How Your Muscle Fibers Might Predict Overtraining


All runners, according to a popular school of training thought, can be divided into two categories: slow-twitch and fast-twitch. Physiologically, this idea rests on pretty shaky ground. The old view that there are only two types of muscle fiber has long been supplanted by the realization that muscle fibers exist on a continuum between explosive but easily fatigable and slower but more fatigue-resistant, and can even alter their characteristics with training. Still, the two-types-of-runner theory is one of those ideas that brings to mind the statistician George Box’s aphorism: All models are wrong, but some are useful.

That’s my takeaway from a new study in the Journal of Applied Physiology, from a group led by Phillip Bellinger of Griffith University in Australia. The study involved 24 highly trained middle-distance runners (16 men, eight women) who did three weeks of their normal training, three weeks of “overload training,” then a one-week taper, with various tests of physiology, performance, and health before and after each stage.

The big goal of the study was to look for indicators that warn if you’re at risk of overtraining. The three-week overload block involved boosting total mileage by 10 percent in the first week, 20 percent in the second week, and 30 percent in the third week. If all goes well, this type of increase should push you into a state of “functional overreaching,” which temporarily reduces your performance but then produces a big increase in fitness once you recover. The problem is that it’s a very fine balance. If you don’t push quite hard enough, you don’t overreach and miss out on the subsequent fitness jump; but if you push too far, you risk non-functional overreaching, which can set you back for weeks or even months.

Knowing when you’ve crossed that line would be very helpful, but studies so far have mostly failed to find consistent and reliable markers of overreaching. This particular study cast a wide net in search of potential markers. They tested running performance with an incremental treadmill-running test to exhaustion; they assessed subjective fatigue and psychological wellness; they measured resting metabolism and body composition; they looked at cardiac properties like heart-rate recovery; they took blood tests to assess 23 different markers of stress, inflammation, and so on.

The results that emerged from this mountain of data: pretty much nothing. Exactly half of the subjects were classified as functionally overreaching, based on a significant decrease in their treadmill performance after the overload block. The rest felt more tired than usual, but were able to maintain or even increase their treadmill performance, meaning they were merely “acutely fatigued.” Both groups lasted significantly longer on the treadmill after the one-week taper, which tells us that they weren’t non-functionally overreached. But there were no significant differences between the two groups in any of the measurements listed above.

There was one other measurement that produced better results, though. The traditional way of assessing muscle fiber composition (i.e. fast-twitch vs. slow-twitch) is with a muscle biopsy, which basically involves carving out a small chunk of flesh for analysis. High-level athletes aren’t big fans of this technique. (No one is, actually, but athletes have more reason than most to refuse to give away chunks of muscle.) In the past few years, a non-invasive method of estimating fiber composition has been developed, using a technique called proton magnetic resonance spectroscopy to quantify levels of a molecule called carnosine, which turns out to be roughly proportional to the amount of fast-twitch fibers in a given muscle. It’s not a direct measurement, but by comparing the spectroscopic measurements to what you see in the general population, you can get a sense of where someone lies on the fast-twitch/slow-twitch continuum.

Sure enough, the runners classified as overreaching had a significantly higher proportion of fast-twitch fibers. Or to put it another way, runners with more slow-twitch fibers were better able to handle a significant increase in training volume without overreaching, which suggests more generally that they’re better equipped to handle longer stints of higher mileage without starting down the slippery slope that leads to overtraining. Both groups were more slow-twitch than fast-twitch, as you’d expect from a group of endurance athletes, but the non-overreached group was farther along the continuum. (The data was expressed as z-scores: the overreached runners were shifted, on average, 0.44 standard deviations toward the slow-twitch end of the spectrum; the non-overreached runners were 1.25 standard deviations in the same direction.)

As it happens, this is more or less what traditional coaching wisdom would have predicted. Some people seem to thrive on lower volume and speed work, and regress when they try piling on too many miles. Others thrive on miles and burn out on speed work. These are the runners who would be labeled fast-twitch and slow-twitch, respectively, in the two-types-of-runners worldview. Now there’s some scientific evidence to back up the idea that (a) not everyone responds equally to the same training plan, and (b) some of that variation is associated with the characteristics of your muscle fibers. A crucial point: these differences aren’t necessarily self-evident. Prior to the study, the two groups were logging essentially the same amount of weekly mileage (42.4 miles in the group that ended up overreached, 43.7 in the other group), with nearly identical distributions of easy, medium, and hard running. 

There are a few caveats to note. One is that, in a study that tested dozens of possible ways of predicting overreaching, there’s always the risk that the one positive finding occurred merely by chance. We won’t know until this result is replicated in other groups. A somewhat worrying detail, in that respect, is that they tested muscle composition in the two muscles of the calf, the soleus and gastrocnemius, but only found a significant relationship between fiber type and training outcome for the gastrocnemius. The researchers suggest the gastrocnemius may be more important for your running stride, which is why its fiber type matters more—but the null result for the soleus should give us pause.

On a related note, it’s worth recalling a Belgian study I wrote about last year that claimed to have discovered that a hormone called GDF15 predicted overreaching in a study similar to this one. The new Australian study found no differences in GDF15 levels between their two groups. There are various possible explanations, including the fact that the new study used near-elite athletes whose GDF15 levels were already way higher than the recreational subjects used in the Belgian study. But it could simply be that when you test a big enough number of parameters, you always get some false positives.

Then there’s the practical side. This particular study looked at middle-distance runners who were averaging just over 40 miles a week in their normal training. It’s far from clear that muscle fibers tell you anything useful about whether a 10K runner who is already putting in 100 miles a week should increase their mileage. Elite running coach Steve Magness and others have argued that each event has its own continuum from slow-twitch to fast-twitch—which makes it tricky to figure out whether you’re, say, a slow-twitch miler or a fast-twitch 5K runner. After all, muscle fibers aren’t the only factors that determine what event you’re best suited for and what training you respond to.

In the end, the simple version of the muscle fiber story is still in the wrong-but-useful category. Yes, we don’t all respond best to the same training, even if we’re preparing for the same event. Yes, we’re each predisposed to do better in some events than others, and muscle fibers play a role. But they’re not destiny. A few years ago, researchers at CSU Fullerton published a study of two identical twins, one of whom injured his ankle in high school and stopped exercising and the other of whom became a marathoner and triathlete. They had the same genes, but at the age of 52 the endurance athlete had 94 percent slow-twitch fibers and the non-athlete had just 40 percent slow-twitch. There may be two types of runner, but with sufficient time and effort, you can choose which one to become.


For more Sweat Science, join me on Twitter and Facebook, sign up for the email newsletter, and check out my book Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance.

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