Success After ACL Surgery: Know Your Numbers

Success After ACL Surgery: Know Your Numbers

September 01, 2023

Following ACL Reconstruction (ACLr), it is incredibly important that you re-develop your quadriceps strength on your surgical leg to both protect the repaired ACL, as well as return to your previous levels of performance in the gym or on the field.

Most physical therapists do incorporate strength testing in ACLr rehabilitation, but unfortunately, many of them rely only on manual muscle testing. Manual muscle testing is a type of “strength” testing that relies on the patient kicking into the physical therapist’s hands, with the physical therapist then assigning a 1-5 strength score.

This may seem like a quick and easy way to measure quad strength after an ACL surgery, but unfortunately, it is wildly inaccurate, and if your physical therapist relies on manual muscle testing to determine your quad strength after surgery, your risk of re-injury will go up.

At Barbell PTP, our approach involves a dynamometer, a straightforward tool enabling
accurate and objective strength testing after ACL surgery. With this tool, we can calculate two very important numbers that can guide the rehab and return-to-sport process.

These numbers are the Limb Symmetry Index (LSI) and torque-to-body weight ratio (TTBW).

The Limb Symmetry Index involves comparing injured and uninjured side strength as a percentage, typically aiming for 90-95%. However, around 80% often serves as a benchmark before introducing running and jumping drills. While LSI is crucial for evaluating strength symmetry, it alone is insufficient as an independent strength measure, as it doesn’t take into account a patient’s strength relative to their body weight.

This is where our second strength measurement comes into play, which is the torque-to-body weight ratio. This calculation evaluates how much torque (force rotating around the knee) an individual can generate from the quadriceps relative to their body weight.

The ideal torque-to-body weight ratio for ACLr rehab stands at 3 Nm/kg, roughly
corresponding to body weight. Consider the following case examples:

Athlete A is a rugby athlete 9 months into ACLr rehab with their local PT. Athlete A performed a
series of jumping tests in addition to a manual muscle test performed by their provider. Based on the results of their manual muscle test “feeling strong” on both sides, Athlete A was discharged from PT and cleared to return to Rugby. Athlete A’s rehab provider was not able to accurately measure quadriceps strength using their hands, leading to premature discharge of Athlete A who had an actual LSI of 70%.

Athlete B is a soccer player 6 months into recovering from ACLr. Athlete B found a provider
that was able to objectively test their quadriceps strength with a dynamometer and calculate
their limb symmetry index. Athlete B’s LSI was calculated at 95% and they were cleared to return to soccer practice. However, because they failed to calculate the torque to bodyweight ratio, they did not reveal the fact that both quads were weak after the surgery, as their torque to body weight ratio was 1.3 Nm/kg.

These two cases show why it’s incredibly important to calculate BOTH numbers so that you can see symmetry side-to-side, but also strength compared to bodyweight.

If you want to rehab to the highest level possible and get back to training in the safest and most effective way possible after ACL surgery, you need to make sure that you know your numbers.

If you’re looking for help after an ACL injury or ACL surgery, enter your information below in our “Ask A Specialist” area and we’ll see if we can help you out!

Prone Quad Stretch

This exercise is useful to improve your knee flexion flexibility, with a special focus on lengthening the quadriceps muscle as much as possible, as part of the quadriceps muscle attaches above the hip joint.


To perform this stretch, you’re simply going to lie on your stomach with a belt/towel/strap wrapped around your foot and held in both hands, and use your arms to pull your heel as close to your butt as you can (just like in the heel slide)


Hold this stretch for at least 60 seconds, and perform three to five sets, one to three times per day.

Heel Propped Knee Extension Stretch

This exercise is the main stretch that we use to improve knee extension range of motion, which is very important for proper walking and running mechanics.


To perform this stretch, you’re simply going to sit or lie down with your heel propped up on an object, with a weight placed on top of your knee (as is seen in the video) or suspended below the knee via a backpack or other similar object.


For this stretch, we usually have our patients start with a five minute long sustained stretch, and build up to a 10 minute long sustained stretch one to two times per day.

The next step in recovering from an ACL injury or ACL surgery is quad activation, which we are going to cover in our next blog post!


If you’re dealing with an ACL injury or surgery and are looking for expert help solving your pain and getting back to your full potential, fill out the form below to request a free phone consultation with one of our ACL rehab experts!


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