In this episode, we discuss the cross education effect: the idea that we can impact the strength and performance of the operated limb by training the unoperated or unaffected limb to help create less of a deficit in the early phases of your ACL rehab.
What is up guys? I hope you are having a great day wherever you are listening to this, hope you’re crushing your ACL rehab. And if you’re working with someone who is going through this process, I hope that you are serving them well.
Now, without further ado, let’s dive into today’s nerdy episode. Today’s episode is all about the cross-education effect in ACL rehab. Now, is it just me or do you guys also get super excited to talk about nerdy concepts like this? Just me. Okay, I get that. I have a whole podcast based on ACL, so it makes sense. The cross-education effect in ACL rehab, I remember hearing about this specific term cross-education from Eric Cressey a long time ago, back in undergrad. He talked about when you train this unoperated side or this uninjured side, the other limb can actually have some benefits from it. I was like, this dude is crazy and is making some stuff up.
And then as time has gone on since then, lots of research has come out to help validate and to help dive into more of this concept and some of the benefits and mechanisms from it. You guys are asking, what is it? The term is cross-education or sometimes referred to as cross-transfer. We’ll be referring to it as cross-education. And in most texts, that’s what you will see. It’s basically strengthening the contralateral or the uninjured limb, typically while the other limb is immobilized or can be loaded and trained normally, like after an ACL injury or right after ACL reconstructive surgery.
For a simple example, if you have torn your left ACL, then your right side is the unoperated, uninjured, contralateral side. It’s the opposite side of where the injury basically is. And that’s where this cross-education effect can come in and training that right side in order to help with the left side and the deficits that occur. And so this goes back to research by scripture back in like 1894. That’s pretty old. And there’s been lots of studies since then, especially in the past 20 years to show its impacts and the mechanisms that we’re actually about to dive into.
Why is cross-education so important? There are tons of problems that come after injuring your ACL or even after reconstructive surgery. And we need to make sure we could do as much as possible to help minimize whatever those problems are. One of the main problems is strength loss. And you guys all know this, and we’ve talked about this before. Strength is such a huge foundation of this process. You can’t really run, you can’t jump, you can’t cut, you can’t do a lot of things that you want to do if there’s not a foundational level of strength there. And of course, we’re trying to hit certain metrics of 70%, 80%, 90% of limb symmetry. As well as to body weight ratios to make sure you are set up best for the ACL rehab process and for return to sport. And with strength loss, our goal post-op is to help minimize that problem. And the goal of cross-education is to help attenuate this. It helps to decrease the strength loss, and it creates less of a deficit.
Harput, in 2019, a research study cross-education improves quadriceps strength recovery after ACL reconstruction, a randomized controlled trial. They found a strength gain of 28% and 31% in concentric and eccentric cross-education groups. They essentially trained the uninjured side and they had benefits of 28% to 31% in strength. That’s a pretty big difference, and that’s compared to the control groups.
Another study in 2013 by [Papendro?] definitely just butchered that name, so I’m sorry. However, this group found that providing a high neural training stimulus to the unaffected limb can also help reduce the loss of neural function in the affected limb. This is pretty cool with a big topic that is of discussion, which is AMI (arthrogenic muscle inhibition). If we can get better input and neural stimulus to the muscle, especially the quadriceps, then we can hopefully help with attenuating and improving the strength deficits that are happening. And this study shows that through cross-education exercises.
Last study, contralateral strength training attenuates muscle performance loss following anterior cruciate ligament reconstruction, a randomized control trial. Lots of words here. This is by Claire Minshull in 2021, just a couple of years ago. They found that high-intensity cross-education strength training attenuated the postoperative decline in quadriceps peak force, and should be considered in early-phase ACL rehab. The cross-education significantly attenuated the decline in the quadriceps peak force of that ACL-constructed limb at 10 weeks compared to sham or upper body stretches, which they compared it to. And the decreases were pretty significant in the group that did the cross-education on the unoperated side.
There was a 16.6% decrease in strength compared to the group that did the sham or the upper body stretches, which had a decrease of 32%. So that almost cut that strength loss in half. You’re going 16.6% versus 32%. If we take that from a hundred, that 16.6% is going to get you close to 84%. If you’re talking about the 32%, then that’s going to get you close to the 68%. So that’s a pretty massive difference there and almost cuts that percentage in half. If we were to subtract, let’s say, from a hundred percent. I’ll gladly take an 84% quadricep strength starting versus a 68% quadricep strength starting. As you can see, there’s research that supports this and helps us to make sure that that drop in the strength post-op or post-injury is not nearly as significant as compared to when you can cross-educate and train it. That can help decrease that deficit, which is massive.
The next point I want to talk about of why it’s important is the whole concept of deconditioning. Strength falls into this. But one of our main goals for post-op ACL and post-injury is to reduce the deconditioning that happens after. Usually that injury or your post-op and essentially you are limited to what you can do. And so then therefore you deconditioned or your capacity decreases from multiple areas: power, strength, and range of motion. So many different things, especially muscle size, that’s one of the biggest things that we see. You guys all know this. When you have your surgery, you immediately look down and you’re like, my leg looks like a hot dog now. I remember my quads being like, where do they go and I need them back.
One of the cool things about cross-education is that this was pretty cool because it was paired up with blood flow restriction training. This was a study by Bowman in 2019, and they found a significant increase occurred in thigh girth. You’re looking at 0.8% versus. 2.3% and knee extension strength 3% versus 8% in the non-tourniqueted BFR extremity compared with the control group. Basically, what this means is that they put a BFR unit on the unoperated side. They did exercises on that said side. They saw improvements in thigh girth and knee extension strength compared to not using BFR on that unoperated or contralateral side, which is pretty cool compared to the control group. This is using blood flow restriction training, which I’m a huge fan of in ACL rehab, especially early when you can’t load that operated side as much. But here you’re seeing a cross-education effect for both size and strength, which is just wild. And so we’re continuing on with some of the reasons why it’s important.
Symmetry comes to mind as well. If our strength is better, if we decrease that deficit, then the symmetry is going to be better. And while symmetry can have its pros and cons and people have their own opinions of it, it’s still of a lot of value in this process. It allows us to compare side to side. And then also take that into consideration with absolute values relative to body weight. And we’re still always trying to chase symmetry in this process alongside so many other criteria. And what we want to make sure is that the unoperated side does not get weak. The analogy that usually we use is two flat tires. You don’t want to compare one flat tire, which is, let’s say the ACL-operated side to another flat tire. That’s just going to be a very poor comparison.
Let’s say the unoperated side got significantly weaker, well, the max of that is at a lower threshold. And then if you take the operated side, that’s at an even lower threshold. When you bring that up to a weak, unoperated side, that’s going to be essentially two flat tires. We don’t want that. The benefit of the cross-education is that you’re training the unoperated side which will also help to maintain its peak strength levels, which then therefore is going to help give an accurate estimation of symmetry.
And the last piece as to why it’s important is the psychological or the mental benefit. Getting in the gym and doing exercises that are challenging compared to some boring post-op exercises, and can help with motivation. So that’s very important because the psychological weight of this process can be a lot, especially early post-op. You’re dealing with pain. You can’t move as much. You’re typically away from something that you love to do. And so it can get very mentally draining. If we could still train at a higher intensity on our unoperated side, it’s going to make you feel like an athlete, is going to make you feel like you’re challenging yourself, and it’s going to help break up that monotony that you feel with post-op exercises after ACL injury or ACL reconstruction. These are the big buckets that will play into why is cross-education so important.
Now, let’s briefly dive into the mechanisms, or how does this work? And one of the biggest misconceptions is that this doesn’t necessarily improve muscle size or hypertrophy of that injured side; it’s more neural mediated. Our neurological system is the thing that is kicking in to help with all of these benefits and the things we see in the research. Now that BFR study is pretty interesting because it does show a size increase. But across the research for cross-education, you don’t see as much of a size or muscle preservation as compared to strength preservation or attenuating the strength. And a lot of this comes back to the mechanism of why cross-education works so well. If you’re expecting your quad to blow up or get massive or even not shrink as much, that’s less likely with this process and is more of the strength and the input that is coming into the muscle itself.
Let’s talk about the neurological mechanisms that are at play that can help to answer a little bit of why this cross-education effect is so valuable. Number one is the motor unit recruitment. Different motor units are activated to produce a given level and type of muscle contraction. This increases with the cross-education effect, and that increases the voluntary activation of the muscle of the operated or injured side.
Next is the firing rate, and this is through central neural mechanisms or the brain, motor cortical output, activating both the contralateral and ipsilateral sensorimotor cortex. And then we’re looking at other neural changes, neural changes that happen at the spinal and the supraspinal pre-motor networks. These are all pieces that play into essentially neural mechanisms, adaptations, and inputs, that can help to explain a little bit of this cross-education effect. Instead of thinking locally at the muscle in those muscle fibers, things are changing, getting bigger, it’s more related to the power source, it’s more about how is that input coming in from the brain to the spinal cord, to the motor nerves. And all those pieces together, that is going to play into why that cross-education effect is transferring over.
Also, as a side note, there are other research studies out there that show a lot of this cross-education effect that is not only in the lower body but also in the upper body with our elbow flexors and different contraction types. There’s a lot out there. Today, I wanted to make it specific to the lower body, especially ACL reconstruction focused, obviously because we love the ACL and trying to make it as best as possible for all of you out there.
A lot of you are probably thinking, Ravi, this is all cool. But how do I actually implement it? And one of the things that’s big for me, and especially for this podcast, is that we are very practical and we give you action items and not just tell you, oh, here’s this cool idea. Now, go forward. I want to help give you some strategies to be able to utilize this and some things to think about. Let’s consider this an ACLer who has ACL reconstructive surgery, but also has a meniscus repair. And a lot of times surgeons will limit them with weight-bearing somewhere around four weeks. So that means you can’t put weight through that foot and that leg for four weeks. So that is going to have a significant impact on the joint loading, as well as being able to strengthen due to a lot of the restrictions. This is where this cross-education effect can be even more valuable because, instead of waiting to put weight down on the foot, you can make sure you train the unoperated side. And making sure that you reduce that difference in the muscle strength, potentially help with the muscle size, helping to just kind of stay engaged with the rehab process, especially for someone who is non-weightbearing for four weeks, maybe even longer.
All of our ACLers do lots of work on the unoperated side, especially post-op. To make sure we don’t lose that strength, but also to promote this cross-education effect. One of the things to consider is that if that deficit is less, then it’s easier to get back into shape after surgery if there’s less of that deficit. It makes it less of that uphill climb if you will. Or if you’re talking about a hole, instead of feeling like you’re in a deep hole, that hole is pretty shallow and you can come out of it pretty quickly, as opposed to taking longer and longer to get there.
Let’s talk about contraction types for a second. In a lot of these studies, they’ll say concentric or eccentric. Concentric just means kind of pushing away or where the muscle is shortening. So think about when you’re doing a seated knee extension, where you’re kicking away versus lowering it from straight. Lowering it from straight is eccentric work versus straightening it out is concentric work. Same thing with a squat. If you’re squatting, when you lower, that’s the eccentric portion. And then there’s the concentric, which is coming up from the squat. Every single movement in a muscle has a certain type of portion of contraction. With a lot of this cross-education research, there are benefits to both concentric and eccentric; with eccentric seeming to have a little bit more of a superior benefit compared to concentric.
Now, let’s talk about some different methods with general strength and conditioning and principles that can help you to implement some different types of variations into this. Lifting something heavy so that’s where that concentric/eccentric can play in. Lifting something light and fast and then something like an overcoming isometric where you’re pushing into something like, let’s say you’re pushing into a wall and you can’t move it. If you’re doing a knee extension, you just set it on a weight that you cannot move. And let’s put it at maybe a 60-degree angle and kick out into that for a set amount of time.
There are a lot of different methods that you can implement to hit some of these; moving something heavy, moving something light and fast, and moving something that you basically can’t overcome in order to increase max force and speed. While these different methods help with the cross-education effect and with the neural mechanisms, the main thing that we see in the research is the heavy and eccentric pieces. And that’s where we want to probably put most of our focus because that’s been validated a bit. So that doesn’t mean the other methods won’t help. It’s just if you want to keep it super simple.
Let’s say, for example, you are doing leg press, knee extension, leg curl, and let’s say calf raise. And you are trying to do this on your unoperated side, your early post-op. And instead of just laying there and doing post-op exercises, that’s great for the operated side. But let’s keep the unoperated side training at a high peak level of force and maximize that cross-education effect. And the way that you could do this is by doing three by five reps at an RPE of 9 to 10. And make sure that you plug in a three to six- second eccentric on that non-operated side. So that means if you’re doing seated knee extensions, you’re going to do three by five reps, RPE 9 to 10. Pick a weight that can challenge you a lot. And then you’re going to go out and do five reps. And with each rep, you’re going to lower three to six seconds. And then you’re going to make sure you complete through that, and then you could plug away other exercises for that non-operated side as well. And you can do something like this two to three times a week with rest days in between. Obviously, you’re doing your post-op or post-injury exercises are limited on that side and you’re doing your cross-education work on the unoperated side.
Now, one side note here. Don’t go ham on this, make sure you build up. And that you also are considering all the other things that this leg has to do, especially because you can’t necessarily put weight on your ACL injured or operated side. So that means that you want to balance that recovery. Don’t end up aggravating that unoperated side because you end up doing too much. And then you got to deal with one knee that’s bugging you and then you have your ACL operated side, and that’s just going to suck. Make sure you’re smart about this and that’s where having a very good plan in place, and having someone to guide you to do that work for you is so valuable. Make sure you are strategic about implementing cross-education in your ACL rehab. So that is going to do it for today.
In review, cross-education is good for you. It builds character. But it also helps to reduce that strength loss, it helps with that deconditioning process, it helps us with our symmetry, might help us with some muscle mass, and definitely helps us with some of the psychological and mental barriers that we face. I know that I dealt with that stuff right after my two ACLs. And this type of stuff can make a difference to make you feel like you’re doing more, instead of just simple post-op exercises. And I know that stuff can be monotonous, but there are ways to train around the surgery or the injury. To keep that conditioning up, to keep yourselves motivated, and to make sure that you are making the most of the time and this opportunity that you have.
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