ACL Surgery Recovery

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ACL Rehab and Return to Sport

How Do I Know When It Is Safe To Return To Sport After ACL Surgery?

Coming back to activity and returning to your favorite sport following an ACL repair can certainly be a daunting task. It has likely been quite a while since you have been at your usual activity level and the knee might not feel ready. Looking at information out on the internet can also be overwhelming with a seemingly endless number of post-op protocols out there. This can certainly make it confusing to know when it is ok to do what and what are the risks associated with different activities. While no one can absolutely guarantee an ACL re-injury will not occur, at Union we follow evidence based practice and can help guide you through objective and measurable testing to see when it is safe to return to dynamic activity.

Injury Rates/Causes

With over 300,000 ACL repairs performed every year in the US it is one of the most common significant sport-related injuries, one that can sideline an individual for a significant period of time. The ACL is an important internal stabilizer of the knee and many of the injuries occur with the athlete twisting over a planted foot, performing a sudden cut or jumping and landing on an extended knee. There are a number of factors, many of which are modifiable, that can influence the likelihood or risk of ACL tear. One of the ways the ACL stabilizes the knee is by preventing the tibia (shin bone) from sliding forward. If one has a significant imbalance between the muscles on the front of the knee and the muscles on the back of the knee then some of that stability is lost. Another modifiable factor is one’s neuromuscular control. In other words, how well one can adapt and respond to quick changing surfaces, directions and speed when playing sports.

ACL return To Sport Testing

There are multiple components and tests we use when determining if an athlete is ready for full sport participation and all must be met.

1. Lack of symptoms: First, the individual must not show any remaining symptoms. The knee must not have any loss in range of motion, there cannot be residual swelling or pain before, during, or after activity

2. Time: While there is not a specific day or milestone where everything becomes clear to “go for it”, a patient must wait AT LEAST 6-9 months (more likely 9-12+) as the graft matures.

3. Strength: We use dynamometry (use of a tool to measure force output) to ensure that the rehabbing knee is strong enough to ensure stability in return to sport. The injured knee should be able to demonstrate at least 90% strength of the opposite side. Additionally we look for a good hamstring:quadricep ratio. Ideally the hamstring should produce 85% of the force that the quads can. This is important because like the ACL, the hamstrings can help prevent the shift forward of the tibia (shin bone) on the femur (thigh bone).

4. Functional Testing: Reliable, measurable tests that are used to compare and assess the injured knee relative to the uninjured limb are an important and useful tool for helping decide when it is safe to return to sport. The injured side should measure at least 90% of the uninvolved side.

○ Hop Tests: Four jump tests are used to assess dynamic ability. i. Single hop: starting on one foot, jump and land on the same foot ii. Triple hop: similar to the single hop but performed three consecutive times iii. Crossover hop: like the triple but crossing over a centerline with each jump iv. Timed Hop: jumping on one foot, measure the time it takes to cross 6m

○ Lower Quarter Y / modified star excursion balance: standing on one foot, reach out as far as you can to the front and to the side/back (like the shape of an upside down Y)

5. Outcome measures: These are questionnaires that help establish how confident and comfortable one feels on their injured limb. We often use the Lower Extremity Function Scale (LEFS) but others like Anterior Cruciate Ligament Return to Sport after Injury (ACL-RSI) are excellent as well.

Limits Of Testing

These tests are very helpful in determining readiness but they are not perfect. For one, they don’t account for in-clinic vs. real life conditions. The clinic will not have a slick patch after a rain that your soccer pitch may have or a hidden root under some leaves that may be lurking on your trail run. These unexpected events can test your knee in unexpected ways that are difficult to reproduce in the clinic. When doing the testing in the clinic the athlete will typically be “fresh” and won’t account for fatigue that develops during play and the decreased control over the knee that comes with being tired. The tests are also measuring the injured leg against the non-injured one. Over the course of rehab, because the patient has not been as active as they typically were, the opposite limb likely has become weaker as well. Therefore, any measurements are not typically a person’s “true baseline”.


Very commonly athletes are pushing to return to their sport after an ACL repair and each month can seem like an eternity. It’s very important to take note that each month of waiting to return to sport can decrease the rate of re-tearing significantly. Part of what takes so long to return to sport from this injury and surgery are the changes that are happening to the graft over time. When an ACL repair is performed the graft does not simply replace the old ACL but rather it acts as a scaffolding as the body replaces and remodels the tissue in and around the graft. Tissue turnover during this phase accounts for why the graft may be at its weakest at some time between weeks 2 and 10. This remodeling phase, in some studies, can take as long as 24 months. Full maturation doesn’t typically occur until at least 9-18 months. In short: even if your strength is great and you can jump and land on one foot, there is no shortcutting the time it takes for tissue to remodel and a return to sport too early can put you at risk for having to start the process all over.

Role of Blood Flow Restriction Training in ACL Surgery Rehabilitation

Blood flow restriction (BFR) training is an excellent tool in the strengthening and recovery process from ACL repair. In short, BFR can facilitate strengthening at much lighter loads than typically required thereby making it safer for post surgical tissues. Not only can it elicit strength gains at lighter loads, but it can also help speed that strengthening process. For a more in-depth discussion of BFR please follow this link to a past blog post:

Blood Flow Restriction Training

How Union Physical Therapy in Seattle Can Help Get You Back to Your Sport

It’s not all bad news though! Many, many people come back and are able to perform at a high level. Even a few of us here at Union have had ACL repairs and are back to doing our sport. At Union PT we are outdoor athletes ourselves. We understand the unique challenges faced by your sport or daily life. We are trained in manual therapy and movement analysis making sure you will get the most out of each visit. Not only will we address the immediate issue but we’ll also look “upstream” to see WHY you are here with that problem.


About the author

Jon Sparks, PT, DPT, CSCS is a physical therapist at Union PT in Seattle. He is experienced in treating acute and chronic industrial injuries, postoperative rehabilitation and orthopedic injuries. He enjoys staying up-to-date with evidence-based treatments. Outside the clinic Jon is thoroughly obsessed with rock climbing. When not climbing, he enjoys traveling, exploring new restaurants and snowboarding