by Tyler Tice, PT, DPT, MS, ATC


After a primary anterior cruciate ligament (ACL) reconstruction surgery, 25% of patients 25 years old or younger that return to high-risk sport sustain a second ACL injury. Currently, there is conflicting evidence surround the return to sport timeline following ACL reconstruction surgery. One study demonstrated no difference in strength and hop test performance between athletes that successfully returned to sport vs. athletes that sustained a second ACL injury. Another study demonstrated that athletes that had more symmetrical quadriceps strength and returned to sport less than 9 months after surgery had an 84% reduction in knee injury rates. Due to this, many questions regarding the protective effects of delaying return to sport and achieving symmetrical muscle function remain unanswered. The goal of this study was to investigate the association between sustaining a second ACL injury and 1) time to return to sport, 2) symmetrical muscle function, and 3) symmetrical quadriceps strength.


Patient demographics and results from 5 muscle function tests were obtained from a rehabilitation registry. Inclusion and exclusion criteria can be found in the original article. The 5 muscle function tests were the following: isometric or isokinetic knee extension, isometric or isokinetic knee flexion, single-leg vertical hop, single-leg hop for distance, and single-leg side of hop. A questionnaire was sent to athletes between the ages of 15 and 30 found on the rehabilitation registry who were involved in knee-strenuous sports prior to their injury and subsequent ACL reconstruction surgery. The questionnaire was utilized to determine time of return to knee-strenuous sport. The primary outcome for this study was whether or not the athlete sustained a subsequent ACL injury following their return to sport.


In this study, 159 athletes with a mean age of 21.5 years old were included. Of the participants, 64% were female.

Athletes who sustained a new ACL injury returned to knee-strenuous sport, on average, 10.1 ± 3.3 months after ACL reconstruction, compared with 12.7 ± 4.8 months for athletes with no new ACL injury.

Ten of the 33 athletes who returned to knee-strenuous sport earlier than 9 months after reconstructions sustained a new ACL injury.

Twelve (67%) of the second ACL injuries occurred in athletes who returned to knee-strenuous sports between 8 and 9 months after ACL reconstruction.

Athletes who returned to knee-strenuous sport at 9 months or later after surgery had a lower rate of new ACL injury compared with those who returned earlier than 9 months after ACL reconstruction.

Athletes who returned to knee-strenuous sport earlier than 9 months had an approximately 7 times higher rate of new ACL injury compared with those who returned at 9 months or later after surgery.

Achieving symmetrical muscle function in 5 tests or symmetry in quadriceps strength was not associated with new ACL injury.


Athletes who returned to sport before 9 months following ACL reconstruction surgery were 7 times more likely to experience a second ACL injury compared to athletes who returned to sport at 9 months post-surgery or later. Despite some athletes returning to less knee strenuous sport, findings in this study were similar to previous research. Additionally, there was no association between symmetrical quad strength and rate of re-injury, which contradicts previous research that has demonstrated a positive relationship between muscle function and re-injury rate. A few factors that may have affected this finding is the relatively small sample size, few athletes within the sample size having symmetrical muscle function, and different athlete populations being included in the study. An explanation that may support this finding is that all athletes in this study achieved a limb symmetry index (LSI) of 90% or greater which may have been protective against a second ACL injury, which is consistent with previous research. LSI is commonly measured in ACL rehabilitation to assess symmetry of quadriceps strength.

A key limitation of this study is that only 18 athletes included sustained a second ACL injury between both groups. This makes it difficult for in-depth analysis and assessment of multiple risk factors to be made.

Take Home Messages:

As youth athletes continues to become more competitive and the number of young athletes returning to sport following ACL reconstruction surgery, more concrete evidence regarding ideal milestones and return to sport timelines is needed to reduce re-injury rates. Over the years, research has demonstrated that delaying return to sport can benefit the athlete and reduce the likelihood of re-injury following return to sport, which this study supported as athletes who returned to sport before 9 months post-surgery were 7 times more likely to sustain a second ACL injury compared to athletes who returned after 9 months. Overall, for long-term knee health and to enable young athletes to be able to participate in their chosen sport for as long as their bodies can tolerate, delaying return to sport following a major injury like an ACL tear is more favorable compared to returning to sport earlier on in the recovery timeline.


Beischer, S., Gustavsson, L., Senorski, E. H., Karlsson, J., Thomeé, C., Samuelsson, K., & Thomeé, R. (2020). Young Athletes Who Return to Sport Before 9 Months After Anterior Cruciate Ligament Reconstruction Have a Rate of New Injury 7 Times That of Those Who Delay Return. The Journal of orthopaedic and sports physical therapy50(2), 83–90.