the Limits of Performance

Usain Bolt is the fastest man alive when it comes to foot speed - at 28 miles per hour. Zoom, Zoom! But how many miles per gallon does he get? What is his caloric efficiency? Measuring the human equivalent of miles per gallon is tricky, since we each use different amounts of energy depending on what we are doing.

It's obvious that we burn more calories when we are running than if we are sitting behind a desk. The rate at which your body uses energy at rest is called your basal metabolic rate (BMR), and it varies from person to person depending on gender, weight, and other characteristics. The rate at which you use energy increases as your activity level increases. The ratio between your active metabolic rate at any given time and your BRM is known as your metabolic scope. The closer to 1 that number is, the closer you are to your resting rate; the higher that number, the more energy you're using. Humans like Usain Bolt generally max out at a metabolic scope of 5, but some animals can scope as high as 7.

Rephrasing the miles-per-gallon question then, how do you find the human body's maximum sustained metabolic scope - the point where the body's rate of energy use over time outweighs its ability to absorb food and turn it into energy. To answer the question researchers studied six runners in the 2015 Race Across the USA, a 20-week, 3,080-mile foot race from Los Angeles to Washington D.C.

In the study published in the journal Science, researchers measured the BMRs of the six runners by having them drink water that was enhanced with harmless but rare isotopes of hydrogen and oxygen - specifically, deuterium and oxygen-18. As the isotopes came out in the runners' sweat, urine, and breath, the researchers could measure how much carbon dioxide they produced and, therefore, how many calories they were burning. The team made these measurements before the race began, during the first week, then during the final week.

What they found was that the runners' metabolic scope went from 1.8 before the race to 3.8 after a week of running. But by week 20, it had leveled off to 2.8. From the data, the researchers could draw the obvious conclusion that the plateau was due to the runners' bodies simply using less energy. Some of that was due to them just losing weight and running fewer miles per day as the race wore on, but the remainder - about 600 calories per day - couldn't be explained by any obvious factors. Their bodies, it seemed, were adjusting to ensure they could cover the long road ahead. If they'd kept at their original energy usage, the researchers write, they would have petered out around 10 weeks. But they didn't.

The researchers analyzed the runners' data alongside data that had been collected from similar long distance endurance events, including the Tour de France, triathlons, shorter ultramarathons, and Arctic expeditions. In all cases, the participants' metabolic scope started high, then plateaued after about 20 days to settle around 2.5. After that plateau, the human body has to turn to other sources of energy besides food - namely, its own fat stores.

After the recent astounding 1:59:40 record marathon time by Eliud Kipchoge in Vienna, many of us are thinking about the limits of performance, not only in the marathon, but in whatever sport we consider. The studies above of metabolic scope serve to remind us that there are indeed physiological limitations to performance. Improvements in athletic performance will necessarily be by thinner and thinner margins. The body, in its wisdom, anticipates need over long endurance events and moderates caloric demand to enable an extended performance. Those of us who are practicing athletes are already aware that the body, like an automobile, has a set capacity that can only fuel so much effort before we either have to resupply calories (food) or begin the less efficient metabolic process of breaking down our own fat reserves. All we can do is hone these limitations and perform within their parameters. We will always only be able to go so far and so fast. After all, we're only human!