Comeback SZN: Return to Sport after ACL Reconstruction

I hate social media. Isn’t that ironic? I deleted virtually all of my personal accounts not too long ago – after deciding that I was tired of seeing posts of ‘that random kid from high school’s’ opinions on US politics or pictures of my neighbor’s sister’s husband’s cousin’s friend’s daughter’s baby’s gluten-free Waldorf salad from the Cheesecake Factory.

I will say, however, that creating the @drjhubbsy Instagram account has been pretty freaking cool. Every day, I get to interact with people from across the state, country, and globe and meet them somewhere unique in their own ACL journey. I can almost guarantee a daily “when will I be able to play sports again” direct message, comment, or mention. While I cannot offer medical advice over such a platform, I have decided to answer that question for you all here. Remember that ACL rehabilitation varies from surgeon to surgeon, so make sure you are communicating with your sports medicine team before trying any of the subsequent ideas at home.

You might recall from one of my last posts that the first few weeks of ACL rehabilitation are kind of really boring. The fun begins after you have reduced your pain/swelling, increased your range of motion, and regained neuromuscular control of your quad muscles. Return-to-run (RTR) is usually initiated somewhere around the 12-week mark, and this acts as the transition from the impairment-focused tasks in early rehabilitation (i.e. ROM, isometric strength) to the functional, sport-specific tasks that characterize more advanced rehabilitation (i.e. springing, pivoting, cutting). With return-to-run placing relatively low demands on the healing knee, many doctors and rehabilitation clinicians cite time as the only determining factor for when a patient is permitted to start running. A study by Rambaud et al. (2018) showed that, aside from time, the most frequently reported criteria for RTR are full knee range of motion or >95% of the non-injured knee plus no pain or pain <2 on visual analogue scale; isometric extensor limb symmetry index (LSI)>70% plus extensor and flexor LSI>70%; and hop test LSI>70%. While many clinicians can get away with using time in determining RTR status, return-to-sport (RTS) is and should be a little different.


When will I be able to play soccer again?

While it’s not often a conversation I cram down patient or client throats, it is possible for some to never play soccer again, or at least not to the same level of quality and/or competition. A study by Ardern et al. (2014) showed that approximately 81% of patients return to any kind of sport, 65% return to their pre-injury level of sports participation, and only 55% return to competitive sports.

Six months used to be thought to be the magic number following ACL reconstruction, with some even attempting accelerated rehabilitation in 4 months in order to get an athlete back for her senior season or [insert other extenuating circumstance here]. In recent years, extensive research has been completed in the realm of RTS following ACL reconstruction due to unacceptably high re-tear rates and risk of re-injury. Studies have found that risk ACL graft rupture ranges anywhere from 6% to 25%, whereas the risk of contralateral ACL injury ranges anywhere from 2% to 20.5%. I’ve even seen some research citing overall second ACL injury rates reaching upward of 49 percent! But fear not; The Delaware-Oslo ACL Cohort study found something pretty interesting. They found that ACL re-injury rate can be reduced by as much as 51% for every month RTS is delayed until 9 months after surgery, after which no further risk reduction is observed. With current research supporting at least 9 months of rehabilitation, it is important to note that some patients require up to two years to fully restore function and confidence in their injured knees. Aristotle once said, “Patience is bitter, but it’s fruit is sweet.” I knew the Greeks had it all figured out.


But why are re-tear rates so high? 

Perhaps the problem is two-fold, with incomplete rehabilitation and unmet return-to-sport criteria being the two easiest answers. First, limited insurance coverage often acts as a barrier to healthcare, limiting physical therapists from treating ACL patients beyond the 8 or 12-week mark. This leaves young athletes to ‘go it alone,’ so-to-speak. Many PT’s also lack the knowledge of advanced strength training and periodization, leading to less optimal care of these athletes in late-stage rehab, as well as a breakdown in communication between PT’s and sport performance professionals. As physical therapists, it is important to have good working relationships with strength coaches and other fitness professionals alike, so that we can continue to guide recovery once an patient is no longer approved additional physical therapy visits. During this stage of recovery, athletes should absolutely perfect their jumping, landing, pivoting, and cutting biomechanics with recovery of full strength and balance prior to progressing back to sport full-tilt. Instead, what often ends up happening is an athlete receives substandard care or incomplete rehabilitation due to lack of communication, leaving her back on the field with a predisposition to re-injury her newly reconstructed ACL.

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The second answer to this question honestly makes me cringe. Some medical professionals regard time as the only criteria for return-to-sport following ACL reconstruction. This is negligible from a practitioner’s standpoint and is honestly, simply unacceptable. If an athlete walks into a doctor’s office after six or nine months and immediately receives the green light without any subjective or objective measurement taking place, there is something seriously wrong. Unfortunately, nearly 79% of ACLR patients return to sport without passing any strength and/or hop test discharge criteria. It is well-accepted in the healthcare community that both time-based and functional RTS criteria be met prior to allowing a patient to resume athletic participation. The only catch is that we still do not know the optimal combination or battery of RTS tests and criteria, with different measures including isokinetic strength testing, functional hop and agility tests, clinical assessments, and related subjective questionnaires. Return-to-sport criteria is not standardized and varies considerably from clinician to clinician. (Keep reading for my plug at the end where I encourage you to invest the best five dollars you may ever spend.)

Different research has advocated for the use of different tests and in different combinations. Below you will find a pretty comprehensive list of objective and subjective RTS criteria. I’ve listed them in my order of priority, but this is just personal preference.

Strength Testing: With muscle strength deficit being associated with potential risk of future knee injury, increasing muscle strength of the quadriceps and hamstrings is a key factor for successful RTS after ACL reconstruction. Limb symmetry index (LSI) of the injured and uninjured leg, and hamstring to quadriceps (H:Q) ratio of the injured leg are the two most often measures of strength taken post ACLR. Isokinetic testing is currently the ‘gold standard’ of measuring strength after ACL reconstruction, however, many have called into question its functional relevance to sporting and training situations. Furthermore, some research has highlighted the fact that isokinetic knee strength has not sufficiently been validated as a useful criterion measure for RTS.

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With LSI, it is expected that an athlete’s involved leg be at least 90% as strong as her uninvolved leg before returning to sport. It is important to note that research has also shown LSI values to be invalid.  During the 6 to 12 months of rehabilitation, it is believed that the strength and proprioception of the uninvolved leg actually diminishes, making LSI values inflated and inaccurate. LSI values may overestimate knee function following ACLR, which is why researchers have advocated for the collection of baseline strength and functional output soon after initial ACL injury, instead of waiting months for the involved leg to heal and catch up. And what if you have torn your ‘good’ leg’s ACL before too? You might as well just throw LSI out the window. A study by Zwolski et. al (2016) stated that the use of LSI’s during strength and performance tests may not be an appropriate means of identifying residual deficits in female patients after bilateral ACLR at time of RTS. Instead, these researchers suggest the comparison of strength performance values to the normative values of healthy controls in the population. Hamstring to quadriceps ratio is generally a little bit less controversial. Clinicians often aim for the hamstrings to produce 50-80% as much torque as a patient’s quadriceps, or an H:Q ratio between 0.5 and 0.8.

You can usually find research to support or refute just about anything. While some of the above measures have been deemed ‘invalid,’ it is imperative for clinicians to gain an objective measure of quadriceps and hamstring strength prior to releasing an athlete back to sport. This is where clinical decision-making comes into play. How will you assess your athlete? Grindem et al. (2016) showed that for every 1% decrease in quadriceps strength below the 90% LSI cut-off, risk of knee re-injury increases by as much as 3%. Another study by Johnson et al. (2018) showed that on average, ACLR patients show a 28.5% decrease in strength relative to the uninvolved leg at seven-and-a-half months post-op due to diminished motoneuron recruitment or decreased motor-unit-firing frequency. Kyritsis et al. (2016) found that for every 10% decrease in the hamstring to quadriceps strength ratio, there is a 10.6x higher risk of sustaining an ACL graft rupture. Yes – you better believe I’m having my patients hit those nordic hamstrings! While much of the research harps on quad and hamstring strength, it is also important to remember gastroc, gluteal, and core strength/stability.

 Hop TestingOne-legged hop tests have been around since their creation by Dr. Frank Noyes in 1991, and have acted as a frequently-cited RTS criteria in sports medicine. This series of one-legged hops assesses an athlete’s ability to generate and attenuate force. This series includes the single-leg hop for distance, triple hop for distance, triple crossover hop for distance, and 6-meter hop for time. Again, the general rule of thumb is to achieve at least 90% LSI, but we know this logic is inherently flawed.

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Subjective Measures: There are many ways to subjectively gauge an athlete’s knee function, including but not limited to the IKDC, KOOS, Lysholm, Tegner, and Cincinnati Knee Scoring Scale. Each of the aforementioned standardized functional outcome measures have different scoring systems, however, none of these tests have proved more valid or reliable than the others. Other subjective measures that have proved prudent in RTS include the Tampa Scale of Kinesiophobia (TSK-11), Pain Catastrophizing Scale (PCS) and the ACL Return to Sport After Injury Scale (ACL-RSI). All of these scales assess an athlete’s confidence and mental readiness to return to sport. Some research has shown that athletes with a TSK-11 score of 19 or less at the time of RTS are 13x more likely to suffer a second ACL tear within 24 months. Self-reported fear of movement/reinjury after ACLR at the time of RTS has been receiving more attention as of late and is an important area to assess prior to allowing the resumption of athletic participation.

Other measures frequently implemented before return-to-sport include the Star Excursion Balance Test, the agility T-test, and fatigue-state hop testing (where an athlete performs the four aforementioned hop tests after reaching a general fatigue level of 7/10 on the VAS scale). While there is currently no standardized recipe of RTS criteria following ACLR, Kyritsis et al. (2016) found that not meeting six clinical discharge criteria before RTS is associated with a 4x greater risk of rupture. These six criteria include: isokinetic testing at 60, 180, and 300 degrees/second, single-leg hop test, triple hop test, triple crossover hop test, the agility t-test, and completion of on-field sports-specific rehabilitation.

As you can probably tell, there is not just one test that can determine whether you are ready to return to sport or not. Rehabilitating from a torn ACL is a huge investment of time, energy, and money. Wouldn’t you want to protect your investment by determining that your risk of re-injury was minimized before stepping back onto a playing field? Performing and passing as many of the tests listed above as possible is one way to mitigate your risk. Find a sports medicine clinician in your area to administer these tests and tick the boxes.

Two Melbourne-based Sports Physiotherapists by the names of Randall Cooper and Mick Hughes developed The Melbourne ACL Rehabilitation Guide just this past year. This 30+ page eBook is the most comprehensive RTS Guide I have seen to date, as it encompasses almost all of the tests mentioned above. It is designed for both people undergoing ACL rehabilitation, as well as their treating clinicians. Click here to spend those five bucks you won’t regret.

Thanks for reading, and until next time – be well.


  1. Kyritsis P, Bahr R, Landreau P, et al Likelihood of ACL graft rupture: not meeting six clinical discharge criteria before return to sport is associated with a four times greater risk of rupture Br J Sports Med 2016;50:946-951.
  2. Grindem H, Snyder-Mackler L, Moksnes H, et al. Simple decision rules can reduce reinjury risk by 84% after ACL reconstruction: the Delaware-Oslo ACL cohort study. Br J Sports Med 2016; 50: 804–808.
  3. Ardern CL Taylor NF Feller JA, et al. Fifty-five percent return to competitive sport following anterior cruciate ligament reconstruction surgery: an updated systematic review and meta-analysis including aspects of physical functioning and contextual factors. Br J Sports Med. 2014;48(21):1543-1552.
  4. Zwolski C., Schmitt L. C., Thomas S., Hewett T. E., Paterno M. V. (2016). The utility of limb symmetry indices in return-to-sport assessment in patients with bilateral anterior cruciate ligament reconstruction. Am. J. Sports Med. 44 2030–2038.
  5. Undheim MB, Cosgrave C, King E, Strike S, Marshall B, Falvey É, Franklyn-Miller A. Isokinetic muscle strength and readiness to return to sport following anterior cruciate ligament reconstruction: Is there an association? A systematic review and a protocol recommendation. Br J Sports Med. 2015;49:1305–1310.
  6. Wellsandt E Failia MS Synder-Mackler L. Limb symmetry indexes can overestimate knee function after anterior cruciate ligament injury. J Orthop Sports Ther. 2017;47:334-8.
  7. Palmier-Smith RM, Lepley LK. Quadriceps strength asymmetry after anterior cruciate ligament reconstruction alters knee joint biomechanics and functional performance at time of return to activity. Am J Sports Med 2015;43:16629.
  8.  Paterno MVSchmitt LCFord KR, et al. Biomechanical measures during landing and postural stability predict second anterior cruciate ligament injury after anterior cruciate ligament reconstruction and return to sport. Am J Sports Med 2010;38:196878.
  9. Rambaud AJM, Ardern CL, Thoreux P, et al. Criteria for return to running after anterior cruciate ligament reconstruction: a scoping review. 
  10. Müller U, Krüger-Franke M, Schmidt M, et al. Predictive parameters for return to pre-injury level of sport 6 months following anterior cruciate ligament reconstruction surgery.
  11. Macrina L. Return to play testing after ACL reconstruction. Mike Reinold Website. Accessed February 16, 2019.


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