How To Get Your Knee Straight After ACL Surgery

It’s something you’ll hear over and over again during recovery after ACL injuries and surgery.  We’ve gotta get your knee straight.  In most cases, it is THE number one priority immediately after injury and then again after surgery. 

Getting your knee straight after ACL surgery (or more technically known as “full knee extension”) is correlated with better short term AND long term outcomes. This means that the sooner you’re able to straighten your knee, the more smoothly your recovery process will go, and the less likely you will be to have painful knee symptoms down the line.

I probably don’t have to say it (because you’ve heard it so much before), but getting the knee straight after ACL surgery is serious business.

This article will take a look at the muscles involved with getting the knee straight, the muscles that might KEEP the knee from getting straight, and an exercise that you can use to slowly but surely get all of the muscles working correctly to straighten the knee.

The primary muscle responsible for straightening the knee is the quadriceps.  In order for the knee to straighten, the quadriceps must contract.  During contraction, it must create a force that is greater than the muscles opposing the motion (which we will get to in a second), the force of gravity (which can vary with position or if weight is added), and the force of any additional resistance applied to the muscle (maybe through a machine or band).

After ACL injuries and ACL surgery, the nervous system (control system of our body) holds the quadriceps in a VERY guarded state. It keeps the quadriceps from creating much force at all, which is why it is so hard for you to straighten your knee.

There are other, smaller muscles involved with straightening the knee that are held in the same “guarded” state after ACL surgery as well.

At the same time, the nervous system also “guards” against knee extension by keeping a different set of muscles from lengthening.  Think of these muscles as the opposing team in a tug of war.  The harder they pull, the more difficult it is for the quadriceps to move the rope (or straighten the knee).

The muscle most responsible for keeping the knee from straightening is the hamstring.  This muscle may feel tight after ACL surgery, which is a normal response. A tight hamstring muscle causes the knee to bend by pulling the top of the shin backwards. Additionally, a tight calf muscle may also limit your ability to straighten the knee. It can cause the knee to bend by pulling the thigh downwards towards the ankle.

In order to straighten the knee, we’ll need to keep these opposing forces in mind.  The quadriceps must activate and create force that is greater than the opposing forces of the hamstring and calf. If the hamstring and calf are too tight, the quadriceps may not be able to overcome those forces.  Therefore, the hamstring and calf must simultaneously relax (or allow for lengthening), as the quadriceps activates.

We’ve established that getting your knee straight isn’t about just one thing.  It’s not about JUST getting the quadriceps stronger and it’s not about JUST stretching the hamstring and calf.  It’s about both. Once you’ve been cleared for weight bearing, here’s a simple exercise (demonstrated in the video above) you can use to explore the relationships between these muscles and how they affect your ability to straighten the knee.  (If you haven’t been cleared for weight bearing, I recommend you check out this article on getting the MOST out of your quad sets, which will also help you to straighten the knee after ACL surgery.)

Stand with the feet 6-8” apart, toes pointed straight forward, and chest away from the sternum. Keeping the knees as straight as possible, press your heels straight down into the ground, and rotate the hips anteriorly as far back as possible. Once you hit end range, try to straighten the knee just a bit more, and then return to the starting position. Throughout the exercise, maintain the ability to push straight down through the heels (so you aren’t sitting TOO far back on heel) and think about pulling the belly button “in.”

As you move through the range of motion, pay attention to what your body is feeling.  Do your hamstrings feel tight?  Are you having trouble finding the ability to push straight down through the heels?  Are you able to squeeze your quadriceps just a bit more to straighten the knee? Find ranges of motion that are challenging, but in which you are able to maintain good form.  Gradually build on these ranges to help the muscles restore “normal” length-tension relationships that allow the knee to straighten more naturally.