Quantifying the Benefits of Drafting for Runners

For a brief moment back in 2017, drafting for runners was a sizzlingly hot topic. Eliud Kipchoge had just narrowly missed the two-hour barrier in Nike’s Breaking2 marathon, and speculation was rampant about the supposed aerodynamic benefits of the big digital clock mounted on the pace car in front of him.

In the end, an independent analysis concluded that the car probably didn’t make much difference. Instead, it was the runners themselves—rotating teams of six pacemakers in an arrowhead formation—who eliminated most of the air resistance. At least, that’s what a couple of studies from nearly half a century ago suggested. But how much difference did the pacers actually make? No one could agree, and there was surprisingly little scientific data to shed light on the question.

Researchers apparently took note. A new study in the Journal of Biomechanics, from a group led by Fabien Beaumont at the University of Reims Champagne-Ardenne in France, is one of several recent attempts to bring new science to the debate, providing more evidence that drafting really can make a difference even for marathoners.

The study uses a technique called computational fluid dynamics to simulate the drafting approaches used by Ethiopian star Kenenisa Bekele when he ran 2:01:41, just two seconds off Kipchoge’s world marathon record, at the 2019 Berlin Marathon. Bekele had three pacers running side-by-side until the 25K mark. Based on video of the race, the researchers determined that Bekele spent most of that portion of the race in one of three positions about 1.3 meters (just over 4 feet) back: behind the central pacemaker; behind one of the side pacemakers; or between two of the pacemakers.

Here’s what those four positions look like:

drafting
(Photo: Journal of Biomechanics)

The simulation enabled the researchers to calculate the air pressure experienced in each configuration. Here are two visualizations of the results, with red indicating increased pressure and blue indicating decreased pressure:

drafting
(Photo: Journal of Biomechanics)

What matters to a runner is the difference between the pressure at their front and the pressure at their back. Compared to running alone, running behind pacemakers reduces the frontal pressure (less red) and increases the pressure behind you (less blue). Interestingly, that means that the pacemakers themselves get a slight advantage when someone drafts behind them, because the pressure behind them doesn’t drop as sharply. This is well known to cyclists, but perhaps more surprising to runners: everybody benefits in a pace line, though the biggest benefits by far go to the follower.

The best of Bekele’s three formations is when he was behind the central pacemaker, but only by a tiny margin. Those results were nearly indistinguishable compared to running behind the side pacemaker—which makes you wonder what the results would be for running behind just a single pacemaker.

But running between two of the pacemakers was not nearly as good. By the researchers’ calculations, you feel a drag force of 7.8 Newtons running in still air at just over two-hour marathon pace (4:35 per mile). (For context, a medium-sized apple weighs about 1 N, so imagine being tugged directly backward by the weight of a bag of apples.) Running between two pacemakers drops the drag force to 4.8 N; running directly behind a pacemaker gets you to between 3.3 and 3.5 N.

What we really want to know, of course, is how much faster Bekele went thanks to shedding those 3 or 4 Newtons. While Beaumont and his colleagues don’t give a time estimate, they do make some calculations about how much energy he saved. That requires making some assumptions about how efficiently runners convert energy into mechanical power—a topic that remains controversial even among biomechanists.

I asked Wouter Hoogkamer, a biomechanist at the University of Massachusetts Integrative Locomotion Lab, for his thoughts. To answer the “how much time does it save?” question properly, he suggests a slightly different three-step approach that sidesteps the mechanical power debate:

  1. Calculate how much force is pushing you back. That’s what this study did, using computational fluid dynamics, and its drag force results (roughly 4 N with drafting, 8 N without) are consistent with other estimates of air resistance in running.
  2. Figure out how much extra energy it takes for runners to overcome that force. This is the tricky part.
  3. Determine how much you have to slow down because of the extra energy you’re burning. This was the topic of a paper last year by University of British Columbia researcher (and former Olympic steeplechaser) Shalaya Kipp (on which Hoogkamer and University of Colorado biomechanist Rodger Kram were co-authors), so it’s a solved problem. If you know how much extra energy you’re burning due to air resistance, or how much you’re saving due to drafting, you can calculate how much slower or faster you’ll go at a given pace.

So the second step is the hard part. Imagine you’ve got an elastic band attached to the small of your back, tugging you very gently backwards with a force of a few Newtons. How much extra energy do you have to spend to maintain your pace? Because running is such a complex motion, there’s no obvious and easily calculable answer. Instead, Hoogkamer says, the most practical thing to do is measure the relationship directly by hooking up pulleys and rubber bands on a treadmill in the lab.

That’s exactly what he and his colleagues have done, but the results have yet to be published. One interesting preview detail: it turns out that some people are consistently “better” at this than others. In other words, as you apply increasing force with the elastic band, their energy consumption (as estimated by oxygen consumption) only goes up a little bit. Others have much bigger increases. This suggests that, just like the controversial benefits of Vaporfly shoes, some people will benefit far more than others from drafting.

Without that missing piece, I don’t think the current study can fully answer how much time Bekele saved or lost due to drafting. But it nonetheless offers some useful comparisons between different drafting positions. Most notably, running behind but between pacemakers—as elite marathoners frequently do, even when setting world records—is measurably worse than tucking directly behind. Of course, it’s also less comfortable to be directly behind, since your vision is obstructed and you risk getting tangled up with the back-kick of the runner in front of you. But if you want the biggest aerodynamic edge, you’ll have to get used to it.


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Original source: https://www.outsideonline.com/2415147/kenenisa-bekele-drafting-benefits-study?utm_campaign=rss&utm_source=rss&utm_medium=xmlfeed

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