We’ve all seen it: the acute ACL patient who drags their leg into the clinic, unable to squeeze their quad to save their life. We call it Arthrogenic Muscle Inhibition (AMI), and for years, we’ve treated it as a stubborn, long-term enemy. But a fascinating new study from the American Journal of Sports Medicine suggests we might be overcomplicating it.
Researchers analyzed 300 acute ACL patients and found that while AMI is incredibly common (affecting over half of patients), it’s also surprisingly fragile. In fact, they found that nearly 80% of cases could be fully reversed in a single session with simple exercises. In this episode, we dive into the ‘Red Flags’ that predict quad shutdown—including one common sleeping habit you need to ban immediately—and how to flip the switch back on for your patients.
Good timing on this podcast episode. I’m going to be hosting a free webinar with Dan Pope and Kevin Coughlin on the 3 biggest mistakes we see after ACL reconstruction. Join us on May 4th at 8:00 PM EST:
To view more episodes, subscribe, and ask your questions, go to mikereinold.com/askmikereinold.
#AskMikeReinold Episode 385: Risk Factors of Arthrogenic Muscle Inhibition
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Show Notes
• Incidence of and Risk Factors for Arthrogenic Muscle Inhibition in Acute Anterior Cruciate Ligament Injuries: A Cross-Sectional Study and Analysis of Associated Factors From the SANTI Study Group
• Online Evaluation and Treatment of the Knee Course
Transcript
Mike Reinold:
Welcome back everybody to the latest episode of the Ask Mike Reinold Show. We are here at Champion PT and Performance, answering your questions. Anything you want to talk about, head to mikereinold.com, click on that podcast link and ask away. Today, we have another journal article review, and Brendan Gates is going to take the lead on this one. I thought this was a great article. I really liked this. There’s a couple cool things I thought in this, but just a quick introduction to exactly the title and everything so I get this right. And the link will be down in the show notes. The Incidence of Risk Factors for Arthrogenic Muscle Inhibition in Acute ACL Injuries: Cross-Sectional Study and Analysis of Associated Factors. And this was published in AJSM in 2024. And if you notice in these reviews, we’re trying to pick some really good influential articles that are changing our practice, but also from reputable journals, kind of keep that in mind.
So Brendan, what’d you think of this article? What are your thoughts?
Brendan Gates:
Yeah, I really liked this article. I think they did a really good job of planning some things that I think we see clinically quite often, but quantified it really well. I think before we move into a little bit more of the review, just wanted to quickly touch on what is arthrogenic muscular inhibition. I’m going to try to see if I can put this together in a relatively simple way. But to my understanding, AMI or arthrogenic muscular inhibition is a protective neural response to a knee joint injury where there’s altered sensory input from the knee, which then drives inhibitory signaling at both the spinal and the cortical levels, so brain and spinal cord. And this shuts down the quad activation and it promotes the hamstring flexor reflex, which then in turn leads to a loss of knee extension, so a neural block rather than a mechanical or structural block.
But as we look through this, in the paper, what they sought out to do is they said, “We know AMI is common after knee injury, we know it’s associated with quad atrophy, weakness, knee extension loss, but then other things like stiffness, problems with gait, scar tissue, and some more.” But what they said they didn’t know was how common it actually is in clients, and who is at the most risk, and whether they can clinically detect and reverse it. So what they did is they used this new AMI classification system, the Sonnery-Cottet AMI Classification, and they set out to basically see how often it occurs, how reliable the classification was, and to look at risk factors, or maybe better described as red flags, that are associated with AMI. So what they did was they took 300 ACL injured patients that were within six weeks of their injury, and they were evaluated at the clinic, and they were given patient-reported outcome measures, a physical exam, and then they had their AMI graded by two independent surgeons.
We can look through it a little bit more specifically, but in short, the grades go from grade 0, which is the quad fires normally and the knee straightens fully, all the way to the other side of the coin, which is grade 3, which is a chronic fixed loss of extension that does not resolve without surgery. And then, in between those two bookends, there were four grades that were varying severities of reversible AMI. And so what did they find? They essentially found that out of those 300 patients, 170 of them presented with some severity of AMI, none in the grade 3 level there. So these were all reversible. That’s 56% of all the ACL patients that they looked at. Out of those 170, 135 of those patients were able to reverse their AMI in just one visit using some simple exercises. They used some hamstring fatiguing exercises and then followed by quad reactivation exercises, and they were able to combat their AMI in that first session, that’s 79%.
The other 35 had to use other methods like biofeedback and motor imagery, things like that, using a little bit more time and a more structured approach, but they were able to reverse it. So this proved that AMI is common, but it’s not fixed damage. It’s neural inhibition in these cases. They also found that the AMI patients had significantly worse outcome measure scores, so they felt and they functioned worse than people who did not have AMI. And then they identified some risk factors, or what they eventually termed red flags, more likely, that were associated with AMI. So they found that if you had swelling, if you had high reports of pain, if you used crutches, if there was a pillow under the knee, if it was a multi-ligamentous injury or if the duration between when they had their injury and when they were seen for the first time was a short period of time, any of those, you were two to three times more likely to have AMI present. And again, these aren’t causes of AMI, they’re just associated with AMI likely being suspected. Interestingly enough, the only “risk factor” associated with a lower risk of AMI was actually a previous ACL injury. And so that’s ipsilateral or contralateral to the investigated ACL injury that we’re talking about now. And so does that suggest maybe some CNS adaptations? Maybe. I’m sure we’ll talk about it, but I thought that was really interesting.
Clinically, what do we take away from this? We know AMI is likely present after an ACL injury. So if you have that effusion, pain, you’re using crutches, the pillow under the knee, multi-ligamentous, that short duration between injury and evaluation, those are all things that should say, like, “Hey, we probably have AMI present here. Let’s address it before we move, and we waste time.” It tells us that again, with those exercises, they referenced the paper to combat or reverse that AMI in those 79% of clients that came in, and they just used a PNF contract relax technique with the patient in prone, and then they did a short arc quad variation to get the quad functioning. So very applicable to what you can do in clinic. It’s not taking very advanced techniques, I don’t think. So hopefully that’s something that we can use, and listeners can use, to work on that.
The biofeedback, I think, is interesting. At Champion, I know people use the
So that was a lot, tried to fit it all in there, but I’ll let you guys talk, I think. I have more thoughts, but would love to hear yours.
Mike Reinold:
Great summary, Brendan. That was awesome. So I think some big findings here. I love the pillow thing. We actually have definitive evidence on having a pillow under your knee, which I think feeds into the rationale and their whole protocol in there. But let’s start with this.
Len, I mean, are you surprised that 57% of people have AMI?
Lenny Macrina:
No. I mean, it depends on how you define it. AMI, as we’re seeing in the definitions, it’s a little broad, meaning they’re going to have muscle inhibition basically because of the surgery. It’s just to what extent, and is it reversible? Will it stay chronic and prolonged? I don’t know. I thought it would be bigger. I don’t know. Most people can’t do a quad set actively or get that heel pop and all that that Brendan just talked about. But it was interesting, the hamstring fatiguing stuff that’s kind of emerging. In my head, and what we’ve always gone after, is getting that hyperextension through some passive stretching, hamstring stretching, stuff like that. But it was interesting, that hamstring fatiguing stuff, which I’ve dabbled with a little, and maybe I need to add it more in to get the hamstrings to… And even just vibration. It’s not even just doing a hamstring fatiguing protocol with exercise. I think it’s just vibration to get the hamstrings slightly inhibited to get the quads activated and allow the knee to get into that full extension.
So it’s little things like that that made me think a little bit more than what I currently do, which is stim, BFR, biofeedback, heel prop with weight on the knee and maybe gearing more towards, I think, a hamstring fatiguing, as they call it, may be kind of a new key for me in how I treat my acute ACL. So I thought it was an interesting paper. Classification system, fine. I think it’ll evolve over time, but I think most people are going to have this. I think it’s beneficial for most people to go after some of the concepts that they talked about. And it made me think a little, which was good.
Mike Reinold:
Yeah. I thought the classification was pretty broad and subjective.
Lenny Macrina:
Yes.
Mike Reinold:
Some of the nitpicking between the various types is really hard to differentiate, but I think it hits the main things. But yeah, totally agree on that. The hamstring concept, we talk about soft tissue vibration, like hamstring type stuff, contract relax, like Brendan said, just a PNF. I just thought it was really cool. But to me, did you see the video in the article? They literally showed just fatigue out the hamstring, which by the way, is probably going to take a little bit of time to fatigue out the hamstring, but fatigue out the hamstring, and then all of a sudden the quad worked better. It was crazy to see, considering they did nothing for the quad.
Lenny Macrina:
Right.
Mike Reinold:
And there’s so many reasons why you can have this, building into this. So it’s like, we’ll add more to it, but just by working on hamstring, the quad worked better, and you could see it in the contraction. They went from a rectus dominant contraction to a VMO/VL kind. You could see it, and you could see the patella actually tracking. So I thought that was really interesting. But what else? What do you guys got?
Kevin, you want to jump in? Did you have something?
Kevin Coughlin:
Yeah, I was just going to say that when Brendan talked about the risk factors, it was cool to see how many of those are modifiable from a PT education and treatment perspective. So if it’s a short duration from the injury to the evaluation, we know that over time, if we’re reducing swelling and pain, which were other risk factors, that it will likely get better if we educate them on “here are some things to avoid” or a certain pain number to avoid. Or during our exercises, monitor for swelling, and we’ll make sure that we’re doing the appropriate things to not prolong the swelling and maybe make this grade 1 go on to a grade 2 or grade 3. And then also just crutch education. I know we all do this education about sitting with your heel propped up with nothing behind your knee. And even though it feels better to lay with something behind your knee, we know that’s going to lead to issues with regaining extension and prolonging this issue.
So I think that’s awesome. I mean, there’s a lot that we can intervene on. And then, as the study showed, if someone is developing AMI or has this present from the time of surgery, it seems like a lot of them can be resolved through those different techniques. So I think, like Lenny said, it’s good to think about and have on your radar and keep doing some of the stuff we’re doing, and make sure that this doesn’t prolong or worsen a little bit throughout rehab.
Mike Reinold:
Yeah, I love it. And the crutches one. Let’s talk crutches because you said that was modifiable, but Len, I mean, we’re starting to see some younger clinicians on social media starting to advocate extended crutch use after ACL, which is crazy to think. But I think this is an article showing that that would be bad. What’s your thoughts just on the crutches? Because I think we all know about ditching the pillow. I think it’s just great that we can say that there’s a study out, for reference, not to put a pillow under your knee, but what do you got on the prolonged crutch use? Because I never would’ve thought we’d be having this conversation based on social media, but what do you think?
Lenny Macrina:
Yeah, I’m torn. I think sometimes when I see people using crutches, and it comes down to my coaching and helping their gait, they tend to not put full weight on it. They kind of bend their knee. They kind of create almost like a flexion contracture by walking on a toe instead of getting that heel contact. And it’s just like, we think we’re doing well with putting crutches under the person to un-weight it a little, but then it modifies their gait, and it creates this kind of cycle that I’m trying to work through. I tend to be, “Let’s get rid of the crutches. Let’s keep you in a brace. Let’s unlock the brace, but the brace is there just to kind of help you.” But I’ve seen people’s gaits improve better, faster when they’re in that brace, but no crutches versus the vice versa that we’re kind of seeing a little bit more right now.
And I can’t get people out of the brace quicker. I’m not having people exercise in a brace and PT. At four weeks, that brace is almost non-existent, and they are using it if they’re in a crowded area. Otherwise, they’re not really using it around the house or anything else, really. So I don’t know, I’m torn with this prolonged use of crutches that I’m seeing right now, and trying to work through it and see. I’ll have feedback sooner as I get more patients that are going through this right now. But I think it almost is not helping us as much as we theoretically think it will.
Mike Reinold:
Not helping and potentially with some evidence like…
Lenny Macrina:
Slowing things down. Yeah. For sure.
Mike Reinold:
Why wouldn’t you? You’re using the crutch as a crutch, I guess.
Lenny Macrina:
A crutch.
Mike Reinold:
Crazy. Anyway, Dan, what do you got?
Dan Pope:
I always wonder sometimes if this is kind of like a chicken or the egg kind of thing. So if someone has more pain, they’re more likely to use a pillow. If they have more pain, they’re more likely to use a crutch. If there’s a worse injury, maybe there’s more swelling, and subsequently, they have more AMI. Was it really the pillow or the crutches that caused the AMI? Maybe. You know what I mean? So I always wonder about that too, but maybe it does give us a little bit of more firepower to get people moving and just let them know, like, “Hey, it’s probably better to ditch this when you can.” But I think the other thing is probably we don’t want to throw the baby out with the bath water. If the crutch is going to allow someone to walk a little bit better, reduce some stress in the knee, maybe they start having less swelling, less pain because they’re offloading it better and their mechanics are still good, I would say maybe that it is something that’s useful, but you probably have to read the patient that’s in front of you.
The one thing I will say, too, about this study is that it’s kind of nice to know that, okay, well, if you’re really painful and you come in, you’re having a hard time, we probably need to focus more on that quad. So I think that was a little bit of a clinical takeaway for me that if I have a patient that kind of presents this way, and it’s like, “Okay, well, we probably need to be more focused on getting your quad stronger and making sure that you’re kind of prepared for your surgery.” Because I think, Brendan, these are pre-op patients, right?
Brendan Gates:
Yep, that’s right.
Dan Pope:
Okay. And the other thought I had is, to Lenny’s point, it’s like only 50% of people have AMI prior to surgery. I guess that makes sense. That’s always surprising to me, after people have an ACL tear complete, that they sometimes can look pretty good, present pretty good. But after the surgery, pretty much 100% of people, I would assume, have AMI. Just because the nature of the surgery and the damage it causes. So maybe it just speaks to, I don’t know how to word this without making it sound poorly, but the damage that is caused by doing surgery, which obviously it ends up being beneficial, just speaks to what happens in surgery in relation to what happens when you actually tear it the first time.
Lenny Macrina:
Can we touch upon the previous history of ACLs that showed a significant decrease? I think there’s something to that, and I don’t know what we’re talking about here. Is it that their system is already down-regulated from the previous ACL, or is it like the mental component of “I’ve gone through it, I know what’s coming up, and I’m okay with it. I know what to expect in pain and all that.” Not that you can control your body that way, but is there some kind of somatosensory kind of issue that we’re seeing in patients that already have experienced it and like, “Ah, here we go again, but I’m fine with it, let’s go,” versus the, “I’ve never had an ACL tear, everything gets shut down, it’s traumatic, it’s bad.” I don’t know. I don’t know what to think of all that, and I want people to help me with that.
Mike Reinold:
I don’t have an answer. I mean…
Lenny Macrina:
I know, right?
Mike Reinold:
…Probably both. What about you, Brendan?
Brendan Gates:
Yeah. So from my understanding from the paper, they made the assumptions that it showed either a CNS adaptation from the first injury, so your brain pretty much found a way to work around the AMI because it had experienced it in the past, and then you have these neural adaptations. They kind of hedged a little bit and said there could also just be local joint changes, like there’s less nerve input or there’s damaged proprioceptors too, so that changes the sensory input. But I also wonder too… I’ve treated people in the past with AMI, and you get them on the NMES, you get them with a strap, and you’re really working on that more cortical inhibition, and you’re working on getting them to put more effort into contracting their quad, they can pick it up within a session and then maybe have to work on it some more. But I wonder too how much of that is a learned behavior from the first time as you go around, but that’s at least what the paper said.
Lenny Macrina:
Because they also said it was ipsilateral or contralateral. So it doesn’t matter if it was the same knee or the other knee, they did better overall. So it’s not like there was damage if there was the ipsilateral knee, but the contralateral knee. I don’t know. It was interesting, the finding, it just made me think.
Brendan Gates:
Yeah, I think they had said in the paper, if it was ipsilateral, there’s a lower risk compared to the contralateral, but both of them were factors that were associated with less risk of AMI.
Lenny Macrina:
Yeah, yeah.
Mike Reinold:
Makes sense. Yeah.
Lenny Macrina:
Yeah.
Mike Reinold:
What else? Anybody got anything else on that one? I liked that article. That was pretty good. Brendan, good review. Appreciate that one there. I think this is good evidence to support some of the things we’re saying. I think we kind of know all these things to an extent, but I think the hamstring protocol was a little bit more highlighted for me. To Lenny’s point, we do hamstring soft tissue, we started doing more vibration, but yeah, sure, it looks like that hamstring inhibition is probably more related to some of these factors, and maybe we give it credit sometimes. So it’s definitely part of a comprehensive package that we can do to work on these.
And then for those stubborn ones, then we know we got to dig a little bit deeper, and maybe those are the people we start throwing some other things at. So it’s kind of a way for us to tell if you can clean this up, to Brendan’s point, just by some quick motor control education during one session, or fatiguing out the hamstring, and it cleans up a little bit, you got a little bit easier road to go down. But if you’re struggling, maybe we need to double down on some of the stuff we do early.
So, great stuff. Thanks, Brendan. Really appreciate it. If you’re liking these articles, please let us know, comment on social media, these types of things. We see them. Love to hear more, the feedback on these articles, but we’re having fun. So hopefully we’ll keep doing them, and please rate, review, subscribe, Apple Podcasts, Spotify, whatever you listen to, and we’ll see you on the next episode.
