In this episode, we discuss Limb Symmetry Index — a common metric used in ACL Rehab and research. We dive into why it is important and how to best utilize it.
What is up team, and welcome back to another episode on the ACL Athlete Podcast; it’s episode 111. And hopefully, you’re not hearing as much heavy breathing because we are in the new office space for the ACL Athlete. Although I can’t tell you that the space was the reason why there was heavy breathing, we shall see. What’s also funny is that there’s a guy outside blowing our neighbor’s yard. This is something that if you’re tuning in new to the podcast, has been a revolving theme since it started because of being at the apartment. Anyways, if you guys hear that, it’s back. I think it’s pretty fitting for this episode, which is all about limb symmetry index or LSI. And this is something that is a very interesting topic that I wanted to dive into today. I’ve been doing a lot of thinking about this specific measure and what it is and its use of it in our space and in ACL research and rehab and return to sport. We’re going to dive into it, the pros, the cons, and just some examples of this and how it can be utilized and things to watch out for.
LSI which is the Limb Symmetry Index. It’s often referred to sometimes as the quad index or hamstring index. But it’s basically a metric used super often in the ACL research and other research and for return to sport for ACL. The value could be represented by numerous qualities such as strength or a particular muscle group like the quadriceps. Or even something like performance testing, like a hop test, for example, like a horizontal hop test or a single-leg vertical jump. We’re always gathering and seeing, especially if it’s a single leg or just one side doing it to be able to get symmetry numbers in order to understand how is the other side doing. Use a lot in the ACL research. Probably the most common of everything discussed as a metric. That’s if we’re lucky in the setting that you’re in and the clinic and it makes sense if we’re trying to compare the side that got hurt versus the side that is not hurt. And that’s how we’re able to get some numbers. And for us, with our remote athletes and the people we see in person, there are a variety of KPIs or key performance indicators that we track. Symmetry is one of many for a lot of these different tests and measures we have.
What exactly is the limb symmetry index? It is the ratio or comparison often reported as a percentage comparing one limb to the other side. Again, as I had mentioned, it is used a lot in research in general, whether it’s the upper limbs, the arm versus the other arm, or certain muscle groups. But we reference it a lot in ACL rehab to be able to get some comparisons. But it’s basically how the ACL injured side or the operated side after ACL reconstruction compared to the uninjured or unoperated side. Now, this is assuming that there has been only one single ACL injury. If there have been multiple, then that is a little bit of a different story because there’s a complexity to the history, but you’ll still use that as a reference. For example, I have an ACLer right now. We referenced her latest operated side as the injured side because that’s what she’s currently rehabbing from the other side, which is the first ACL injury that she had.
But with this said, we are going to compare, for example, for most of these situations, an injured side versus an uninjured side or an operated side versus an unoperated side. It could be leg to leg as I had mentioned, or it could be muscle groups such as the quads. So one quad to the other quad. And the calculation is pretty simple. LSI is equal to the affected limb value over the unaffected limb value times a hundred. And that basically just spits out a percentage. So that helps us to know, okay, how does the affected limb value as a percentage compare to the unaffected limb value?
For example, if you’re post-injury or post-op ACL on the left side, that’s your affected limb. As I mentioned before, we’re going to assume this is your first, so your right limb is your unaffected limb. Let’s say you do a quad strength test and on your left side the output is 30 kilograms, and on the right side is 60 kilograms. Then your quad lsi or your limb symmetry index is 50%. Or a different but simpler way.
You’re basically 50% as strong on the ACL-injured side as you are compared to the unoperated or non-injured side. You’re at 50% of the strength. Now, imagine the ACL injury just happened. Then ensue the events of taking care of the limb, being able to start to walk again, visiting the surgeon, and then maybe getting into physical therapy. There’s some time that has evolved. I’m imagining that also your lifestyle has changed a little bit. You’re probably not in the gym as much. You’re dealing with the pain, trying to get the knee under control. You’re not doing your normal activities, exercising, or strengthening.
What happens after that injury? Not only does the strength of that injured side decrease, but typically in most cases, the uninjured side also starts to decrease. You’re not doing your same level as output as before, so it’s not uncommon for that uninjured side to drop in strength. And so that’s where we want to talk about how the affected limb and the unaffected limb matter in this ratio. For example, the unaffected limb can decrease in strength, as I had mentioned when you think about these details post-injury and especially post-op. There’s so much going on that athletes aren’t typically prioritizing getting in the gym and getting super strong again. It’s, let me manage this injury or this surgery and being able to just survive really, let’s be honest. So with that said, the decrease in that strength can overestimate the LSI in the affected limb. It’s important to consider, especially in this process, how long it’s been since that ACL injury because the decline in strength and activity, in general, are important factors. And it’s a relative value that we’re getting here.
For example, if you can only kick out, let’s say nine kilograms on that affected side, and then you kick out 10 kilograms on your unaffected side, you’re at a 90% LSI. Usually, in this process when you’re thinking about clearance for return to sport, the number that we’re typically trying to chase in most ACL research references is 90%. We want to try and get within 90% of that unaffected side. So 90% LSI is what the gold standard is, which I think should be even more than 95%, 100%, and 110%, if possible. But 90% is typically what we’re chasing. But if you kick out nine kilograms on the affected side and 10 kilograms on the unaffected side, you’ve got 90%. And that might sound good as a 90% metric, but guess what, your strength is not great. It is pretty low overall for both sides, and we want that to be super high for people who are kicking out. If you’re looking at this from a pound standpoint, you just measure those numbers and multiply them by 2.2, so really you’re kicking out 18 versus 20-something pounds per leg.
That’s not a lot for a knee extension test, realistically. And the analogy we like to use is thinking about two tires on a car. When you’re looking at the two tires on your car. There’s air in those tires. Imagine there’s one tire that has full air, and then you have the other tire that has half of the amount of air. Then that’s roughly a 50% LSI. But let’s just imagine that that full tire had come down in its air to only 50% full. Well, you got a 100% symmetry right now, if both are half full and you’re comparing each tire to tire, they’re pretty much even. That’s a hundred percent.
Here’s the problem. They’re both kind of flat, so we end up having to compare one to the other. And it gives us this false sense with the symmetry index of 100%. But realistically, you have two flat tires. You want to make sure that the tire that you’re comparing it to, or let’s say for the quad that you’re comparing it to is at its peak value and at its strongest value, and that’s going to be what’s most important to anchor the symmetry and especially the ACL injured side or the operated side to. And so that’s where we want to make sure that we catch the symmetry. Because a lot of times what will happen is clinicians or even surgeons or whoever’s in this process, if you’re lucky, we’ll hopefully be measuring symmetry and doing it right.
But if they’re measuring it and they’re just like, okay, yeah, you’re at 90% symmetry. You’re cleared to go, a good return to the sport. But we need to make sure that we understand what those numbers’ absolute numbers actually are. Because percentages are basically a relative value which brings me to storytime.
This is a story about an ACLer I saw recently and just started working with. She’s a college athlete. Came in one year post-op. And she’s had some issues with her knee. Really complex pains, weaknesses, and a lot of factors playing into the mix here. She was actually cleared for non-contact drilling for soccer and just playing. And she felt herself, it wasn’t right. She was scared of moving around, but she had gotten clearance from it, which is crazy. And then I asked her, I was like, was there any testing involved in this process? And she said yes. It had taken her a second for her to figure out what the testing was. But she said that, of course, kicked into the hand, which was so annoying. But the other thing that she had mentioned was a leg press test. And it showed an okay symmetry for her on a leg press test. What they did was they test one leg and then they tested the other leg and they measured the symmetry of it.
Here’s the crazy thing. When we tested her quad-specific strength using a handheld dynamometer, she was at 68%. Her being a year out, doing non-contact drills with a cutting and pivoting sport at a high level, and she was dealing with pain and swelling, it’s just not a good recipe. So whoever cleared her, in all honesty, it wasn’t right. And we had a pretty transparent conversation about her situation and where she’s at, and she was fully on board with it. It was the big reason why she decided to pull herself out to work on herself and to be able to build up her knee because she didn’t want to reinjure her knee and she felt like she wasn’t getting the attention she needed.
And so this was basically poor testing, but also relying on symmetry for a global test, like a leg press. And I don’t hate a leg press. The leg press can provide some value. But here’s the thing. A leg press is a compound-based movement. And what I mean by that is multiple joints are moving, so multiple muscles can contribute to the output of the movement. There are a lot of athletes that we can see where they can have a good output and they shift straight to their hamstrings and their glutes and never really utilize their quads. And our bodies are really good at figuring that out, especially if we need to pass a test. Hop testing is another good example. But this was something that was just crazy when they had cleared her. And she was like, “Nope, I don’t feel safe.” And the numbers speak for themselves. She was at 68% LSI and we even measured it relative to our body weight, which is an important metric to anchor this. She was only at 1.74 Newton meters per kilogram body weight, where she needs to be at roughly close to 3.0 Newton meters per kilogram body weight.
We’ll talk about that here soon. But the point here is that it just wasn’t accurate. And she was released to do some pretty higher level activity with a symmetry number that wasn’t really showing her true potential. And that’s where we need to make sure we’re checking in and seeing what that number is specifically rather than just the symmetry. And one really cool research article I’m going to bring into this is by Wellsandt in 2017. It is limb symmetry indexes can overestimate knee function after ACL injury. They followed 70 athletes pre-injury, post-injury, six months out, and even two years after. It was a really cool study and at six months post-op, out of the 70 patients, only 28% of these patients met greater than 90% epic criteria, which is estimated pre-injury capacity criteria. This is basically just taking measures from the uninjured side, pre-injury. And they compared it six months later to the involved limb. It’s a pretty cool metric that I’m going to talk about here in a second. But basically, out of these 70 patients, 28% of them only met 90% or greater of that criteria on all strength and hop tests. However, 57% achieved greater than 90% LSI criteria on the strength and hop test. You’re looking at 28% of these people passing those EPIC criteria. But the same group of people, 57% of them pass the LSI criteria, so that’s really interesting.
It basically means that the epic criteria were probably more difficult and based on the numbers they needed to hit, people weren’t hitting those numbers. But more than half of them were achieving, at least the LSI. Further in this study, 11 of the 70 patients, 15% of these people sustained a second ACL injury during that two-year follow-up period. An 8 out of the 11 passed the 90% or greater LSI criteria on all strength and hop tests at six months post-op. Six of these eight, however, did not pass the greater than 90% of the EPIC criteria on all strength and hop tests at six months post-op. In their conclusion, they basically said that being able to pass 90% or greater of the EPIC criteria was superior in predicting a second ACL injury than the current method of greater than 90% of LSI.
This is a very interesting study. And we’re not going to get into the details of why EPIC is better than LSI. But it’s more important to understand that this was overestimated for a lot of these athletes, especially just using LSI alone. It could be a dangerous place because it gives you this false sense of thinking, I have this really good data or these metrics, but then if we’re just comparing side to side and you’ve got a weak side you’re comparing it to, then that makes it tough because the LSI itself, Is giving you a false sense of security, just like I had given you the 9 kg versus the 10 kg, 90% is great, but both legs are weak. This is just more important to pause and understand what it is we’re gathering from the limb symmetry index. And it’s just one of these things wherein the space is thrown around, let’s hit 90% clear for sport. When we dive deep into why re-injury rates happen, we know it’s very bad care, and not enough testing, and a lot of this comes down to what metrics are we gathering. What type of testing are we doing, and what are the interventions being done to meet these metrics? And so that’s where LSI is in the mix. And I don’t want us to get too carried away and think LSI will solve all of the world’s problems for ACL rehab.
But it’s a helpful part of the mix for sure. And you’re probably listening to this podcast being like, wow, Ravi really hates LSI. We don’t hate LSI. It’s actually a very helpful metric to have and we use it a lot with all of our athletes. But it’s along with other key performance indicators and values for our athletes on their athlete dashboard. And in full transparency, most clinics and most physical therapists and surgeons aren’t using symmetry that much. And it’s one of those things where it is a time thing. It’s more effort and work. They may not know about it. They may have a certain process where they’re like, this is good enough based on their protocol.
But even using symmetry is something that we still need to get into. Because a lot of the ACL research references it, so it’s something important to incorporate into the mix. Because we do have a lot of data that shows if athletes are at their strongest and are pushing it with both legs, then therefore 90% symmetry is a very good value, if they are also relative to their body weight. And that’s the thing that’s important to know here symmetry has a lot of value. We just got to make sure we anchor it to a really strong leg, or a really strong quad, or a really strong hamstring. Because otherwise, we do need to be careful because it gives us a false sense of security of strength or performance. If your clinic or a physical therapist or surgeon’s office is doing testing through this process, that’s awesome.
I think it’s just important to make sure that we are specific about what those metrics look like, what we are being tested on, and how that is informing our practice and programming to make sure we get back to the goals you have set. Oftentimes, if they are, they’re not testing the right things or doing it the right way. And I’m not trying to be a negative Nancy here. You guys have heard me before and the reason I’m bringing this up is because it’s important. I care about it a lot. I’ve fallen at risk for it. I’ve had other ACLers who have fallen at risk for it, who have come into our system, where we hear these stories and we talk to other professionals and we ask them what they’re doing.
We are in this space. We’re hearing it every single week. It continues to be the thing. And we see that with re-injury rates. And so the reason I bring this up is because it’s my podcast. Two is because I just give a damn. And I think it’s really important to be transparent with you guys to share what I see. And so then that way you don’t turn into another statistic. And if you’re a physical therapist or coach or surgeon listening to this, being able to make sure that the people you come in contact with are not another statistic. This is the thing that we have to do to be able to educate and to help inform our people in order to make sure that we make good decisions.
Now, that I’m off my little tangent, other options that can be helpful in this process, as mentioned before in this Welldsndt study is the epic or estimated pre-injury capacity test, so that can be helpful if we can get that information right after an injury, but that’s not always the case, just depending on what that cascade of events looks like. But if it’s possible to be able to get your strength right after you get hurt on the uninjured side, that’s awesome. That will be so helpful to be able to help base the rest of the process. We try to do that as quickly as we can. If someone comes in, they’re like, I tore my ACL. We want to try and test their unoperated or uninjured side to get that baseline so we can potentially use this epic. Testing and measures to be able to see where they’re at later in the process and how they match up.
The other piece to this that I think really helps anchor this is using it relative to your body weight and comparing it. One of those things is Newton meters per kilogram body weight. You heard me say that when I was testing this college athlete’s quad strength Newton meters per kilogram body weight is basically an output of torque. And so this allows us to basically measure specific to you as a person, and I’m glad it’s gained a lot of traction over the past couple of years, and it is currently really the gold standard in the realm of ACL rehab, especially if you can hit 3.0 Newton meters per kilogram of body weight for your quad, you’re in really good shape because that roughly comes out to being able to push roughly your body weight and your LSI can’t really help you here. It’s a metric that is specific to you and your body weight, and it’s why we love it. You can’t outrun it here. Sorry for the bad pun for ACL rehab. But it’s just something that can be very valuable, especially if we’re strength testing, hand handheld dynamometers, or using an isokinetic test. These could be very valuable to make sure we get some pretty solid numbers and sure, we could still use symmetry as a comparison. But we get to use your body weight, which is something that won’t change nearly as much compared to a strength level that can drop drastically after this process happens.
The other option I also want to mention is normative data. Normative data can be very helpful. There’s research studies that have been done on specific populations, genders, and things of that nature where it can give us a helpful range of being like, okay, well, where are these athletes in this specific age range, sport, gender, to be able to see this is the normative data that you see for someone who is uninjured. Therefore, you also have a target among your peers and other athletes, so that could also be a very helpful option in this process.
Now, the last thing I want to touch on here is just some of the other things to consider to think about. We’re talking a lot about numbers here. Not only are the numbers important, but the feel or that quality or the qualitative value or the components in this process. So that’s going to be that look and feel of this process and movements. You could do a knee extension-type movement, and create an output, but how does that feel? And also can we measure that? Often this gets missed if we get too stuck on the numbers and the data. It’s important as a physical therapist, as a coach, and even as an athlete to be able to not only assess the numbers that you’re getting but also the feel or how it looks and the quality of it; it’s important to have both.
All right, team, as we wrap up this episode, I’m sure this one hurt your brain a little bit, and hurt mine. But it’s just something that’s important to consider in this process of understanding what LSI or Limb Symmetry Index is. It’s utilized so much in ACL research and can be very helpful. But you don’t want to put all your eggs in one basket. You could always talk to your physical therapist, your coach, or whoever you’re working with, of what these metrics look like. Are we measuring them? And if they have no idea, then that might be a question mark on who you’re working with. Or what they’re actually measuring, or is it just this protocol that they’re running you through which is very common?
It is just something to be able to evaluate and not put all your eggs in one basket, as I had mentioned. This LSI can be overestimated as the WellSandt article mentioned, especially depending on your own personal journey, whether you had a long time between your injury and the surgery. Maybe you became more sedentary or maybe you kept your levels up and that LSI is very spot on. But each individual journey is going to be so different and what we often see is that people decrease their activity just cuz they have a knee that can no longer do as much of what it could before when there’s an ACL injury. Just something to consider. We also discussed other options to be able to factor in like the epic or the estimated pre-injury capacity. Being able to compare it to your body weight, which I think is the gold standard that we should be taking on in this process, especially for our quad strength, our hamstring strength, and even compound movements. It’s helpful to have some points to anchor to, to know, okay, what are we aiming for? To be able to put force into something and to be able to express it in a dynamic fashion on the field for activity, sports, whatever that may be. And then the other thing to consider is the normative data. Normative data can be helpful. These are all different places that you can go to, not only anchor yourself to but also utilize LSI along in the process.
Hope that was helpful. If you guys have any questions on this topic, you could always reach me at my email email@example.com or just ping me a message on Instagram at ravipatel.dpt. We also had an impostor who tried to steal my identity on Instagram, so I’ve received a number of messages. Thank you to all of you who have messaged me and told me. But if there is an underscore in the Ravi Patel DPT and there’s an underscore between that. They’re an impostor, report them. And if they’re asking and saying, hey, like, my main account got hacked. This is my new account. They’re impostors. It’s all a lie. Help save the profile here so other people aren’t starting to get targeted for this. Anyways, hit me up wherever you can. And if you’re on here and you got value out of this episode, please leave a five-star review so we can reach more and more ACLers, clinicians, coaches, and just people who are trying to learn more about the process. And we could share this, able to expand education and be able to help people make their own decisions versus relying on an external system. That’s going to be it for today, guys.
Thank you all so much for listening. This is your host, Ravi Patel, signing off.
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