The Tortoise, the Hare, and the Kilocalorie: You Burn the Same Number of Calories Running Fast or Slow

FASTER: science of triathlon, science of running, running forces

One of the more interesting, and perhaps unbelievable, scientific principles brought up in FASTER is the fact that a human uses the same amount of energy to run a given distance no matter how fast he or she runs. Seriously! It may seem harder to run faster (and it is) but it’s not because it takes more energy. This has some consequences for runners who run to lose weight, doesn’t it? Let’s take a look.

The generally accepted rate of caloric burn in the scientific community is that it takes 1 Kilocalorie to move 1 kilogram of mass 1 kilometer (1kCal/kg/km). This is also known as “Margeria’s Law,” after the scientist who first observed the trend in research conducted in 1963. Today’s blog post is a quick math exercise to show that it works.

FASTER: science of triathlon, science of running, running forces

Let’s set up our experiment. We have a triathlete weighing 148 lbs. He’s going to run two miles on the track. His pace for the first mile will be 8:00 min/mile, the second at 6:00 min/mile. Because we’re on a flat we assume 0% grade on the course. To quickly make the necessary rate conversions:

148 lbs = 67.1 kg
1 mile = 1.61 km

According to Margeria’s Law:

(1 kCal / 1kg / 1km) x (67.1 kg) x (1.61 km) = 108.1 calories burned per mile

Keep in mind that this is true regardless of our running speed. Now let’s compare that to more familiar methods of calculating our calorie burn. We’ll use that mainstay metric of triathletes everywhere: VO2! To calculate caloric expenditure by that route, we’ll refer to the approved equations by the American College of Sports Medicine. There are quite a few steps involved in this process, so we’ll break it down.

First, we have to convert our running speed to meters per minute (m/min). Our pace for our two miles then becomes:

8:00 min/mile = 201.2 m/min
6:00 min/mile = 268.2 m/min

The first thing we do is to calculate the required VO2 for our triathlete to run at the designated speeds. The equation is:

VO2 = (0.2 x speed) + (0.9 x speed x grade) + 3.5.

Plugging in our numbers (remember road grade is 0 here, so the second term is also 0), we get a VO2 of 43.5 mL/kg/min for our slower mile and 57.1 mL/kg/min when we pick up the pace on the second mile.

Our second step is to calculate how much oxygen we use per minute by multiplying our VO2 by our mass. Notice that the VO2 term has a “per kilogram” term in it. Multiplying it by our total number of kilograms eliminates that term and tells us how many total milliliters of oxygen we use each minute. For the first mile, our 67 kg triathlete uses 2920.2 mL/min. On the second mile, he uses 3835.8 mL/min. To convert this to an actual caloric expenditure per minute, we divide by 200.

2920.2  200 = 14.6 cal/min
3835.8  200 = 19.7 cal/min

Now all we have to do is multiply these rates by the amount of time it took to complete each mile

14.6 cal/min x 8 minutes = 116.8 calories
19.7 cal/min x 6 minutes = 115.1 calories

In terms of calories, there’s virtually no difference between “base” pace and “all-out effort”. Why? It’s a rate thing. To simplify the math, it’s the difference between burning 500 cal/min for 10 minutes, or 1000 cal/min for 5 minutes. Half the time, twice the burn rate. Either way it amounts to the same thing. While time and rate change in relation to each other, the one thing that remains constant is the distance.

Furthermore, after all that math we’re only 7 or 8 calories off of what Margeria’s Law predicts. The error is approximately 7%. Considering that there are multiple other methods of calculating energy expenditure and that actual results will vary based on factors unique to the individual, we’re right on the money.

There you have it! Proof that scientists don’t always make things more complicated. In this case, the physicist makes it easier than the physical trainer!

But why? If it takes the same amount of energy to run fast and to run slow, why is running fast so much harder? Learn why in FASTER.

If you’re interested in getting faster, you’ll be fascinated by FASTER: Demystifying the Science of Triathlon Speed. In Faster, astronautical engineer and triathlon journalist Jim Gourley explores the science of triathlon to see what truly makes you faster—and busts the myths and doublespeak that waste your money and slow down your racing. With this knowledge on your side, you can make easy changes that add up to free speed and faster racing.