For Young Athletes, Returning to Sport Before 9 Months After ACL Surgery Carries a Much Higher Reinjury Rate

Young athletes are often the most eager to get back to their sport after ACL reconstruction — and, unfortunately, they are also the group at highest risk of tearing the graft or the other knee's ACL. This Swedish study looked specifically at how the timing of return to sport related to new ACL injuries in this high-risk group.

The researchers followed 159 young athletes (average age around 21.5 years, about half of them female) who had returned to knee-demanding sports after their first ACL reconstruction. Along the way, athletes completed muscle function testing, including comparisons of quadriceps strength between the surgical and non-surgical legs, and the researchers tracked who went on to suffer a second ACL injury.

The central finding gives the paper its title: athletes who returned to sport before nine months after surgery had a rate of new ACL injury approximately seven times that of athletes who delayed their return to at least nine months. That is a large difference, and it echoes earlier research from the Delaware-Oslo cohort suggesting that the months leading up to the nine-month mark are a period when the knee and the athlete may simply not be ready for the demands of cutting and pivoting sport, even when they feel good day to day.

For a young person in rehab, the message is not that nine months is a magic number that guarantees safety — it is that rushing back early carries a real, measurable cost. Biological healing of the graft, rebuilding strength that matches the other leg, and retraining sport-specific movement all take time. Treating the first nine months as a floor rather than a target, while working through a structured testing process, is the safer path this research supports.

Key findings

• The study followed 159 young athletes (mean age about 21.5 years, half female) who returned to knee-demanding sport after primary ACL reconstruction.
• Athletes who returned to sport before nine months after surgery had a rate of new ACL injury approximately seven times that of athletes who returned at nine months or later.
• The study also examined muscle function testing at return to sport, including quadriceps strength symmetry between legs.
• The findings reinforce time-based caution for young, active patients, alongside strength recovery, when planning return to sport.

Clinical relevance

It gives rehab teams concrete evidence to counsel eager young athletes on timelines, supporting a patient, criteria-plus-time approach that physiotherapy and supervised strength programming are built around.

Prepared by the Volt Health team as an educational summary of the cited publication. It is not medical advice; assessment and treatment decisions should always be made with a qualified health professional.