Podcast #130: Tendons with Gerard McMahon

Jake Tuura · March 5, 2025

https://podcasts.apple.com/us/podcast/jacked-athlete-podcast/id1462537296?i=1000697939678

“When you work with athletes, probably frustration of mine is people focus a lot just on muscle and forget it’s part of a muscle tendon complex, a muscle tendon bone complex.”

Tendon: “The tissue has its own metabolism. The cells need energy to renew the extracellular matrix and stuff like that so there’s an energy requirement and where there’s an energy requirement, there’s a need for metabolism, you know, but it’s just not on the scale that muscle is because, you know, I shall know tendon is a fibrous structure. It’s not a contractile. So it’s a passive structure. So it’s not got those huge ATP requirements that a muscle will. But it’s still got a lot of things happening within it that require energy to be part of their processes.”

“Tendon strain is just the relative change in the length of a tendon compared to its original length, or its starting length, which would depend on the joint angle that you have. So yeah, it’s dimensionless.”

“Strain itself is not uniform within the tendon. It’s different depending on the hierarchical structure you’re talking about, as you go down from total tendon through fascicles, the fibrils, stuff like that.”

“The total strain at the tendon level is not the same total strain that’s been felt at the cellular level or the fibril level or anything like that. It’s diminishing as you go down through the hierarchy.”

“Whole tendon strains, say like 10 % in the Achilles or patellar tendon or something like that. But then when you go to an in vivo study, they’re talking about the cells being strained by one or 2%… but that’s ultimately what they feel in vivo. It’s representative of that.”

“It functions in so many different ways with different purposes. At times, we want it to be able to store energy and release it at times you want it to act a wee bit stiffer and just be efficient at returning energy to the system. in order to respond and act in different manners, it’s bound to have to have some sort of complex structure to it. It can’t be very simple in nature.”

“Yeah, well, I suppose that is the sort of key thing is the rate of strain application. That does affect its behavior. And just like you said, the more rapid it is, it does act in a different way. It acts stiffer. Because otherwise, you’ll dissipate some of that energy.”

“During all these hopping activities, jumping activities, peak strains are like 14 % in the patellar tendon. But during strength training, you’re probably getting peak strain of only 7 or 8%. But yet, the strength training is what ultimately will make the tendon stiffer.”

“There seems to be a minimum time frame that the strain has to be applied for. So it’s not all about strain magnitude. It is, but as long as there’s a minimum threshold of strain duration.”

Plyos not increasing stiffness: “I’ve had this sort of discussion with several people. I think plyos, you’re probably getting something. It’s just our ability to find it and detect it. Another thing, tendon’s incredibly hard to study because we don’t really tend to biopsy tendons as often as much as muscle because you’ve damaged the tendon pretty much and that’s that. There’s not much coming back from it after that.”

“And Kubo also did a paper of one second on two seconds off versus four sets of 20 seconds holes. So the whole volume was equated. And again, they find that the 20 seconds was much better than the one second. In fact, the one second didn’t see any stiffness adaptations at all.”

“When that force is applied through the tendon, it is some of that sliding and rotating of fascicles. Ultimately, it takes a little bit of time to filter down through the matrix to actually get to the cells, which are going to be the ones that are going to respond to that strain. So I think it just, it’s literally the strain finding its way through that hierarchical structure down to the cellular level takes time.”

“If you’re trying to strain one to 2 % at the cellular level or 6, 7 % strain at the whole tendon level. When we’re developing that amount of strain, it usually takes quite a high percentage of our MVC to apply that strain, you know, maybe 80, 90 % of MVC. But to get to that, that’s already at somewhere like 200 to 300 milliseconds to get to that force level out of your one second. So, you know, you’re a third of the way through your second before you’ve developed the force. And then it’s still to get through that hierarchical structure of tendon as well. So probably by the time of one second contractions over, you’ve probably spent very little of that one second actually straining any cells.”

“When we looked at sort of the application of Keith Baar in vitro work, where he said, the tendon cells take 10 minutes to react and they seem to be unable to react to any mechanical stimulus [after that]. Whereas when we do the training studies, we find that that’s not the case at all in vivo in humans. Strain magnitude is very much the dominant factor.”

“In vitro, you’re literally dealing with the cells, but in like we’ve just said, there’s a huge hierarchical structure of force transmission. You know, none of those cells have an interfascicular matrix or anything like that. So that’s where a lot of in vitro work falls down. It’s not representative of the in vivo tissue, it’s just got no application. Literally no application at all.”

“We can’t really give recommendations based off mechanistic work. It should be used to try and show how this comes about, but not actually guide the entire thing itself. It’s the reverse of that. Your training studies is what should guide you, really. And then try and use mechanistic work to understand why they do the things they do.”

“We do get tendon creep in humans. Absolutely. With consecutive loading cycles, you’ll get more creep, but it hits what you call sort of an asymptotic point where it doesn’t creep any further.”

“There was significantly greater creep by holding the contraction for 12 seconds than there was after four seconds. The tendon just kept on increasing strain that way.”

“There seems to be a sweet spot for tendon strain… If the movement’s too quick, the strain just doesn’t get to the cells or it doesn’t really get there for any length of time or maybe any sort of real magnitude either before it’s unloaded again. If you strain an average strain, in an average, so if you squat, you’re probably lasting a good few seconds to perform a squat. So the average strain during that four seconds it will probably be high, you might hit your 10 % peak strain, but on average over that four seconds, it could be maybe five or 6 % strain, but versus a 10 % peak strain in a jump that’s lasting maybe a couple of hundred milliseconds. So to me, there seems to be potentially a sweet spot where if it’s too quick, it’s not seeing the strain.”

:So to me, there’s probably a sweet spot of maybe, I don’t know, three to 12, 15 seconds or something where you’ve probably got a window of opportunity because it is linked to the muscle and what torque the muscle can produce too, where you’re probably going to get the highest strain. So it makes sense to me to cycle strains or cycle high strain and those sorts of timeframes.”

Longer ISOs: “I think you probably get the tendon adaptation because you went through the first 15 seconds of that under a high force anyway.”

“For healthy tendon to improve mechanical properties, I don’t see any value in longer, longer isometric holds north of 15 seconds. Yeah, I would say that’s just going to be heavily impacting muscle metabolism and motor unit recruitment and stuff like that there rather than doing anything for the tendon.”

“The thing about the change in mechanical properties and blood flow restriction is you would have to have a completely different method of how tendon adapts to allow almost metabolic stimulus to make the tendons adapt. The only way you could really see that is a general sort growth factor type response. They’ll share interstitial fluids. They’ll be bathed in the fluids around the cells will be bathed in similar fluids and stuff with fluid movement. So unless those growth factors are present from the muscle end and help then know you still you know IGF 1 and stuff like that there is pretty important for tending to you and so unless you’ve got some of those growth factors having a systemic effect in terms of the muscle and tendon muscle tendon complex and that could be the only sort of way I could see that happening I don’t I don’t see any other mechanism for it but that’s the thing is blood flow restriction and mechanical properties.”

“You’ve got your mechanotransduction pathways pretty much established for the likes of strain. But with BFR, we’re scratching our heads going, OK, if this does work, how does it work? And if it does work, that’s really exciting stuff. But I think the evidence is underwhelming in total.”

“We may be able to observe tendon adaptation not at high percentages of MVC is because if you’ve got a compliant tendon that’s able to stretch, you know, it’s not very well adapted. You could be straining it significantly enough to get adaptation, but at only a moderate amount, you know, 50, 60, 70 % of MVC as opposed to having to go to 80, 90, 100 [in a stiff tendon].”

Stiff and compliant tendons: “If somebody’s completely untrained, they’re not going to have well-developed mechanical properties of their tendons unless they’re genetically quite good.”

“When you’re untrained or relatively untrained, you may not need to go to those high percents of your MVC to necessarily get sufficient high strain to adapt.”

“A tendon will lose its stiffness over time if it’s not being loaded. So the only way to un-stiffen or de-stiffen or whatever.”

“If you want to make a tendon more compliant, give up the resistance training.”

Deeper vs. Shallow knee flexion: “So the force on the tendon or the stress on the tendon was the same, but the strain was different… The forces at the tendon were the same. But the strain was different because you’d passively strain.”

“Essentially what it does is it isolates the stretch away from the force. So it’s the effect of stretch is pretty much isolated at that stage because the two are intertwined usually.”

“Having the muscle tendon complex lengthened before you apply force to it could be or is additive in terms of mechanical properties.”

“Just put your knee in a more flexed position and use half of your maximum force capabilities versus you having to put 80 % in on a more extended position. But you’ll ultimately get a better adaptation with the more flexed position and a lighter relative effort.”

Stretching: “Maybe straining it passively might do something when you’re relatively untrained. But I think that’s a sliding scale away then… the higher training age you have or higher training status that you have like anything else that becomes harder to adapt.”

“Tendons are an anisotropic material, which means it’s fibrous and the fibers are aligned. When you’re doing shear wave elastography and those waves are propagating along those fibers. The fiber alignment is important for the measurement, essentially. So possibly with the fibers being absolutely all over the place in tendinopathy, I’m not entirely sure if that affects the validity of shear wave elastography.”

“If I’m going to hang my hat on wanting to know something from a measure, I’m not going to want it by tendon at rest because it’s got stress-strain curves, everything changes with load. And until you measure it, you don’t know what it’s going to look like.”

“Females tend to get a vast majority of their stiffness increases at lower force levels versus males which tended to seek higher increases in stiffness towards the top end of the force elongation curve… How that affects training prescription, one may lead you to think that for females you may need to do some heavier loading potentially to tap into that portion of the force elongation curve. Because essentially, if a lot of sporting actions are taking place up there, but you’re not getting the same increase in stiffness, maybe there could be some performance improvements to be made there.”

“Most people are primarily concerned with volume around the muscle and the tendon is just along there for the ride for whatever the muscles doing. You know, if they’re doing high volume for maybe muscle size or strength, stuff like that, you know, the tendons just ends up along for the ride in the middle of all that.”

Volume: “I tend to stick to just two to three sets of five, six reps… And there’s nothing I’ve ever seen that would lead me to believe that’s drastically inferior to any other type of training.”

Gerard on Twitter: https://x.com/gerdy_mac?lang=en

Gerard on IG: https://www.instagram.com/musclemechanicslab/

Podcast #130: Tendons with Gerard McMahon

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