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Knee Extension Strengthening Exercises After ACL Reconstruction

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On this episode of the #AskMikeReinold show we talk about whether or not we perform knee extension exercises in the open kinetic chain after ACL reconstruction, and if we still limit them from 90 to 40 degrees of flexion. To view more episodes, subscribe, and ask your questions, go to

#AskMikeReinold Episode 199: Knee Extension Strengthening Exercises After ACL Reconstruction

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Show Notes


Mike Reinold: On this episode of the Ask Mike Reinold show, we talk about knee extension strengthening exercises after ACL reconstruction, and really about that whole 90/40 concept.

Mike Reinold: Hi Joe, what do we got for a question today?

Joe: We got Matt F from down by the river: “When using single leg extensions after ACL reconstruction, do you have patients perform them in a shortened range from 90 to 40 degrees or do them in whatever range is pain free? If you do shortened range when will you open that range up?”

Mike Reinold: I mean one of the best questions we’ve had in weeks right there and how well articulated by Joe right there. It was dramatic, I felt some suspense with that. That was fantastic. And I will say I feel like every now and then we’re getting these, what are they, pseudonyms, is that what it is pseudonym? So is that Matt Foley?

Mike Reinold: In his van down by the river?

Lenny Macrina: I get your Tommy boy reference.

Mike Reinold: Close but you know what my guess is? It’s just all the same person and they just don’t want to keep asking questions under their name. I like it. Good questions though. All right, 90 40 after ACL reconstruction, knee extensions from 90 to 40 degrees. Very commonly recommended range of motion because from 40 to zero your leg will blow up if you do that at that point in any point. Len, historically why don’t you tell us a little bit about maybe the origin of that?

Lenny Macrina: Historically, yes. Historically it’s been limited after an ACL. There is thought that the tibia does translate anteriorly a little bit more in the last degrees range of motion from 40 to zero. So if we’re trying to limit anterior translation of the tibia on the femur after ACL reconstruction, then we want to block or stop the patient from going those last 30 or 40 degrees. I mean I’ve tried, I even spoke to Kevin Wilke and he still does it and I still respect their precautions too, I just pulled back on my precautions with that. I do it I just do it a little earlier than I used to. I don’t wait the 12 weeks.

Mike Reinold: Do what? The full range?

Lenny Macrina: The full range of motion, full active.

Mike Reinold: So 90 40 historically there is a little bit about, does obviously just bio mechanically, that the pull of the quadriceps pulls the tibia anteriorly, which will then put a little bit more strain on there. Well, let’s talk about strain. You talked about motion, but what’s the strain?

Lenny Macrina: The strain that I’ve seen in some of the studies that are out there, and ischemia has a nice review, I think that was JOSPT 2012 that he basically captured all the literature at that time. So it’s eight years ago and Dr. Bennion put a lot of strain gauge studies out back in the day, back in the nineties and early 2000s.

Mike Reinold: In vivo.

Lenny Macrina: In vivo, correct and showed 3% strain to maybe 4.5% strain for most of the open chain stuff. That was using roughly 10 to 20 pounds of force so an ankle weight, that we would do three, four, six weeks out of an ACL surgery typically. So with that, because I’ve seen that, I tend to do it sooner than the 12 weeks because I don’t think that magically at 12 weeks things are anymore somewhat healed than at six weeks.

Lenny Macrina: We know the graft takes years to mature, one to two years if it’s an autograft. So I do it sooner because of the Bennion studies and that ischemia review paper, that I highly recommend you guys read, and I haven’t seen the increased translation. Not that I’m doing KT test on people, I haven’t seen the issues that we think could potentially happen.

Mike Reinold: That’s a good point.

Lenny Macrina: Again more so I see a lot of patellar tendon grafts, I will go a little slower in general with a hamstring graft because of the soft tissue fixation, but with the patellar tendon graft to bone and bone I’m definitely more aggressive in how I treat those people post op.

Mike Reinold: So I think bio mechanically again where this started was the Bennion Fleming studies, they obviously had a lot of strain gauge type research that showed some strain with comparatively open kinetic chain versus closed kinetic chain, that’s where this all started. It’s open chain versus closed chain. And what they showed was closed chain just didn’t have that anterior stress, that strain I should say, it didn’t have it, and open chain at terminal knee extension did. So they said, let’s stop. Then we started to learn a little bit more. So one thing we did note, as they published subsequent studies, is that when you increase the amount of weight on your knee extension the strain goes up linearly as well. So it’s an interesting kind of component. Maybe you can do full range of motion, but with less weight at the beginning. That’s an important part.

Mike Reinold: But then what really started happening is we started opening up the window a little bit and saying, what other activities should we look at? And then all of a sudden things like walking and going up and down the stairs and riding a bike and some of the basic EDLs that they do also had some small amounts of anterior strain. And I think that’s where people started saying well wait a minute, if just some of the EDLs that we do have a little bit of a strain, then is the strain that is observed from 40 to zero degrees, is it clinically relevant? And I think that’s the question that came up. So, I don’t know. I don’t know if we know the answer. I do know that a lot of well-respected people probably still follow those guidelines a little bit.

Mike Reinold: Are we struggling? Should we change these things? I think the answer is you probably could go all the way to extension, but you certainly probably don’t want to do that with a ton of weight. And you got to be careful how you advise people nowadays, especially on social media and 30 seconds at a time that somebody might misinterpret your statement to say that you can do anything all the way up there. I think that’s a little bit different. Let me ask you a question and maybe we’ll go back to the strength science of this, but why would we want to go all the way to zero? What’s the benefit? What do you guys think? Anything strength-wise? What is it that that does that we can’t replicate with another exercise?

Dave Tilley: I will say just in straight knee extensions, great for the quad. I don’t know that you couldn’t get the same thing maybe with a straight leg raises or something along those lines, but it is obviously a very good isolation exercise. I think the trouble with closed chain exercises, people are so good at compensating, you might be able to compensate around it so I can see why the knee extension is good for someone who is just not very good at isolating their quad.

Mike Reinold: I hate to say it, but I don’t think you’re going to find a better way to increase the EMG of your quadricep with something other than knee extension, especially at terminal end range.

Lenny Macrina: But keep in mind the first few weeks after surgery, I don’t think you’re going to make or break the rehab by doing a 90 40. It’s not like if you do 90 40s only for the first month or six weeks that their quads are never going to come back. I think they’re over-exaggerating. My position that I usually go is I wait a month at least and then I’ll start having them do active or some resisted full knee extensions. But again, if you look at some of those Bennion studies in the strain gauge, some of the curves, the strain on the ACL in full do an open chain active range of motion, even weighted, is very similar to doing a closed chain mini squat. So are we having people do mini squats? Yes. And the strain is pretty close, 3 to 4% in a closed chain position too. So we still have people to mini squats.

Lenny Macrina: If you look at a study that came out, I forget what it was, a few years ago, it was an MRI study looking at the strain on the ACL. They showed just walking was a 13% strain on the ACL. So that begs the question, what percentage strain are we tolerating and we want to put on the graft that we know is trying to mature. It needs strain on it for the collagen to mature. So what’s the percentage strain to tear the graft? I think the research is still mixed. If we do it at 3 or 4% strain in general with most of our exercises is that 25% of the total strain took to tear the graft? I think we think the graft tears at about 15 to 20% strain on it or the ACL tears at about 15 20%. But again that research is still-

Mike Reinold: So we’re quite sub-threshold on that. So I think here’s the thing right now is, I don’t think the debate of whether or not to do full knee extension week two with a one pound ankle weight, I just don’t think it’s this important. So I don’t think this or this exercise is that important. So here’s how I envision it right now. I think most importantly is make sure you’re getting your passive knee extension. That’s number one. Then make sure that you can contract the quad with your knee fully extended. So a quad set, a straight leg raise, like Dan said, that’s the function we want out of quadriceps contraction at full extension. As long as you have that, I’m pretty happy with that. I’d say let’s probably still start at 90 to 40 and gradually add some linear load to that because we know at least it won’t put that much strain on the graft.

Mike Reinold: The things they’re going to do around the house and in their daily lives is going to strain the graft a little bit. I don’t think we need to pick an exercise that’s also going to strain it at this point. But then once you get down the road a little bit, you start opening the window. Like Lenny said, he waits about a month. Maybe he’s doing it with full range of motion. Maybe you’re slowly loading it. That’s fine. But then I think the real question comes down to is, when do you let them get after it in a knee extension machine down the road. And that’s probably looking at, I don’t even know, is that three months you mentioned 12 weeks.

Lenny Macrina: That’s the accepted.

Mike Reinold: I think that’s when you start but I don’t even think you go crazy. It’s almost four months before you start getting real aggressive on that. So I think the science is changing our opinion a little bit here. I think it’s changed our mind and I think we could probably do a little bit more, but I’m just not sure it’s worth it. I’m not sure if we have to. I’m not sure if we’re struggling with anything that this is going to solve. So I don’t know, I just think it’s not the end of the world if you do it, but I also don’t think by you doing it it’s doing that much benefit.

Lenny Macrina: No, I think we struggle with the last three months of the rehab. I think we get to focus on that, more so than the first three months of the rehab. Get the motion back, like you said, especially extension and then really be able to focus on those last three months of rehab when the strength gains going to be huge and required in the functional aspect of returning.

Mike Reinold: If you’re four weeks out though, I would much rather you go 90 40 with a lot of weight if you can do it than to go all the way up with little weight, if that makes sense. I’d rather you go 90 40 with more weight and then we’re getting our closed chain going on there. So that’s a little bit about the history of that a little bit of where it came from and I don’t know if we know the answers right away, but I definitely think there’s some workarounds that will get the strength back, that will still make sure that they have terminal knee extension but not put that little bit of strain just in case. Who knows, maybe we’re wrong?

Mike Reinold: These old studies like Vivo, trust me, they didn’t have a huge end. They literally put strain gauges in healthy people in their ACL while they were doing exercises. Think about that for a second. Those are like med students. Pretty funny. Imagine that if you got a freaking needle in your knee and you have to do an exercise. So it’s interesting studies. So good question because I think that’s one of those, is that an outdated kind of concept? It may be a little bit, but probably not enough that that should be a big focus of us. So if you want to go up a little bit I think you probably could, but I think you just got to be careful loading it in that position. That makes sense? That a good answer maybe? Awesome.

Mike Reinold: Great question, appreciate it so much. Head to, click on that podcast link and you can ask us more questions like that. Anything you guys want to talk about. Hopefully we can get them on a future episode. Thank you so much.

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