ACL Rehab Exercises: Why Knee Extensions Are Essential

ACL Rehab Exercises: Why Knee Extensions Are Essential

ACL , Education

Critial components of ACL Rehab exercises

Article Overview

  • Why quadriceps strength is essential in ACL rehab
  • Why traditional “functional” strength exercises like squats are insufficient
  • Why the knee extension is one of the most important exercises in your ACL rehab program
  • Examples of exercises you can incorporate

Why Quadriceps Strength Is Important in ACL Rehab

Regardless of the injury, we take a similar approach to looking at a long term rehab program.

Step 1: What does the patient want to return to?

Step 2: Identify key performance indicators (KPIs) and requirements to accomplish that goal

Step 3: Assess the patient’s current status and identify deficits

Step 4: Build a plan to address deficits and restore KPIs to return to their activity

When assessing ACL patients and looking at the literature, the glaring deficit across the board, regardless of age, ability level, graft selection, etc. is quadriceps weakness.  From right after the injury and right after surgery, there is an immediate loss of quadriceps muscle function (largely due to a phenomena called arthrogenic muscle inhibition).

Unfortunately, the loss of quadriceps function and strength doesn’t stop there.  At 6 months post-op 65% of patients still have a significant quadriceps strength deficit and this can linger up to 9 months, a year or even longer in some cases.

Ok, so ACL injury = quadricep strength deficit

Why is that important?

I will try to briefly summarize why this is important, but if you want more detail on the specifics, I would highly recommend reading Erik Meira's blog on the topic.

In a Nutshell:

  • Quadriceps weakness impacts landing mechanics and puts the knee in a high-risk position for re-injury
  • Quadriceps weakness affects the ability to stop and change direction safely
  • Quadriceps weakness increases the likelihood of a second ACL injury
  • Quadriceps weakness increases the likelihood of knee arthritis later in life
  • The list could go on, but clearly it is important

Myth Busting

Despite all the evidence supporting the importance of quadriceps strength, and the fact that knee extensions are the most effective way to target quadricep strength deficits, historically rehab professionals have not incorporated knee extensions into rehab programs.

Wait . . . What? Why?

There are two myths that have been spread and are used as excuses and reasons to not utilize knee extensions.

Myth 1: Knee Extensions are not safe and put stress on the new ACL Graft

There were a few scientific articles in the 1980’s that suggested that the knee extension exercise would put stress on the new ACL graft and could in fact cause the graft to loosen.   This opinion became very popular and widely sited as a reason to not include knee extensions in ACL rehab.

In fact, it became so popular that it was even taught in school to new clinicians! I was taught this myth in school, and early in my career I did not incorporate knee extensions because of this reason.

However, despite its popularity and how it has become common place and accepted as fact, it isn’t true.

Research started in the 1990’s and continued now, has consistently shown knee extensions to be safe.  While there is some stress on the ACL graft, it is not any more stress than the graft experiences during normal walking.  So if you think it is safe to walk after ACL surgery, than you should also agree it is safe to incorporate knee extensions.

Myth 2: Knee Extensions Aren’t “Functional”

I could do a whole separate blog or podcast episode on the fact that “functional” is a junk term with no meaning that is overutilized in the rehab community to justify poorly loaded exercises.

In this case, the argument is athletes and people largely function in an upright, weight bearing position, therefore all of their rehab should be completed in an upright, weight bearing position.  So they should only do squats, lunges, and balance exercises.

I would definitely not disagree that athletes should rehab on their feet and complete traditional strength exercises like squats, deadlifts, lunges and single leg squats. These compound strengthening movements are heavily incorporated in all of our rehab programs; but, exclusively using these exercises is not sufficient.

Human beings are master compensators, when given a task to accomplish, they will find a way to do it despite their deficits.  If they have quadricep weakness, and they are tasked with a squat or a jump, they will utilize their other leg or a hip dominant strategy to accomplish the movement.

The ability to compensate is great, because they can do important things like walk, go up and down stairs, get up and down from a chair even with a significant quadricep strength deficit.  However, when it comes to targeting a quadriceps strength deficit, it creates problems.  It means we cannot solely utilize these exercises, because when you are recovering from ACL surgery, you will find a way to off-load your quad and avoid using it.  This can create movement pattern differences and long-standing quadriceps weakness if it is not addressed in isolation.

To Review

Knee extension strength is a common problem in ACL patients, improving it can reduce risk of injury, increase performance, decrease future risk of long-term knee issues.  It is safe and necessary to work on isolated knee extension strength.

So What's Next?

We do knee extensions!

These exercises are not exciting, they are simple to do, but must be loaded and progressed appropriately.

Here are a few examples of some of our favorite variations.







All of these exercises are just examples, they need to be incorporated at the proper time, with the right loads for each individual.  If you are recovering from surgery, we always recommend consulting with your rehab professional before including any new exercises


If you are currently recovering from an ACL injury, and are not working on knee extension strength in isolation, I encourage you to talk to your rehab provider about why.  I hope I have made a clear case on why targeting the quadriceps muscle is absolutely pivotal in your ACL rehab.  Our favorite saying (stolen from a mentor Erik Meira) is, “it is not only the quad, but it is at least the quad.”

At Kinetic Sports Medicine and Performance, we specialize in helping people recover form ACL injuries and return to their sports and activities at a high level.  If you would like to learn more details about our process, check out our ACL Rehab page here.  Also, follow us on Instagram @kinetic_smp as we are consistently sharing content related to ACL rehab.  Finally, we are always happy to discuss your situation in more detail, contact us today.