Episode 17 | The Most Important Muscle in ACL Rehab

What’s up ACL athletes, and welcome back to another episode. Today, we are talking about the most important muscle in ACL rehab. But before we get there, I wanted to talk about this recent interview I had with Ibrahim Ugradar. He came onto the podcast, and we recorded the episode.

He’s a semi-professional soccer player in England. And this episode was just a deep dive into his ACL story, having to deal with Covid, and how that was a setback for his recovery. Some of the mental battles that he faced, which we dove really deep into. And then just some advice that he had about his recovery. He’s about five to six months out now.

I think that this could be incredibly valuable for anyone who is going through this process, and if you’re coaching people through it as well. It’s just so humbling to see an athlete get after it. But he doesn’t shy away from some of the hardships that he had during this time and still that he faces today. This will be coming out next week, so keep your eyes peeled on this episode. 

But now back to the important part of this episode, the most important muscle in ACL rehab. I put out a poll on Instagram that asked: What is the most important muscle in ACL rehab? And one choice was hamstrings, the other was quads. And for those of you who are not familiar with those terms. The quadriceps, “quads,” as we call it a lot of times, is the front thigh muscle. And the hamstrings are the back thigh muscles. What we ended up getting was a 20% vote for the hamstrings and an 80% vote for the quads, which made me incredibly happy. I know I have a lot of clinician friends as well. Hopefully, that did end up pulling that number over. But there’s still people who think that the hamstrings are important. So that’s why this podcast episode is going to touch on this because we need to know what is the most important so we can prioritize.

Now, the main argument that you’ll hear is that the hamstrings are the primary antagonist to anterior tibial translation. And what I mean by that is that the ACL itself helps protect your bottom bone, your shin bone, from moving forward on your high up bone, your thighbone. Just imagine that bottom bone just moving forward; that’s anterior tibial translation. We call it the tibia.

Now, the ACL is strained during that motion, and there are some other movements combined that cause it to tear whenever it does. But usually, the anterior tibial translation is a big component of it. And the hamstring, because if you think about the place where it attaches to the back of the knee, if you imagine that back muscle contracted, it helps to prevent or reduce that tibial translation. And while the hamstrings are important, they are not the most important. And this is based on the science and research of ACL rehab and reinjury rates.

And we’re going to actually dive into some of this stuff. But the quadriceps are the most important muscle in ACL rehab. It’s one of the components of the quiet knee that I’ve talked about in previous episodes. And a lot of times, what people will say is that they’re fearful of stressing the graft. Sometimes it can almost be avoided. And so let’s strengthen our posterior chain, or our hamstrings and glutes, to be able to minimize stress on that graft. But this actually does not need to be a concern, and it kind of needs to stop more than anything. We need to load the quads as early as we can and as safely as we can. And hopefully, this episode will shed some more light on it. But let’s kind of get this out of the way. This is not and/or; it’s not the hamstrings or the quadriceps; it’s an and. 

One of my mentors used to say “and not or.” I’m saying a lot of these words here. But it essentially means that all of these things are important. The hamstrings are important, the quadriceps are important, your calves are important, your glutes, adductors, abductors, tons of muscles – they’re all important. Now, if we’re about to rank these from, let’s say, 1, 2, 3, 4, number one is always going to be the quads. Number two, hamstrings, glutes, all of these other muscles are going to fall into place. But hopefully, after this episode, you’ll know that the quads are the most important muscle. And if the training is not focused on that in all the other muscle groups, then no matter what else you do, it’s going to fall short. 

We’re going to break this down into some of the research because I want you to hear about this, not just from an anecdotal point of view or just, “Oh, this is what people have seen.” But this needs to be grounded in the research. Because that is what’s going to tell us, first and foremost, how to guide our clinical decisions and also what you’re just going to be doing in rehab. That’s how we know we’re doing things right. And it’s the body of that research, not just one specific article. We’re going to dive into some practical reasons. Because I always want you guys to take away practical stuff from this. It’s really important to be able to know, “Okay, I’m done with this podcast. Now, what do I do moving forward?” Whether that’s talking to my physical therapist or even if you’re trying to take this on your own, what are some ways that I can start implementing and thinking about this instead of just following this protocol or just what my PT says?

I want to start this first part of this podcast off by saying, “Sure, I’ve learned some things from my own experiences that are valuable, whether it’s from my own two ACL surgeries and recoveries.” Or, is it treating a bunch of other athletes who have gone through this process themselves? But I just want to stress that I’m not sitting here recreating the wheel in any way, and all this stuff is just my own experience. I am learning from other people, and I am really just standing on the shoulders of giants who have come before me, who have made these concepts easier, who have done so much of the research, who I think don’t get enough credit which helps guide us in this process. So that’s important to consider as we move forward. 

And one of the people that I want to mention is Eric Mira, who is an incredibly brilliant clinician. He’s a big thought leader in the sports and science realm of physical therapy. And one of the things that he talks about – and he says, “It’s the quads until it’s not the quads.” And I couldn’t agree more with a statement like this. Now, let’s see what some of the research says about quad strengthening in ACL rehab. Buckle up, guys. We’re about to go through some stats and some information. And if you take the collective body of ACL research, the most commonly reported in research impairment is a significant decrease in quad strength from early on to years later. There are so many studies out there that have reported this finding, so we’re going to go through some right now. 

The big one is by Lepley (2015). And it states overwhelming evidence links quadricep strength with essential outcomes. Normal walking and running gait function, self-reported success, return to sport, a subsequent knee injury, and long-term progression of knee osteoarthritis. This one bucket is a lot of these things and looks at the relationship. And it essentially finds a strong relationship between all of these different areas and quadriceps strength. 

Toole et al. (2017), less than half of athletes meet quadriceps strength symmetry goals when they’re cleared to return to sport after ACL reconstruction. Gringdam? (2016), reported that 33% of players who returned to sports with a limb symmetry index of less than 90%. We’re talking about taking that injured side and comparing it to the uninjured side and seeing if that’s within 90%. Those people who did less than 90% sustained another knee injury over the next two years upon their return to sport. That’s pretty big—a third of the people did. Whereas only 12% sustained another knee injury if they were above that 90% threshold, and that is looking at that quad symmetry index.

The author of that same study also found that for every 1% of that limb symmetry index, less than 90% equated to a 3% increase in the risk of further knee injury over the two years period. To make that practical, if an athlete chooses to return to sport with a limb symmetry index of 80%, it carries a 30% increased risk of future knee reinjury compared to someone who waited to return to sport with a 90%. You take that 10% and multiply it by that 3% for every 1% increase; that’s where you get that 30% increased risk. I know there’s a lot of math going on in here, but bear with me. I want you to be able to kind of see these different points, not only from the function but return to sport to risk of reinjury. These are all important things to factor in. 

Next up is Schmitt (2012), quad strength was a good predictor of function and performance on the hop test. More specifically, patients with less than 85% quad strength index demonstrated decreased function and poor performance on hop tests. The thing that’s important to note with this one is that it’s the same regardless of graft type, the presence of any meniscus injury, or any other knee pain or symptoms. Greater than 80% quad strength after ACLR is associated with less severe patellar cartilage damage with short-term followup.

Wang (2015), if you’re looking at something like landing mechanics, where you go to jump and you land, and the relationship between weaker quads showed more asymmetry, and that was studied by Schmitt in 2015. How about we look at maybe a different patient population, maybe ones who are not even dealing with ACL-related issues but are knee-specific? Quad strength is a major determinant of both performance-based and self-reported physical function in patients with knee osteoarthritis. 

Maly (2006), Misner (2005), after total knee arthroplasty, quadriceps strength was the most highly-correlated impairment with functional performance. Are you guys seeing a trend here? All right, a few more. A study by Meredith Chaput – shout out to Meredith—from this year, actually. Quadriceps strength influences the patient’s function more than the single-leg hop forward test during the late stages of ACL rehab, which was a really cool study. They saw that there was a relationship between quad strength and functional outcome measures from a patient than a hop test, which we’re starting to see. Strength is starting to become a very prominent figure and role in this.

Ithurburn (2018), young athletes after ACL reconstruction with quadriceps strength asymmetry at the time of return to sport demonstrated decreased knee function one year later. So basically, if they had an asymmetry from side to side in their quad strength, their knee function a year later was not great. And then let’s do one related to running. Iwame (2021) this year as well, which is really cool. And this is one of the things that I use in my return to sport, especially for return to running. Quadriceps strength related to body weight ratio is a significant indicator for initiating jogging after ACL reconstruction. They looked at the quadricep strength, the body weight ratio and used that as a significant indicator to initiate jogging. They looked at roughly three months for people. And that cutoff value was 1.45 Newton meters per kilogram. And those were the people who had a very good outcome—to be able to run without a reoccurrence of a contralateral injury by 10 months after the injury.

What do you guys think? Is quadriceps strength important? To put this all in summary, decreased quad strength is a great predictor of decreased function in many different ways after ACL reconstruction. We looked at this from – not only just your quality of life, from knee osteoarthritis to landing mechanics to running to just being able to return to sport. There are all of these different factors that we look at. And quadriceps strength is at the base of a lot of these things. This podcast episode is focused on showing you why it’s the most important muscle in ACL rehab. If quad strength is so important and well-researched at this point, then rehab programs have to have a very big emphasis on regaining quad strength. But you know what every clinician will be like, “Yup, I do it. It’s out there.” But the research shows us that we’re not really hitting these marks that we need to. 

And guys, I hate to say it, but quad sets and straight-leg raises are not going to get you there. They’re just not. One of the most obvious ways to get after this is actually doing knee extensions. And we’ll talk about this a little bit later in this practical piece. But knee extensions are very valuable, as well as other loading movements, parameters, and exercises that are going to be important to put into your ACL rehab process. I can’t stress knee extensions enough, but this is going to be something that’s going to be very dependent on your situation. But if you’re a clinician, please do not be afraid of them. Just know how to use that tool really well. And if you’re an ACL athlete, you need to have this in your program and ask questions if it’s not there. 

Now, let’s talk about this a little more practically. The quadriceps weakness is there; it happens. People have it all the time. It’s one of the toughest things about ACL rehab. And in this process, I just talked to you about the research. Now, let’s see how this actually translates. If we’re looking at the base of a pyramid, that is the most important. The root of all this is the strength of that quadricep and that knee extension. Strengthening out that leg and that front thigh muscle—how strong is it? And also the speed of how quickly it can create that force.

Before we get into those nitty-gritty details, I want to talk through how this cascade can affect things. If we’re looking at a weakness of the quadriceps, that can lead to bad mechanics. So that can lead to, maybe, landing in an awkward way; maybe you can’t control; maybe you can’t slow down; maybe single-leg type stuff doesn’t feel great. Because you’re just not strong enough to handle those stressors that you’re putting on it. It can lead to compensation, and it can lead to fear, honestly, of a reinjury from something happening. People see weight shifting all the time, where we might shift over to that leg that is not injured, that is stronger. I see that all the time.

I’ve consulted on a bunch of cases where someone is strong but they have difficulty with mechanics. I questioned this a little bit because it can happen. But usually, it ends up because of a lack of quad strength, and they suck at it. 

We need to put some objectivity to it and actually test those numbers and see, okay, where is that quad strength at? And let’s just say it does check out. Then we have to look at the speed of which that strength comes. We talked about this. It’s called the rate of force development (RFD). And that’s how quickly you can produce that force or that torque per unit of time. How quickly can we get that muscle to get to that high peak point per unit of time? Consider these thoughts about strength and speed. If you’re weak and can get there fast, that’s not helpful because you’re getting to a very weak point very fast. So that’s not really helpful for you.

If you’re strong but can’t get there fast, then you can’t really use all of it when you need to. Because if that timing is needed in a shorter period of time, you can’t get to that max strength that you need. So that’s not helpful either. So that’s where you need to focus first, on getting really strong and then adding speed to that. Because, as we just talked about, getting speed first isn’t going to do much if you’re not strong.

Now, once you get strong, the development of speed is very important, especially as an athlete. With multiple tears, you see the quads never get back in the first place. That’s where other compensations and other fears can come in. These are just things to kind of think about. It’s more of a trend that the quads were never emphasized in the first place.

Let’s talk about biomechanics compensations. So picture this: you’re doing a single-leg broad jump where you kind of hop forward. And let’s say we’re doing this on a single leg. I’ve torn my right ACL, and I’m rehabbing. We’re working on jumping forward, and I jump from my right leg by itself onto the right leg and landing. Now, let’s just think about watching this from the side, and let’s see what my body does. Whenever I go to jump, do I land where it looks like there might be plywood in front of my shin, and I just shift my butt back because my shin is very vertical. I just shifted my butt back. We call that a hip-dominant strategy, where we’re trying to load more into our glutes, hips and hamstrings instead of our quads. And I see this all the time. I’ll ask people to even do a squat. And what will happen is, instead of lying, their knees will go slightly forward and their butt will go back where they do a normal squat. They will keep their shin or their tibia very vertical, and they’ll shift their butt way, way back. And that just turns on different muscles, more so your glutes and your hamstrings than your quadriceps.

On the flip side of this, if that person lands in that jump and their foot stays flat, their knees go over their toes and their butt goes back. But it’s more of an even distribution. Then we’re seeing this more as like, “Okay, this is how someone should land.” And they’re actually loading up their quads, as opposed to just shifting their butt back and having that vertical shin. This is just something qualitative that I will look at from the side with athletes and see, are they loading into their quads or into their glutes? If they’re shifting their butt back, then first we need to look at – is it a quad weakness. And again, going back to the earlier points, if that’s not a quad weakness, then maybe we need to train that. Because that muscle can get to that need of force and speed as quickly as possible.

Now let’s talk about some practical exercises because I want you to understand this. And again, this is going to be very specific to you and where you’re at. Don’t just go off and do all these things without some clear guidance from a medical professional, especially if you’re post-op. But you want to think about it in terms of getting strong, and you want to think about it in terms of getting really fast. And the other piece that I would add to this is the contraction type. We have three different contractions within a muscle. The eccentric piece, which is the lengthening of a muscle; the isometric where there’s tension but there’s no change in the length; or the concentric where it’s actually the shortening of the muscle. 

Let’s think about this in terms of a knee extension. If my knee is at 90 degrees and I’m in a knee extension machine, or maybe it’s anchored to a band. If I go to kick my leg out, that is the concentric piece of a knee extension. Then there’s an isometric piece. You could stop at any angle. Let’s say we just stop at the top and hold it. That means there’s no change in link, but that muscle is still engaged and contracted. And then if we slowly lower that leg down, you still feel that muscle working. And that’s the lengthening piece, which we call eccentric. All those components are important. And being able to train those different pieces is important, depending on where you’re at in your ACL rehab process and recovery. Some of those things might be emphasized more, like isometrics early, and then eccentrics when you’re trying to learn load absorption and concentric, maybe when you’re trying to get faster and adding speed. But these are just some things to think about as you’re getting more into the practical components.

For some exercises, the first one that you always start with is quad sets. And they can ramp up to split squats, step up squats. Knee extension is something that I stressed earlier that everyone should be doing in some sort of weight or form, whether it is isometric starting out. And then after they get out, especially after 12 weeks, they’re 100% doing knee extensions for the remainder of the recovery process. There’s not a single ACL athlete that I have that is not doing knee extensions. They will always be doing that because there’s really not any other exercise that I know of.

And that I have talked to other friends and clinicians about, they can isolate your quad muscle like a knee extension. If we’re thinking about more speed and that rate of force development, we’re starting to look into power movements such as plyometrics, where we are trying to create more speed, Olympic lifts, other different movements that we’re adding velocity to, to be able to work on that rate of force development. And then again, this depends on you, what your goals are, your case, maybe you have some other restrictions. It’s going to be important to make sure you’re working with somebody who can help you and guide you along with this process all the way to the end. 

In review, the quads are the most important muscle in ACL rehab. The hamstrings are still important, but they’re not as important as the quads. But that doesn’t mean that we’re not going to train all of these different muscles as you’re recovering. Research supports this based on walking, running, return to sport, subsequent knee injuries, long-term progression of knee osteoarthritis, and quality of life. The list goes on. Strength and rate of creating that strength/force is important. You need that strength first, and then we need to work on getting that to go quickly. Bad biomechanics and compensations can come from this. It’s just something to understand. It’s one of those things where you can hear clinicians say, “Biomechanics don’t matter.” But I disagree completely. 

Now, there’s a sweet spot with everything. It’s not going to be the end-all-be-all of everything. But it will help from a performance standpoint in reducing the risk of injury later. So that does not need to get missed. And especially if that is coming from a weak quad that needs to be addressed. And then we talked about different exercises. This is literally a drop in the bucket of what needs to be done. But these are just some things to think about that I will incorporate into programs with my athletes, depending on where they’re at. 

All right, guys, that’s it. This was a long one, but I think that this needed to be said. Because if you’re listening to this, I think that there needs to be enough education and understanding of why the quadriceps are so important in ACL rehab. And this wasn’t even an exhaustive of all the things. But I hope that this helps show you why it is.

If you have any questions, any feedback, if you want to talk more about this, please reach out to me. Reach out via email at ravi@theaclathlete.com or follow me on Instagram. Reach out to me at ravipatel.dpt, and let’s connect. All right, team. That’s it for today. As always, thank you guys so much for the support and so much for listening. Who wants to listen to me rant about quads for almost 30 minutes? I guess you do so I appreciate you. I’m getting out of here. Thank you guys so much. See you in the next episode.