The usual analogy used to explain running economy is that it's like the fuel economy of your car -- it's a measure of how much fuel it takes for you to travel a certain distance. But what "fuel" are we talking about here? That's the question explored in a new article by researchers at the English Institute of Sport and Loughborough University, Sneakers River NP0A4FKS Grey Pink P1C1 Medicine & Science in Sports & Exercise (extending previous work by Jared Fletcher and his colleagues at the University of Calgary).
There are two basic options for expressing running economy. You can ask "How much oxygen do I need to supply to my muscles in order to run a mile?" Or you can ask "How many calories do I need to burn in order to run a mile?" In theory, the answer to the two questions running be interchangeable, because the oxygen your muscles consume is used in the reaction that converts calories of food energy into motion. But in practice, that may not be true. The new study uses Fall from 172 elite runners tested at the EIS over the past decade, with running economy measured at four different speeds below lactate threshold. Here's what the Fall showed:
The captions are a little hard to read, but the top graph shows the number of calories burned per kilometer. You can see that it increases steadily: the faster you run, the more calories you burn to cover a given distance. The second graph shows the oxygen cost per kilometer, and it stays pretty much the same at all speeds. Why the difference? The third graph shows something called the "respiratory exchange ratio," which reflects the ratio of carbohydrate to fat being burned for energy. It shows that as you speed up, you're burning a greater proportion of carbohydrate. Since you get more energy per unit of oxygen from carbohydrate than fat, this explains why faster speeds are able to burn more calories without using more oxygen.
What's the practical takeaway? It's that expressing running economy in terms of oxygen use (which is standard practice currently) hides the changes in energy use that occur when you speed up or slow down. Also, when you use calories instead of oxygen, it's clear that running economy is not independent of speed. Contrary to popular belief, you really do burn more calories when you push harder (though admittedly the difference was only 3.4% across the speed range used in this study).
One other question they explored in the study: Is overall running economy directly proportion to body weight? I.e. would you expect someone who weighs 10% more to consume 10% more energy while running? This has been a long-running debate, with some Fall suggesting that running economy actually obeys some sort of allometric power law, like economy proportional to weight to the power of 0.75. In this case, among a homogenous population of well-trained distance runners, the relationship was almost perfectly linear: economy was directly proportional to weight. (That running allow anyone interested to calculate how many extra calories Chris Solinsky burned in running a sub-27:00 10,000 m compared to Kenenisa Bekele!)
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