Background The role of muscle strength for the risk of knee injury is not well investigated.

Objective To examine the potential relationship between lower extremity (LE) muscle strength and the risk of traumatic knee injury in young female and male athletes.

Design Case-control study.

Setting Athletes at senior high school sport programs in Sweden.

Patients (or Participants) 226 athletes (43% females) aged 15–19 years participating in ice-hockey, alpine skiing, handball, soccer, basketball or floorball were included. Those with any previous injury were excluded.

Interventions (or Assessment of Risk Factors) A 1-repetition maximum (1-RM) barbell squat test, adjusted for body weight, was assessed by a well-trained instructor at each school, according to “Fysprofilen™”. The strength variable was dichotomized, according to the median, to analyse “weak” vs “strong” athletes.

Main Outcome Measurements Any traumatic knee injury (ACL or other ligament injury, meniscal, cartilage damage, fracture) (yes/no) sustained during the high school years was reported by the athletes on a web-based questionnaire.

Results 115 traumatic knee injuries were reported. The majority (83%) of injuries were in the weak group in females (p=0.0001), while there were no differences between weak and strong groups in males (p=0.589). Females with weak LE strength had 13 times higher risk of sustaining a knee injury compared with strong females (OR, 13.07; 95% CI, 3.53–48.44; p=0.0001). 36 ACL injuries were reported, with a higher prevalence among females (18%) compared with males (5%) (p=0.002). Females in the weak group had 7 times higher risk of sustaining an ACL injury compared with the strong group (OR, 7.64; 95% CI, 1.60–36.52; P=0.011). There were no differences between the weak and strong groups in the male athletes (p=0.348).

Conclusions Weak LE muscle strength appears to be a risk factor for sustaining a traumatic knee injury in youth female athletes but not in males.