Cumulative Overload: myTF’s model for tendon adaptation to running

A couple of months ago I released a new model to predict tendon adaptation to training. The feedback from runners has been overwhelmingly positive, so let’s dig into why tendons need their own overuse injury model, and how it’s implemented in myTF.

The science of tendon adaptation to load

In a previous post I wrote about why the acute:chronic workload ratio (ACWR) can miss some important types of running injuries. Some tendinopathies and bone stress reactions develop too slowly for the ACWR to identify that we might have been doing too much too soon, for too long… And that’s not a small issue: 3 out of 4 running injuries are related to tendons or other connective tissues: Achilles / patellar / hamstring tendinopathies, plantar fasciitis, ankle sprains (Dias Lopes et al. 2012), so the ACWR’s blind spot for injury prediction can be significant for runners.

Research shows that meaningful changes in tendon stiffness require 2 to 3 months of consistent loading (Kubo et al. 2012). That’s much longer than the time it takes for our aerobic system to improve, and this mismatch in adaptation speed is a problem for runners: we feel ready to push on long before our Achilles or plantar fascia have managed to catch up. The figure below shows that tendon stiffness barely changes after 1 month of training, increases slightly in month 2, but only shows substantial improvements after the 3^rd month of training.

Why is the slow adaptation time of connective tissues such a problem for runners? Marathon training plans typically ramp up mileage and speed for around 15 weeks (= 3.5 months), so by the time we start tapering, our tendons have only just adapted to the plan’s first weeks! The net result is that most of the marathon training ramp up happens before our connective tissues are ready for it.

But it’s not just marathoners who need to pay attention to the tendons’ delayed adaptation: busy lives mean that our mileage can vary a lot week-to-week or month-to-month, and it becomes easy to lose track of what our connective tissues are really ready for. Unfortunately, tendons will loose that slow and hard-earned adaptation within a few weeks of reduced activity (Kubo et al. 2010; Frizziero et al. 2011), which is one of the reasons why we shouldn’t quickly return to intense training after an extended break, or a period of lower mileage… Some studies suggest that the slow response of the tendons to training becomes even worse as we age, due to changes in the structure of the tendons (Narici & Maganaris, 2006). So this is something we need to be increasingly mindful of over time.

A 7:28 day ACWR ratio simply cannot see that a training block has been quietly outpacing tissue adaptation for two months. It’s the pattern behind most slow-developing running injuries such as tendinopathies and bone stress reactions: the pain arrives several weeks after the loading that caused it, sometimes after training has already steadied, or even reduced as that initial niggle developed into something worse.

Cumulative overload in myTF

The cumulative overload chart is on myTF’s Trends page. It tracks your training against that slower clock, so it moves slowly by design. One big week barely nudges it, but a multi-week build that consistently outpaces tissue adaptation will push it up and hold it there.

Reading it is straightforward, and follows myTF’s colour coding:

• Blue: you may be losing adaptation, worth knowing before jumping back to old volumes.
• Green: your recent training is consistent with what your connective tissues have adapted to.
• Orange: they’re still catching up, which most runners pass through during a build.
• Red: training is well ahead of adaptation, and an easier week or two should let it catch up.

Something unexpected came up in the user feedback after launching this new model. Matt P. is currently training for his first marathon and learning how his body responds to these long training blocks. Matt noticed periods where he felt bad, sore all over, low energy, tired all the time, and in his words “just generally felt beat up“. When the cumulative overload chart appeared, he went back through his log and found that those rough patches coincided with the peaks on the chart. As he put it, “no other tools that I use capture this aspect of training“. And it makes sense mechanistically: the cumulative overload model integrates your training over weeks, which is also the timescale for general fatigue to accumulate. A training plan that has been quietly running ahead of connective tissue adaptation has usually also been running ahead of everything else, so the chart tends to be high during exactly the stretches where you might feel particularly exhausted.

We need to bear in mind that the cumulative overload model only sees your training, and it knows nothing about your sleep, your stress at work / home, your iron levels or whether you’re coming down with something, so it cannot tell you why you feel rough on a given day. What it can do is show you whether the training itself is a plausible explanation, which turns out to be useful on its own. Matt described it as eliminating some of the questions around how he was feeling on any given day or week: am I getting sick, am I injured in some way, were the past few weeks just too much?

That framing is closer to how I think the whole app should work, not a verdict but just one more piece of context that runners can probe and interrogate. Or as Matt put it, “a great tool to have in the toolbox when you are trying to get a picture of the current state of yourself and your training for an event“.

If you’ve ever had tendon niggles or injuries then feel free to check if your Cumulative Overload chart could have helped you avoid doing too much too soon for too long, and let us know in the comments!

References

Frizziero A, Fini M, Salamanna F, Veicsteinas A, Maffulli N, Marini M, 2011. Effect of training and sudden detraining on the patellar tendon and its enthesis in rats. BMC Musculoskeletal Disorders. 12:20. DOI: 10.1186/1471-2474-12-20

Kubo K, Ikebukuro T, Maki A, Yata H, Tsunoda N, 2012. Time course of changes in the human Achilles tendon properties and metabolism during training and detraining in vivo. European Journal of Applied Physiology. 112(7):2679-2691. DOI: 10.1007/s00421-011-2248-x

Kubo K, Ikebukuro T, Yata H, Tsunoda N, Kanehisa H, 2010. Time course of changes in muscle and tendon properties during strength training and detraining. Journal of Strength and Conditioning Research. 24(2):322-331. DOI: 10.1519/JSC.0b013e3181c865e2

Lopes AD, Hespanhol Jr LC, Yeung SS, Costa LO, 2012. What are the main running-related musculoskeletal injuries? A systematic review. Sports medicine. 42(10):891-905. DOI: 10.1007/BF03262301

Narici MV, Maganaris CN, 2006. Adaptability of elderly human muscles and tendons to increased loading. Journal of anatomy. 208(4):433-43. DOI: 10.1111/j.1469-7580.2006.00548.x