Keith is a Professor in the Department of Physiology and Membrane Biology at the University of California Davis, and Head of the Functional Molecular Biology Lab. The goal of the laboratory is to understand the molecular determinants of musculoskeletal development and the role of exercise in improving health and performance.
Keith completed his PhD at the University of Illinois looking at the mammalian target of rapamycin complex 1 (mTORC1) in the maintenance of muscle mass.
So in terms of injury rates, why are injury rates still on the rise?
”Well, the first reason is that because we get paid on performance (athletic trainers, strength coaches, all of these performance people). And a lot of performance is down to maximizing properties of the musculoskeletal system that actually puts you at an increased risk for injury.
And so what is the delicacy and what’s the real art of performance science is to balance performance against injury rate. Because as far as I’m concerned, I’m going to shift more towards injury rate. I’m going to decrease injury rate because if I decrease injury rate, my athletes are going to have more time in practice. They’re going to be able to have more sessions. They’re going to be trained more frequently. And over time they will progressively get better.
The problem is many coaches and performance directors don’t have that long view because their job is going to be determined in the next six months. So if they don’t win it now, they’re not going to be there long enough to have the opportunity to see the benefits of what they put in place. And so a lot of times what we’re doing is we’re making short-term decisions when we really need to look at long-term progression.
While we still have this system where everybody is judged and the coach is going to bring in his own performance team and all of these things, we’re going to still have this cycle.
Could you give us a overview about the role of the tendon, the function and how they actually adapt, if that’s alright?
”Sure. So I think the best thing to do is to start off by looking at tendons and ligaments, because these two things are often grouped together. And the reason that they’re grouped together is they’re structurally very similar. They’re at least 70% type I collagen and that collagen is supposed to be aligned along the line of force. In a ligament, you’ve got more than one direction of force sometimes, so you get maybe a little bit different alignment than you would do in a tendon.
And what we’ve got in these structures are collagen protein, and that collagen protein is cross-linked together. And that cross-linking is going to alter the stiffness of the structure. So the stiffness of your tendons and ligaments is down to how much collagen you have, what direction the collagen is going and how cross-linked the collagen is.
And so when you have a ligament, what a ligament’s job is to do is to keep a joint from being lax. So is to keep a joint really sturdy. And so the stiffer your ligaments are the better because you don’t want movement within the joint. An example is if we increase the laxity of the knee joint so that there’s 1.3 millimeters of extra give in the ACL, we have a fourfold increase in the rate of ACL rupture.
So anything that’s going to give us small changes in ligament stiffness, or laxity of the joint is going to be bad. And so a ligament, we want it to be as stiff as possible. And that’s because it’s going to connect two bones together and the two bones are going to be super stiff.
If we look at a tendon, the real difference between a tendon and a ligament is a very basic property. A tendon is attaching a muscle to a bone. And so that means on one end, it’s attaching to something very compliant or stretchy. And on the other end, it’s got something stiff. And if you were to give an engineer a job of attaching something that’s really stretchy to something that’s really stiff and hard, they would have night sweats because this is the exact thing that is the most difficult thing to do as far as engineering that structure. And so the tendon is a unique tissue in the fact that on one end it’s stretchy and on the other end, it’s stiff. And so it’s a variable mechanical tissue. That means that the stiffer your tendon is not always the best option, whereas in the stiffer the ligament, the best option, always stiffer; stiffness is better.
Tendon, it’s a little bit different because it has to connect to a compliant muscle.
If it’s too stiff, if it’s stiffer than the muscle is strong, that’s when we get non-contact muscle pulls.
If we just compare female athletes to male athletes, because we said that as stiff as possible is great for the ligament. Well, we know that women playing the same sport have a four to eight times higher rate of ACL rupture. That’s telling us something about the laxity of the ligaments, that they’re less stiff than the men. But they also have 80% fewer non-contact muscle pulls. So what that’s telling us is that when the stiffness is low, we get ACL ruptures. When the stiffness is low, we get fewer muscle pulls.
In contrast, when the stiffness is high, fewer ACL, fewer ligament problems and more muscle pulls. And obviously as a strength or a performance person or a manager, you want to have muscle pulls over ACLs every day. But at the same time, you don’t, you also want to try and eliminate those muscle pulls as much as you can. And that’s where the intricacies of tendons and ligaments and this muscle tendon unit science really take off because to train such that you’ve got stiff tissues for your ligaments, but you can modulate the tendon’s stiffness by using your exercise. That’s really where you’re making your living if you’re a performance or a strength coach.”
What’s the role of the tendon in dynamic performance such as sprinting and jumping?
”My definition of a tendon is it’s something that’s there to protect the muscle from injury. From a standpoint of a performance person, it’s there to transmit force as quickly as possible. Okay, so the stiffer a tendon is, the faster I can transmit the force being produced by the muscle to the bone, and that’s going to increase performance.
So really what I want to do with my tendons for performance is I want to have them as stiff as possible. And the reason for that is that if you think of a weight on your desk and you attach a rubber band or elastic band or a stretchy band, and you pull on the stretchy band, what’s going to happen is it’s going to stretch and the weight’s not going to move. And that’s really what would happen if you have hyper-laxity. If you have really stretchy tendons, you pull on that tendon and the bone, which is our weight on our desk, doesn’t move.
If you now switch that to a rope that’s a braided material, as you pull on it, it’s still going to stretch a little bit, but because it’s a lot stiffer than the stretchy band, now as you pull on it, it stretches a little bit, and then the weight moves. But if I instead have a steel rod there, as soon as I pull on the steel rod, now that bone or that weight on my desk is going to move immediately. That’s basically what we talk about when we talk about rate of force development. When we talk about rate of force development, what we’re saying is how quickly can we get from the message from your brain, to the contraction of the muscle, to the movement of the bone. And that last bit, the contraction of the muscle to the movement of the bone, that’s where your tendon stiffness comes in.
If you want to perform at your best, ideally, you want that tendon to be as stiff as possible. But again, the way that you do that is you’re going to increase stiffness. And then the stiffness of the tendon, if it gets stiffer than the muscle is strong you’re going to have muscle injury. So this is where we’re trying to balance these two things out. We’re trying to balance the performance side, where the higher the stiffness, the better for performance with the potential for injury side, which is if my tendon is stiffer than my muscle is strong, I’m going to get a non-contact muscle pull. And so that’s really where our performance people or performance scientists are earning their money.
So how do we know as sports performance practitioners, if we’re getting that balance right or is it before we get the injury idea?
”So again, what you would do is if you’re at a max performance sport, like you’re a track and field, and you can do everything where you just have to be your best for, you know, for that one event, then what you do is you practice that. And that means in a non world championship, non Olympic championship year, you actually push yourself to the point where you get a non-contact muscle pull. Because that what that’s done is that’s told you, okay, in this individual, what is my ratio of fast movements to slow movements or heavy movements that is going to optimize their performance? And then where am I going to get to that point where if I pushed it too far, I’m going to get a pull? Now, once I know that, I can go back and I can program knowing that in the past, this is where we’ve been. Once we get up towards that level, now I can manipulate training to keep us as close to that without overcoming that.
In a situation like a team sport, where you’ve got a whole bunch of people, what you’re going to find is that’s going to be extraordinarily difficult because each individual has a different set point. And so if you’ve got a whole team, first of all, they don’t have all the same training load because everybody’s going to have positional differences. Second of all, they’ve got different genetics, which makes them either more prone or less prone to injury. And so what you’ve got is you’ve got to really break it down to individualize the training and the performance based work for each individual athlete, if possible.”
How stiff is stiff enough? And I’d like to get you up your thoughts on that as well.
”Again, this comes down to what’s your performance? So if you’re in Rugby Union and you’re one of the big guys, and you just have to absorb a lot of force you don’t need to be extraordinarily stiff. If you’ve got the big, huge guys, so in American football, it’s the lineman. So they’re big, huge linemen, these guys are like 6’6 about 110, 120 kilos. So they’re big. And what they’re doing is they’re absorbing force. I don’t need much stiffness in that athlete.
I like to talk to manual therapists, physical therapists, athletic therapists, who are hands-on, they’ll tell you that there’s two types of athletes. There’s the muscular athlete and then there’s the stiff athlete. And just by touching them they know what type of athlete.
I need stiffness for the people who are going to have high end speed, have to jump super high. Any of these ballistic movement performances, that’s where I need stiffness. And in that situation, what you want is you want the stiffness that’s necessary to perform the movement, but no more. It’s just like flexibility. I don’t want somebody to be so flexible that they’re now hyper lax, and they’re going to increase the risk for injury again. So injury rate and stiffness is a U shaped curve. So if you are very inflexible, there’s a high injury rate. If you are very, very flexible, there’s a high injury rate. And in between, you’re going to get into this kind of shallow area where you’re at the optimal flexibility or at the optimal stiffness, your injury rate is relatively low, your performance is relatively high.
How do I have a quantitative way to say this is it? What I would do, the best thing that we have found so far is to use stuff like counter movement jumps or other things, and look at the slopes of the eccentric impulse. So this is the rate of force development eccentrically. And if you’re going down and up and you can look and you’re seeing big changes in that slope, what that’s telling you is that if you’re increasing the slope, that means you’re getting stiffer. And as you get stiffer, you’re going to find that you’re going to get to a point where you’re going to get a non-contact muscle pull. That for you is now going to tell you where you should be. Again, what we don’t have yet in elite athletics, or especially in non elite athletics, is any type of quantitative measures that say, here’s us tracking it over time. Oh, look, you picked up an injury when you got to this point, this other athlete picked up an injury when they got even less of a slope change. So that means you’re more resilient. You can do more high stiffness work. This person’s less resilient. You can do less.
So what we do is we use injury history a lot of times. And when I get an athlete who’s got an injury history that’s very long, that’s got lots of non-contact muscle pulls, now what that’s going to do is that’s going to change how I’m going to train them. Because I don’t want you to be the fastest player on the team and play two matches over a season. I want you to be the top five fastest players on the team and play every match in the season. And so that’s where I’m going to shift the way that I’m going to train to try and maximize or optimize your performance.”
So in terms of individual differences, is there, is it a huge range?
”There’s a massive range. There’s going to be those two or three guys who’ve pulled their muscle every year. It’s like, oh my God. Yep, he yawned, he pulled a muscle, you know, it’s that kind of thing every time. And then there’s going to be people who they’re a little bit slower. They actually can accelerate a little bit better. So they’re better able to decelerate accelerate, but they’re really bad at their high end speed. Those people tend to be more resilient as far as these non-contact muscle pulls, because their muscle is going to overcome inertia. So your acceleration deceleration, that’s your muscle base. The people who are the fastest people at the top end speed, those are the ones and they have a really hard time slowing down and speeding up.
So, it’s your connective tissue that is going to allow you to continue and to move as high a speed as possible. So if you’re really good at high end speed, but not so good at acceleration deceleration, that’s going to tell me that you’re going to be much more likely to get a non-contact muscle pull. If you’re really good at acceleration deceleration, I’m going to guess that you’ve not had a lot of non-contact muscle pulls.”
A minute ago, you talked about flexibility and this U shaped curve. If people want to be at the bottom and want to make sure that they stay there in terms of building that flexibility, but not becoming hypermobile, what would be your recommendations?
”Yeah, so what we do is, for our flexibility, for our range of motion type of work, what we’re doing is we’re not doing any kind of static based stretching because that’s not ideal as far as how we’re activating the system. There’s a bunch of physical properties that these tissues have, that tendon has specifically, but that matrix has in general. And those are these viscoelastic properties. So that means that the tendon is going to behave both like a liquid and like an elastic solid. And that’s really important for us as a performance measure, because the faster you move, the stiffer of viscoelastic surface becomes.
So if I’ve got a viscoelastic tissue, if I go fast, it becomes stiffer. So we can do these tests in our laboratory where we’ve got a machine that’s just going to pull and it can pull at different rates. And what you can do is you can watch it and it pulls super fast. It’s going to break earlier, but it’s going to have really good stiffness in the tissue. If I pull it really slowly, it’s going to stretch a lot further and it’s not going to take as much force and it’s going to be much less stiff. So if I pull and I hold on a tissue, like a tendon, you get creep, which means I’ve pulled it and then it’s going to slowly come back down. And that’s fine and that’s what you get with static stretching. What we want to do that slightly different is we want to actually continue to maintain the load on the tendon while we’re getting this kind of creep. And that’s called stress relaxation instead of creep. The difference is that when we do stress relaxation, we’re using muscle contraction to continuously load the tendon.
When we’re doing creep, we just go into a position where the muscle tendon unit is longer, or we just hold it there. And eventually it slowly relaxes, but there’s no tension across it. And so the tension of the whole system goes down together. When you use a muscle contraction to do that, now what you’re doing is you’re allowing the tendon to continue to get a load across it. But because the tendon is slowly relaxing, the strong parts of the collagen are relaxing, now what you’re getting is you’re getting a signal from the muscle and a signal from the tendon that correspond to each other. The tendon feels load, the muscle is creating load. When we do a static stretch, what we’re getting is we’re getting a disparate signal from the two tissues. One, the tendon is under load but the muscle’s not under load. There’s no contractility, and so what you get is you get this almost counter-intuitive to the two sensors within our musculoskeletal system, the Golgi tendon organ, and the muscle spindle, those are changing in two different ways.
And so that’s potentially giving us mixed signals that could potentially increase injury rate. And the example I give is our NCAA athletes. So the athletes where you think, okay, if you were to think of an athlete who should have really stretchy tendons, you would think probably of gymnasts. And you would think that these gymnasts are really super flexible. Well, two years ago, 17 NCAA gymnast ruptured their Achilles tendon. And so it’s not about, and so they’ve done lots and lots of passive stretching. They’ve done lots of holds. A lot of coaches actually have them sleep in those little devices that hold the toe back so that they get more flexibility in the Achilles. And yet here they are rupturing their Achilles faster than any, or more than any other athlete group. And it’s likely because they’re doing that passive movement and that passive movement isn’t increasing flexibility. What it’s doing is it’s changing the Golgi tendon organ reflex. And so slowly over time, the Golgi tendon says, oh yeah, this kind of stretch on the tendon or this kind of load on the tendon is normal. So it doesn’t have that really quick reflex that’s going to assist you at protecting your musculoskeletal system.”
Top 5 Take Away Points:
- Risk: reward – a lot of performance is down to maximizing properties of the musculoskeletal system that actually puts you at an increased risk for injury
- Ligaments vs. Tendon – the stiffer your ligaments are the better; tendon is a variable mechanical tissue. That means that the stiffer your tendon is not always the best option
- Role of tendon- to protect the muscle from injury. From a standpoint of a performance person, it’s there to transmit force as quickly as possible
- Know your limits – you actually need to push yourself to the point where you get a non-contact muscle pull.
- Static vs Dynamic stretching- dynamic stretching is better as it applies a stretch to the tendon and continues to apply a load on the tendon.
Want more info on the stuff we have spoken about? Be sure to visit:
You may also like from PPP:
Episode 217, 51 Derek Evely
Episode 207, 3 Mike Young
Episode 192 Sprint Masterclass
Episode 87 Dan Pfaff
Episode 55 Jonas Dodoo
Episode 15 Carl Valle
Hope you have found this article useful.
- If you’re not subscribed yet, click here to get free email updates, so we can stay in touch.
- Share this post using the buttons on the top and bottom of the post. As one of this blog’s first readers, I’m not just hoping you’ll tell your friends about it. I’m counting on it.
- Leave a comment, telling me where you’re struggling and how I can help
Since you’re here…
…we have a small favor to ask. APA aim to bring you compelling content from the world of sports science and coaching. We are devoted to making athletes fitter, faster and stronger so they can excel in sport. Please take a moment to share the articles on social media, engage the authors with questions and comments below, and link to articles when appropriate if you have a blog or participate on forums of related topics. — APA TEAM