Running is a trade-off between distance, time and energy. It’s long been thought that energy and distance are neatly linked, and that runners use a constant amount of energy over a given distance, regardless of the time it takes them to run that distance.
But a study in Current Biology challenges this assumption. In fact, say the authors, it’s runners’ relationship between distance and time that remains constant, not distance and energy.
In other words: unless they’re deliberately racing or training to get faster, people tend to run at a consistent average speed. While people speed up and slow down over the course of a run, their average speed between runs stays constant irrespective of distance.
“We intuitively assume that people run faster for shorter distances and then would slow their pace for longer distances,” says first author Dr Jessica Selinger, a neuromechanics researcher at Queens University, Canada.
When Selinger and colleagues examined data from both lab-based experiments and commercial fitness trackers, however, they discovered that this wasn’t the case.
The researchers examined data from 26 runners on treadmills, looking at their speed and oxygen consumption (from which they could deduce energy use).
They then compared this data with data from runners “in the wild” – using the commercial fitness tracker Lumo Run, which attaches to the waistband. This gave them information from 4600 runners going on 37,000 runs totalling 28,000 hours.
The researchers knew the demographics but not the identity of these users. They believe, based on pace, that most were casual rather than professional runners. “The vast majority of runners exhibited paces consistent with recreational runner status,” they write in their paper.
The researchers found that individuals tended to run at roughly the same speed, regardless of the distance they travelled. For example, if a runner took an hour to go on an 8km run, she’d take half an hour to go on a 4km run the next day. It was only after 10km of distance that average speeds slowed a little compared to shorter jaunts.
The lab-based experiments provided some reasoning for this: it’s all to do with energy conservation. Participants typically settled into a running speed that used the least amount of energy per metre.
“When you go out for a run, you run to have your best fuel economy,” says co-author Professor Scott Delp, a researcher at Stanford University, US. “So, regardless of the distance you travel, you run in such a way that you burn the least amount of fuel per distance travelled.”
In practice, what this means is that unless you’re deliberately trying to run faster or slower than usual, you naturally set a pace that minimises your energy usage. You don’t go faster just because you’re headed for a quick trip around the block, and similarly you don’t slow down if you know you’re headed out for a longer and thus more taxing run (unless it’s a much longer distance).
Selinger says that this makes biological sense.
“Minimising energy expenditure has evolutionary advantages – it allows us to move farther on fewer calories. We share this trait with other animals, be it flying birds, swimming fish, or galloping horses – there’s evidence that we all move in calorie conserving ways out in the wild,” she points out.
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This is consistent with other lab-based data and research on walking speeds, according to the paper, but it’s the first time it’s been shown to be true for runners at a large scale.
“Humans are amazing endurance hunters, so to see how modern-day humans run when they’re in the wild is a really interesting window into how our physiology is tuned, over millennia, to enable us to move over such long distances,” says Delp.
What if you want to speed up? According to Selinger, there are some tricks you can use to overcome your natural pace.
“Listening to music with a faster pace has been shown to help speed up stride frequency, which can then increase running speed,” she says. Running with faster people also can help.