The Limits of Ultrarunning

Tigst Assefa broke the marathon world record by more than two minutes in Berlin last month. That’s mind-boggling. Then again, Courtney Dauwalter broke the course record at the Western States 100-Mile Endurance Run, one of the most prestigious and historic ultra races in the world, by more than an hour. There are still some unanswered questions about what limits endurance in “regular” long-distance races—but when it comes to ultrarunning, we’re still trying to figure out what the right questions are.

This is the blank spot addressed by a big new paper in the journal Sports Medicine, titled “Limits of Ultra: Towards an Interdisciplinary Understanding of Ultra-Endurance Running Performance.” That’s a mouthful, but the key word in there is “interdisciplinary.” Ultrarunning success isn’t just about having a high VO2 max or great running economy—though both those attributes are helpful. It’s also not just about being mentally tough, or hydrating appropriately, or having an iron stomach. It’s all of those things and more, which is why the paper brings together the insights of nine authors from a wide range of scientific disciplines, led by Nicolas Berger of Teesside University in Britain. Here are some of the highlights:

Beyond “Born to Run”

The first point the authors make is that ultrarunning is not natural. Sure, there’s some neat evidence that evolution shaped us to run long distances. But the demands of modern ultrarunning, both in training and competition, are far more extreme than the persistence hunting or scavenging our ancestors might have done. As a result, they write, “we should not view ultra-endurance running as the ultimate expression of our evolved physical nature, nor ultra-endurance runners as proxies for our ancestors.”

In fact, the most relevant evolutionary edge we have might be “phenotypic plasticity,” which is another way of saying that our physical abilities—and indeed our bodies—change dramatically in response to the demands we impose. Maintaining muscle mass, red blood cells, capillaries, and mitochondria burns energy. Since we evolved to conserve energy, we automatically cut back on those traits when we don’t train. But phenotypic plasticity also enables us to respond to extremely high volumes of training, far above what our ancestors ever encountered, that improve ultrarunning performance beyond what evolution ever “intended.”

Does Oxygen Matter?

Models of marathon performance focus on three key parameters: VO2 max, running economy, and lactate threshold. Together, these parameters determine whether you’re able to deliver enough oxygen to your muscles to maintain your desired pace. They don’t tell you everything about who’s going to win a marathon and how fast they’ll run, but they offer a pretty good first approximation.

It’s not clear that the same approach should work for ultramarathons. Oxygen supply and demand matter most when you’re running faster than critical velocity, which corresponds to about 90 percent of VO2 max. Marathons are run very close to this speed, but ultra races are far slower, typically between about 40 and 70 percent of VO2 max depending on the distance. If your muscles are short on oxygen halfway through a 100-miler, you should in theory be able to solve that problem by simply breathing a little harder. And indeed, studies have generally found that factors like running economy and VO2 max don’t predict ultra performance very well.

There are some caveats, though. Many ultra races take place on extremely challenging terrain. When you’re climbing steep hills, you might well approach or exceed critical velocity. If the race is at altitude, there will be less oxygen to inhale. And your critical velocity will almost certainly get slower as the race proceeds, because your form will begin breaking down, your fuel stores will get depleted, and your breathing muscles will get tired. So oxygen does matter, but it’s not the dominant factor it is in, say, a 5K.

Hot and Thirsty

In some ways, overheating should be less of a problem in ultras than it is in shorter races. The heat produced by your body is proportional to the intensity of exercise, so hammering a 10K is way more likely to overheat you than jogging a 100-miler, all else being equal. Of course, all else isn’t equal. Ultras are often held in places like Death Valley, where you can get heat stroke just sitting there. And the long duration means you might be out in the sun for many hours. That makes deliberate heat acclimation, which Berger and his colleagues suggest should involve at least five consecutive days of training in hot conditions, a good move if you’ll be racing in the heat.

The biggest overall risk for overheating, the authors conclude, is if you get dehydrated enough to compromise sweating and blood circulation, which are key intrinsic cooling mechanisms. Of course, hydration is a hot-button issue in its own right. The fastest runners at Western States tend to finish about 3.5 or 4.0 percent lighter than they started—but not all of that weight loss is water. When you’re running that far, the internal fuel stores you burn are also substantial. By one estimate, that can reduce your starting weight by 2 percent over a 100K run.

How do you strike the right balance? The two main options are pre-determined drinking plans and simply drinking to thirst. The authors lean toward the latter plan, mainly because it reduces the potential risk of hyponatremia. If you do make a drinking plan, you should measure your own sweat rate in advance, and build in the expectation that you’ll lose some weight as the race progresses.

Gut Check

Ann Trason famously called ultrarunning “an eating and drinking competition.” The reason is that virtually all competitors experience some gastrointestinal symptoms, and for some they’re serious enough to compromise performance or even force a DNF. This is really where the limits of ultrarunning diverge most sharply from shorter races: there’s a whole skillset here that 10K runners can totally ignore.

The underlying problem is that, during exercise, your body diverts blood away from the gut to keep your running muscles happy. Shutting down digestion is OK for half an hour, but it’s not an option for a full day. So you have to try to keep fueling despite the fact that your blood-starved intestines aren’t working properly. Their leakiness lets toxins into the bloodstream, which may be what triggers nausea. Pre-race nerves make it worse, as do certain types of food. There are no magical cures, but there are ways of minimizing your risks; for more details, check out this deeper dive.

Not all fueling problems are the gut’s fault, though. Flavor fatigue is also an issue, the authors point out, due to “overstimulation of taste receptors.” Marathoners are often heartily sick of the flavor of gels after a few hours, so I can only imagine how you might feel 20 hours into a race. There’s an obvious solution—just eat something else!—but balancing palatability and digestibility is a major challenge.

Two Kinds of Fatigue

The final section of the paper makes a distinction between two kinds of fatigue (or, in their jargon, fatiguability): performance and perceived.

Performance fatigue is what you see if you directly measure muscle strength: after 20 hours of ultrarunning, for example, your maximum leg strength will decline by 35 to 40 percent. But you don’t need maximum strength, or even 60 percent of maximum strength, to run at ultra pace. Your legs are still capable of going on.

Perceived fatigue, on the other hand, is about how you feel—most notably, it’s expressed in the sensation of how much effort you need to put out to maintain a given pace. It’s not independent of performance fatigue: if your muscles lose 40 percent of their strength, your brain will have to send a stronger signal to them to keep your pace constant, which will increase your sense of effort. But it also incorporates other inputs: the cognitive fatigue of staying alert, sleep deprivation, fatigue in your breathing muscles, hunger, thirst, overheating, and so on. This perceived fatigue, ultimately, is what usually mediates your decisions to speed up, slow down, or drop out. That’s true for shorter endurance races too, but for ultras the contributions from all the other non-muscle-and-oxygen stuff are much greater.

In the end, there’s no grand theory of ultra limits. Perhaps the very nature of ultras dictates a messy and multifaceted picture. But there is one sentence in the paper’s conclusion that caught my attention: “Enhancements in ultra-endurance running performance,” the authors write, “will likely come from advancements in managing the deterioration of the systems outlined in this review.” I think that’s the key: scientists can measure whatever they want in the lab, but those measurements are all but meaningless after 12 hours of running. The true frontier of ultrarunning—the one that record-breakers like Courtney Dauwalter are still pushing back in leaps and bounds—is managing the deterioration.

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.