Muscle Interventions After ACLR

So, I see a lot of athletes after ACL reconstruction (ACLR). I also often share my thoughts and opinions on rehab after ACLR. You could even attend one of my courses on the subject (shameless plug). That said, I have a lot of questions about what we are exactly doing here. I’m not saying that anything I write here is valid or correct (small chance of that) – I’m just thinking out loud here.

Let’s start with some things we know.

  • Those who have undergone ACLR tend to have more muscle dysfunction on the involved limb than controls at time of return to play – specifically in the quad.
  • Those who have better restoration of muscle function when they return to sport – specifically the quad – have lower rates of second ACL injury
  • Those who wait until 9 months out from surgery before they return to sport have significantly lower rates of second ACL injury than those who return earlier.

So this is how we as a profession respond to this information:

  • Work hard to restore quad function as quickly as possible.
  • Fight to avoid atrophy so we can have a quicker recovery.
  • No matter what, wait 9 months before return to play.

So we hit them with NMES and blood flow restriction (BFR) and isolated quad loading. We hold them out at least 9 months even if their quad function looks really good. There are other things as well but this is the kernel that I want to talk about.

Now I’m not questioning this approach. I would actually say that if you used this approach, you probably would have pretty close to optimal outcomes. To be clear: I would highly recommend that you use this approach.

But I personally don’t use this approach. Not because I think it is wrong but because I question the thought process.

“But Erik, you talk about ‘It’s the quads until it’s not the quads’ and advocate for targeted, isolated loading of the quads!”

Yes, I do. But my thought process is more nuanced than that. Let’s ask some questions…

Why do we have quad dysfunction after ACLR?

Easy question. We have quad dysfunction because of descending corticospinal and spinal reflex pathway excitability changes. There are also changes in the activations of cortical brain areas. This disuse also leads to muscle atrophy which ultimately results in decreased capacity for the muscle to do work.

No. That’s HOW. I’m asking WHY do we have quad dysfunction after ACLR? Dumb it down for me.

Oh. That is most commonly described as arthrogenic muscle inhibition (AMI).


Feedback from the injured joint environment inhibits the muscle from contracting fully as a complex cascade of nervous system responses.

Dumber for the folks in the back…

The knee says “I haz problems so I will make the quady-quad go sleepy-sleep.”

Thank you. So WHY does the body have this process?

In order to protect itself during the healing process. Task-related activities that put a high load to the quad tend to put a high load to the knee. I’m talking about activities like jumping, hopping, deceleration, etc. Shutting down the quad is a simple way of temporarily protecting the knee from those loads.

So instead of seeing AMI as the cause of the issue, AMI is the symptom. The symptom of a joint that isn’t happy. We need to treat the cause.

“Thanks Einstein, I already address swelling and pain and all of that.”

No. I mean yes, but no. Swelling and pain aren’t the only ways that a knee joint will not be happy after ACLR. If it were, simply getting a “quiet knee” would result in rapid restoration of quad function. We don’t see that here, especially when we are months out. (SIDE NOTE: Obviously strive for a quiet knee quickly after surgery!)

I know you could say that the downtime results in atrophy and it takes a while to get that back. Well, the knee gets “quiet” pretty quickly, usually within a few weeks. But the restoration of the quad takes months, sometimes years, sometimes never. It has to be more than just pain and swelling driving that inhibition.

What about load tolerance of the chondral surfaces themselves? What about healing of the graft fixation? What about maturation of the graft itself? All of these take time and those times are variable from patient to patient.

But that is just talking about tissue healing/conditioning rates. What about the fact that there is now a graft in an ever so slightly different spot than the original ACL? How long for that knee to get used to that thing before it says, “I no longer haz problem”?

Of course, there are also psychosocial issues that can be at play driving nervous system responses – but this post will be long enough without getting into that directly.

“Yeah. That’s why we wait 9 months.”

[Drums fingers on the table] I’m not a fan of a fixed time point as a “ready/not ready” designation. I would assume that there is a bell curve distribution here meaning some people are ready well before that and some people need a lot more than 9 months. We do see this distribution of tissue healing rates specifically so why wouldn’t it also apply to “I haz problems”?

So I think we come back to quad function. Quad function is our real indicator here. That and the ACL-RSI.

“So if I find a way to make them confident and make their quad come back faster, then I can return them to play faster?”

Ok. That’s the important question that I want to answer here; and this answer is the whole point of this blogpost. The answer is, in my opinion, unequivocally…


“I really, really hate you.”

Yeah, get in line. Let me clarify a bit. I believe that returning to play with quad dysfunction is likely causal for a lot of the aberrant movement we see after ACLR and therefore second injuries – read this here.

BUT (notice the size of that ‘but’) I believe that the quad dysfunction we see at that point is caused by two things OUTSIDE THE MUSCLE.

  1. Having someone practice functional tasks like jumping, hopping, cutting, etc before they have full return of quad function. They learn how to “functional” without it. Now, unless you are doing a ton of leg extensions, they don’t have any real load going into the quad to restore it. All of this creates a movement problem as well that correlates to second injury. There is another post for that, but here is the thought in a single sentence:

If you find a way to trick the system into thinking it is restored when it is not, you have a false sense of function.

But I’m here today to talk about…

  1. Having some kind of continued process that’s driving inhibition. A continued perception of “I haz problems” if you will. It’s probably not swelling because you would’ve noticed that. It’s probably something else…

What else is it? I could make a bunch of guesses (joint surfaces, getting used to graft, fear, etc) but honestly, I have no idea. But something isn’t ready. I do believe that it presents as muscle inhibition and a lack of confidence. And you can probably use quad function and the ACL-RSI as a way to track it. Unless you screw those up.

“How can I screw those up?”

Back to the other example. If you have someone get better at functional tasks without a quad you have not addressed the problem, you have hacked the task to go around the problem. You likely gave them a false sense of confidence as well.

I imagine that the same may be true for aggressively trying to restore quad as fast as possible by hacking the muscle inhibition. Essentially tricking the quad to generate more output than the muscle inhibition is “recommending” if you will. I worry that things like blind attempts to go around or hack muscle inhibition might be doing this. So, like my other statement:

If you find a way to trick the system into thinking it is restored when it is not, you have a false sense of function.

I guess I just want everything to come along together. I don’t want a super strong quad with a bunch of other things (including brain) that can’t tolerate the same loads. I worry about an artificially achieved amount of quad output which may no longer be an accurate representation of true quad function.

“So what should we do? NOT address muscle inhibition?”

No, I’m not saying that. And I’m not saying we shouldn’t use things like NMES and BFR – they can be great tools. What I’m saying is that quad function is not the goal; it is the measure. Think less about, “How can I override this system and make it do what I want it to do?” Think more about, “How can I nudge this system in the direction that I want it to go?”

It is more of a conversation. Like this:

Knee: I haz problems so no quady-quad thanks you hello

PT: Like, no quads at all?

Knee: No quad ok thanks you hello

PT: How about you push into this? [isometric in mid-range into dynamometer for feedback]

Knee: Ok yes some quad thank you

PT: Wow! You got a little higher on that one! Last time was 15ft-lbs now you’re at 22ft-lbs

Knee: Ah yes, I can see that I can push 22ft-lbs through my extensor mechanism without a problem

PT: Let’s try for 25ft-lbs

Knee: [Tries] No, at 25ft-lbs I can confirm that something is not right I will hang out here at 22ft-lbs for now

PT: That’s cool

Aaaaaaaand scene!

You will notice that the system is directly reckoning with load and getting feedback about that load. It is perhaps letting the system test its need for muscle inhibition instead of trying to find a way around the muscle inhibition or arbitrarily tricking the muscle inhibition into turning off. Maybe it is just engaging the nervous system on its own terms which may have a psychosocial effect as well? I have no idea. I have no study to refer to here.

In my experience (always makes me cringe) some people take more time than others. I’ve had people that were fully restored in 4.5 months (no thanks to me I’m sure). I’ve had others that required close to 2 years and still others that don’t return at all. Most fall between 8 and 10 months. Anecdotally most people I work with get to 80% limb symmetry pretty quickly and then they “stall” for a couple months with a feeling of “something being off”. Then one day it just seems to figure itself out and goes up to 90+% and they feel great.

And don’t get me wrong, of course you need to train for muscle adaptation with adequate loading principles, but it is more complicated than that.

A different perspective

I feel like a lot of PTs have a view that says:

I do interventions on muscle function so that I can intentionally make them ready to return to sport faster.

My view is one that says:

I do interventions on muscle function so that, when they are ready to return to sport, they do not try to return with residual muscle dysfunction.

The intervention may be exactly the same, but the approach is different.

In summary…

  • It’s the quad until it’s not the quad…but why is it the quad?
  • “I haz problems so I will make the quady-quad go sleepy-sleep.”
  • If you find a way to trick the system into thinking it is restored when it is not, you have a false sense of function.
  • Quad function is not the goal; it is the measure.
  • Nudge, don’t demand.