Back to Blog

Why do you emphasize achieving full knee hyperextension after ACL reconstruction?


The question is, "Why do you emphasize that you should achieve full knee hyperextension after ACL reconstruction?"  And I tell you what, I am shocked at this being such a hot button issue.  The amount of pushback, the number of questions, the amount of, "Wow, I didn't even know that," from everybody from new therapists to people who are experts in the field has been a really interesting and surprising thing to me.  But I kind of figured out why, because I think it's partly about language and partly that I grew up as a physical therapist in school in Indiana, and I'll tell you the reason for that in a second here.

So I think the natural response is, "You want me to hyperextend the knee after ACL reconstruction. Like, wait a second. isn't that one of the mechanisms of ACL injury?  Isn't that how they tear it in the first place sometimes?"  And I totally get that.  But what's interesting is it actually causes a reaction when we say hyperextension. Because if I say achieve full knee extension, everybody kind of just goes, "Alright yeah, I totally agree that you should have full knee extension after ACL reconstruction."  But when you say hyperextension, you go, "Wait a second.  That means be beyond zero.  That means that we're going beyond zero degrees."  And here's the rationale with that.  That's why I actually still use the word hyperextension, because it notes that we are looking for range of motion that is equal to the contralateral limb.  Now, certainly that presupposes that the range of motion in the contralateral limb is normal, that there isn't any sort of injury to the other limb, but for the most part, there isn't.

So what do I mean by hyperextension and how can that be an okay thing?  Well, it matters for several reasons.  It matters because that lack of full range of motion, actually, whether it's flexion or extension, but particularly extension, causes anterior knee pain.  It is a factor in anterior knee pain after ACL reconstruction that the lack of range of motion and anterior knee pain causes decreased quad function.  And that decreased quad function decreases power production.  Now, I don't have a ton of studies on power production, but it's inherent that if you have decreased strength, just because strength is a component part of power, therefore it has to be decreased.  So power production is decreased, and maybe there is an increased risk of tearing that ACL again, or maybe the contralateral one.  There's not a lot of data on that.  There's some in knee flexion, but we're going to walk through this here.

Alright, so I've got to say the history like I said, I grew up in Indiana, which means that Don Shelbourne came down and gave a presentation relatively early in his career to us as physical therapy students.  And I remember this.  I remember exactly where we were at.  And he was really a pioneer in accelerated ACL reconstruction. ACL reconstruction kind of looks a lot the way it does today, partly because of his work back in the '80s.  Early on, we were casting folks after ACL reconstruction and the outcomes were horrible.  But one of the things that he really promoted was that if people don't get their extension range of motion back then we would see a decrease in outcomes.  The outcomes would not be as good.  And this was even back in '89, that quadricep weakness was positively correlated with flexion contractors, that decrease in extension range of motion of greater than five degrees from side to side, patellar irritability when using patellar tendon grafts. Okay, so this '89, this is back when we started talking about this. 

Now, Shelbourne had followed up, looked at a retrospective study of his patients and anterior knee pain.  This is a great conclusion from this study of 600 patients undergoing ACL reconstruction. Anterior knee pain is not an inherent complication of the surgery, particularly with patellar tendon harvesting (the bone patellar tendon bone graft).  For a long time it was thought that is the only reason that you have anterior knee pain.  Like, that's the only reason. But it's actually an increased incidence of decreased full hyperextension postoperatively.  So when we don't achieve full hyperextension postoperatively, that was what was causing the knee pain.

In this cohort of 607 ACL reconstruction patients.  So this is back, like I said, in the '90s, 1997, and this is kind of the era that I grew up in as a physical therapist, as a sports physical therapist and athletic trainer.  So this was really ingrained in me early on, and I probably never questioned it after that, other than the fact that when I would see patients who lacked full knee extension range of motion, whether it was a total knee replacement or an ACL reconstruction, that they would have a lot of problems down the road.

So we were always obsessive about getting that full knee hyperextension within the first week post op, and certainly no later than two weeks.  If at two weeks we still didn't have that full hyperextension, that was a big deal.  And basically I then went through my next 25 years assuming that this was always still the case in the literature, and because I saw it work so well in practice. Well, as I looked into it and presented it and got so many questions about it, I recognized, whoa, wait a second, not everybody knows about this, that knee hyperextension is important.

So in 2012, Dr. Shelbourne looking at 780 patients with a long term follow up, if extension range of motion was within two degrees.  So that's what they considered normal, the hyperextension, that extension range of motion being within two degrees and flexion being within five degrees.  So that's our definition of normal range of motion.  So basically it equals the contralateral leg because that's within the error measure of goniometry.  So what he found there was that significant factors related to OA development. Abnormal knee flexion range of motion early and final, and abnormal knee extension ROM early is the same risk of OA as having a partial medial meniscectomy and articular cartilage damage with the original injury.  Okay, so we're looking at not only does limited knee extension range of motion, not getting full knee hyperextension, cause anterior knee pain in the short term, we are now putting it as a risk factor for development of OA in the long term. 

And so the odds of having osteoarthritis were two times more for patients who had abnormal range of motion at final follow up. And remember, abnormal was not this big, huge discrepancy . . . two degrees hyperextension asymmetry on the operative leg or five degrees greater on the knee flexion range of motion.  And that lack of range of motion is a similar risk factor as medial meniscectomy or articular cartilage damage. So it gives us an idea that both short and long term outcomes are affected.

Now, let's go back to anterior knee pain.  This is a 2020 study looking at 438 patients after ACL reconstruction.  About 6% of them had anterior knee pain. And basically what they wanted to do is say, hey, is this anterior knee pain due to the graft type?  Does that actually cause it?  And what they found that indeed, those with bone patellar tendon bone graft harvest sites did have more anterior knee pain.

So that means that the harvest site does matter compared to hamstring or other graft sites.  Well, as we look at that, knee extension deficit, again defined as a greater than five degrees difference side to side, was also an independent predictive factor.  So what we have to remember with that, with logistic regression, that is an independent predictive factor.  Independent meaning that alone, not combined with bone patellar tendon bone graft, but as it's standalone, if you don't have your full knee hyperextension range of motion, you will have increased odds of anterior knee pain. 

Now, with that said, though, when you have both a bone patellar tendon bone graft and lack of range of motion, now you've really increased your odds there.  Now, I must talk about knee flexion range of motion with this because I see this all the time as well.  I would say the vast majority of patients who come to me, or clients who come to me as performance clients, they are lacking knee flexion range of motion when they come to me.  And that's sometimes six months, nine months, a year, 18 months, two years after their ACL reconstruction.  And those who have a flexion deficit of greater than five degrees had twice the likelihood of a sustaining a graft rupture. So we know in lacking flexion range of motion is important for retears.  This is a big deal, and we need to pay attention to this.  One note on what full knee flexion range of motion is . . . remember, our goniometry has an error of five or ten degrees, depending on the motion that we're looking at.  So really, full knee flexion range of motion should be measured this way (prone rocking position -- childs pose). I look at it as you should be able to get those ischial tuberosities to your heels.  If you can't do that, you don't have full knee flexion range of motion.  The vast majority of people I see come in like that picture under, not this, with that space between the heel on one side, but not on the other. 

For measuring knee hyperextension range of motion, propping up on a towel roll, and I'd like to use my hand goniometer there, prop them up on the uninvolved side till I can just barely slide my hand under there.  So that just skin to table on both sides there, and then checking it on the other side.  To me, that's the most precise way of actually measuring it.  Certainly I want a goniometer on it, but I think I even get more precise that way.  You can also measure distance of heel from the table as well. 

Now, so if it's a risk factor, lack of knee extension range of motion is a risk factor for anterior knee pain, it's also a risk factor for development of osteoarthritis. But anterior knee pain in this 2020 study stated that anterior knee pain is related to quadriceps weakness independent of the type of graft used.  So if anterior knee pain is related to quad weakness, knee extension deficits cause quad weakness and anterior knee pain. You can see how we're in this cycle, this cascade of problems, and now we're going, okay, lack of range of motion, anterior knee pain, anterior knee pain, quad function is decreased.  Quad function is decreased, then our satisfaction and outcomes are decreased. 

Now, I do have to give some considerations, and I think this is where maybe some of the questions come up, because we immediately go, we have a couple of things.  One, what if their other side is not normal?  Well, you have to figure out what is normal for them and use your best clinical judgment with that. The other one that comes up pretty quickly is, what about the hypermobile athlete?  I've seen people with 20 degrees of knee hyperextension.  As a matter of fact, I remember before my ACL reconstruction on that involved knee, I had 20 degrees of hyperextension and ten on my right.  Well, ten is about the limit that I consider, okay, this is what we're going to look at. If we have a hypermobile athlete, a hypermobile person, we are going to, in general, try to get into that five to ten degree range of hyperextension.  So five to ten degrees beyond zero, say they have 20 degrees.  I'm really not interested in pushing that early on.  They will kind of naturally get that back. I do want to monitor it over time, but in that first . . . if we're talking immediate, that first week, first two weeks, I'm going to want to see them in the five to ten degree hyperextension range.  Again, that means beyond zero.  I'm not going much more than that.  That's kind of what we're considering.  They're obviously already mobile there, but we still don't want to have a huge asymmetry. 

The next thing is graft type, I think hamstring grafts in general, the research tells us they have a higher rate of failure and have a higher rate of increased laxity.  So if I've got a person who probably is maybe a little bit more slightly mobile and a hamstring graft, I'm a little more okay with them just getting to zero in that first week.  But when I mean zero, that is a zero degrees with nice good strong quad contraction, not like barely getting to zero or getting to zero with overpressure or that type of thing.  And the only reason I do that again, because hamstring grafts tend to stretch out a little bit in my opinion, and the research bears that out as well. They have increased laxity as well and increased failure rate.  So we want to consider graft type. Hamstring graft . . . I'm not going to go as aggressively toward knee hyperextension initially.  Ultimately I do want to see it though. 

Now remember, I like this summary here.  The combination of extensor mechanism weakness and graft stiffness tends to limit knee extension range of motion with a patellar tendon graft compared to a hamstring graft.  Again, that's basically saying what I'm saying there is this becomes particularly important to get full knee hyperextension immediately with bone patellar tendon bone grafts. The other grafts to consider quad tendon or your BEAR procedures, those two, I have not seen enough of those, quite frankly, to know exactly what those should look like.  I lean toward those are kind of more toward analogous of the hamstring graft. So let's not be quite as aggressive in knee hyperextension. But we know with bone patellar tendon bone, knee hyperextension is critical.  There's enough research out there both in the short and long term that tells us that that's essential.

So, bottom line, it's important to achieve full knee hyperextension, to decrease anterior knee pain, increase quad function, increase power production, increased function and satisfaction is achieved and decreased risk.  And we want to do that within that first week postoperatively and we need to talk about it with the patient preoperatively as well.  And it's particularly important with bone patellar tendon bone grafts that we need to get this early.



Are you looking to gain confidence in taking athletes from injury to high level performance? Looking to simplify the process and gain clarity? Wish you had a community to ask questions and bounce ideas off of? Check out the Coaches Club.

Don't Miss Out on the Latest Research and Insights!

Sign up for The Huddle


By entering your info, you’ll become a member of The Huddle– with FREE access to a weekly newsletter, return to sport and discharge checklists, free content and offers delivered to your inbox. (Unsub anytime in a click.) You also agree to our Terms of Use and Privacy Policy.

This is a blog and information website and it is not intended to replace the advice or care from a healthcare provider. If you are a healthcare provider, this blog and website are not designed to replace formal education or give you authority to practice outside of your jurisdiction’s practice act. It contains my opinion (based on the best available evidence where possible) and may even be wrong. The information also may not be complete or timely. It does not cover all diseases, physical conditions, ailments, or treatments. The information should NOT be used in place of visiting with your healthcare provider or formal continuing education on the subject matter, nor should you disregard the advice of your healthcare provider because of any information you obtain on this website. Discuss any activities presented on this website with your healthcare provider before engaging in the activity. Links and posts by others do not imply my endorsement of the material and the link might not even be the correct one. Any information, content, products, services, advertising or other materials presented on or through such websites is not under my control or my responsibility.
Disclosure of Material Connection: Some of the links in the post above are “affiliate links.” This means if you click on the link and purchase the item, I will receive an affiliate commission. Regardless, I only recommend products or services I use personally and believe will add value to my readers. I am disclosing this in accordance with the Federal Trade Commission’s 16 CFR, Part 255: “Guides Concerning the Use of Endorsements and Testimonials in Advertising.