Background The additional effect of anterior cruciate ligament (ACL) reconstruction on muscle strength and physical performance after a structured exercise programme is not well understood.

Objectives To investigate and compare muscle strength and physical performance test results after a structured exercise programme, in young active adults with acute ACL injury, between those treated with and without ACL reconstruction (ACLR) and to evaluate these test results as predictors of clinical outcomes 2 and 5 years after injury.

Study design Prospective cohort study.

Methods In a treatment randomised controlled trial of acute ACL injury (the KANON-study), 87/121 young active adults underwent two muscle strength tests and five physical performance tests after a structured exercise programme (median 37 (IQR 24) weeks after injury). Results were presented and compared as limb symmetry indices (LSI); endpoints in predictive analyses were having a delayed ACLR over the first 5 years and self-reported knee function (Knee injury and Osteoarthritis Outcome Score; KOOS₄) at 2 and 5 years.

Results Overall, 74–95% of patients had LSI≥90% in the individual tests, with no difference between treatment groups (p=0.08–0.92). Results of the one-leg rise tests predicted KOOS₄ at 2 and 5 years (R²=0.25 and 0.24, p=0.001 and 0.002) and vertical hop results predicted having a delayed ACLR over a 5-year course after injury (p=0.048) in those starting with exercise alone (n=21).

Conclusions After an acute ACL tear, the majority of young active adults regain physical performance and muscle strength after a structured exercise programme, with or without surgical reconstruction. Poor physical performance at the end of rehabilitation predicted worse patient-reported outcomes at 2 and 5 years regardless of treatment.

Registration number: ISRCTN84752559.