How to maximise tendon adaptation for sports performance

As we reference in our first question, the interest on tendon adaptation and its link to sports performance and injury reduction has increased substantially over the last 5-10 years. So we asked Steph Lazarczuk six questions to get a better idea how we can improve our practice if this is our performance goal.

Why has there been such a sharp increase in tendon adaptation research over the last 5 – 10 years?

Until relatively recently, in the context of sports and exercise medicine, tendons were thought to be relatively inert. Once it was shown that they are, in fact, adaptable structures, there was a need to answer both the “how” and the “why.”

In our recent systematic review and meta-analysis, there were no intervention studies that predated 2000 (our inclusion / exclusion criteria are detailed in the paper).^[1] Since then, interventions have been more prevalent with the literature doubling in size every 7-10 years and showing no signs of slowing down.

Second, this is how research goes when there is an area that is topical. Practitioners and academic are constantly asking questions, then publications tend to spark ideas, which leads to more papers, more questions and so on. I’d say we’ve seen the same uptake in projects in other areas recently, too (hamstring injuries, injury prediction modelling, velocity-based training, etc.). I think the ever-expanding literature demonstrates that there are many unanswered questions surrounding tendons, both healthy and pathological. Not only that, but within this vast array of papers there is equally vast variability in the protocol design, which suggests there are multiple schools of thought trying to tackle the same problems.

I don’t think we’re quite done yet and should expect more discussion over coming years.

Based on your findings, what are the most effective training types to increase tendon stiffness?

Fundamentally, resistance training and jump-based training both increase tendon stiffness, but only resistance training demonstrated clear increases in stiffness, modulus (a material property) and cross-sectional area (morphology). This is shown in Figure 1.

The mechanisms explaining the increased stiffness in these training types are likely different. For example, time under tension (load duration) is variable. We might expect resistance training to be more effective because the duration of loading is typically longer than for jump-based strategies, whereas load frequency (rate of loading) would be higher but the duration of the loading period shorter. A longer duration at slower speeds can lead to greater deformation of tendon tissue and the transmission of external tendon strain to the tenocytes, which are key to the adaptation process.^[2–5]

This is not to say that jump-based training isn’t useful as the link to coordination; and the influence of muscle contraction on the tendon’s capacity to deform are highly interlinked. Prior work has shown that jump-based training might have minimal effect on tendon stiffness, but the ability of the muscle to remain relatively isometric during a ballistic task changes the overall stiffness of the musculotendinous unit (MTU).^[6] This is likely to be desirable during activity since this might have implications for MTU efficiency / metabolic cost, power output and the risk of muscle injury.

This links really nicely to the recent Sportsmith articles by Sean Maloney, who discussed the nuance of when training for whole limb stiffness is more appropriate, and Danny Lum, who described the use plyometrics in chasing tendon stiffness.

Does contraction type affect the way that tendons adapt?

This might surprise some people but no, contraction mode does not influence tendon adaptation. Adaptations were typically independent of contraction mode (Figure 1) for each key outcome measure.

I think some of this confusion relates to work in the tendinopathy space where it was suggested that specific contraction types had greater positive effects (e.g., decreased pain) in comparison to other contractions. However, tendons are non-contractile tissues which are exposed to load no matter what type of contraction you perform. They still fundamentally attach muscle to bone, so regardless of whether you are contracting through isometrics, concentrics or eccentrics, the tendon still transmits that force to the bone.

Having said that, some contraction modes might be better vehicles for delivering the required stimulus (high strain) to induce adaptation. High strain intervention studies used very high intensities (90% 1RM), and such intensities might best be delivered during isometric or eccentric actions where high loads are often better tolerated.

So, while the short answer is no, contraction mode is not demonstrably important itself, you still might want to consider how you achieve the more meaningful stimuli in your programming.

How does training intensity and training volume affect how tendons adapt?

Based on our meta-analysis, there was no clear effect for either intensity or training volume. However, the caveat is that the protocols used in the literature are highly variable. While we tried to control for this in our analyses, it was not possible to control for all training variables (e.g., exercise selection, range of motion, velocity, contraction duration), and this may confound results.

We also need to consider that the magnitude and volume of external loading during training doesn’t necessarily directly reflect the stress or strain on the tendon. Tendon loads are likely to vary between individuals, partly due to tendon geometry and morphology (e.g., cross-sectional area), and so the same exercise prescribed to the same intensity might well elicit different responses across individuals.

It’s also worth noting that the threshold for an appropriate stimulus might differ with age. Work by Quinlan et al.^[7] used 60% 1RM loads in adult and elderly populations and demonstrated improvements in tendon properties in both groups. However, while adults plateaued after four weeks of training, the elderly group continued to see improvements over the protocol duration. This might suggest that in an elderly population a lower intensity is sufficient to induce positive change, while in a younger, adult population, greater intensities might be necessary to avoid a ceiling effect to adaptation.

Based on your work and your answers above, could you give us an example of what an intervention would look like if tendon stiffness was the performance goal?

The first thing we need to recognise is that most of the studies in our review used recreational athletes or untrained individuals, and so we might expect the magnitude of adaptation in these populations to be greater than for highly trained individuals. In fact, in one study of already resistance-trained men, no change in stiffness was found following an accentuated eccentric loading programme,^[8] but there were changes in other muscle characteristics and properties, e.g., fascicle behaviour. This suggests that resistance training could be used to maintain tendon stiffness in well-trained populations while seeking other MTU adaptations relevant to your programming. 

In those with a shorter training history, lacking a regular resistance training programme or returning from a lay-off period (injury or restricted participation), resistance training would still likely be my first port of call. As a strategy, it provides clear changes in all tendon properties; and, because it serves multiple purposes within a training programme, we can target several adaptations simultaneously.

We already identified earlier that contraction mode isn’t important for inducing adaptation, but isometrics can be programmed to allow a longer strain duration and you can take these right through to maximal efforts. Eccentrics can also be programmed to have a longer contraction and have the added benefit of being able to use supramaximal efforts. These near maximal, maximal or supramaximal tasks might have a greater strain profile, based on studies that have estimated strain.^[2,9,10]

Interventions in our review were mainly 10-14 weeks in duration, with some individual studies reporting adaptations from four to eight weeks.^[7,11] As such, this is likely the lower boundary for duration, and it would be necessary to provide continued loading for some time to see changes in tendons.

What are the biggest mistakes you see young practitioners/clinicians make, and what advice would you give them to help?

My key advice would be to make sure you are identifying the adaptation you want to induce as your first port of call. This could be a physiological change or a movement competency or skill. From there, identify the stimulus required to provoke that adaptation. Then choose the exercise, task or drill that provides that stimulus.

For me, this is the absolute fundamental of programme design, whether it is for healthy or injured populations, and really links to the question I always ask myself and others: “Why are you doing the thing that you’re doing?” I think it instills and maintains a critical element to your programming, which can sometimes be lacking, particularly when we start drifting towards habit (which is a very human thing to do).

Deciding what adaptation you want often comes from a needs analysis process, and you can really get into the depth of what underpins movement skills.

This is why it irks me when sometimes people write off exercises because they’re not “functional.” If you’ve gone through the process and identified the adaptation you want, and you’ve chosen an exercise that delivers a specific stimulus, programme it in! If it serves a purpose, whether that is from an injury risk or performance perspective (preferably both), then it has a function. Back yourself and your justification.

I repeat: “Why are you doing the thing that you’re doing?”