The highest tertile was used as reference.

Results: 376 players were included without substantial knee injury at baseline (mean age, 13.9 ± 1.1 years), and 71 (19%) reported at least one substantial knee injury during the follow-up. Several associations were found; the strongest was that players in the lowest tertile of knee extension strength had a higher incidence of knee injuries than players in the highest tertile (HRR, 2.28; 95% CI, 1.20-4.38). Players in the lowest tertile of trunk rotation ROM in lunge position half-kneeling (HRR, 0.50; 95% CI, 0.27-0.94) had lower incidence of knee injuries than players in the highest tertile.

Conclusions: Poor knee strength and high trunk ROM were associated with an increased incidence of substantial knee injury in adolescent female football players. Therefore, knee-strengthening exercises during season may be recommended.

Level of evidence: Level II.