Quantify the advantages of the design for runners

For a brief moment in 2017, drawing was a hot topic for runners. Eliud Kipchoge narrowly missed the two-hour barrier at the Nike Breaking2 marathon, and there was speculation about the alleged aerodynamic benefits of the large digital watch mounted on the speed car in front of him.

In the end, an independent analysis concluded that the car probably didn't make much of a difference. Instead, it was the runners themselves – rotating teams of six pacemakers in an arrowhead formation – that eliminated most of the drag. At least that's what some studies suggested almost half a century ago. But how much difference did the pacemakers actually make? No one could agree, and surprisingly little scientific data was available to answer the question.

The researchers apparently took note of it. A new study in the Journal of Biomechanics by 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 into the debate and provides more evidence that drawing really does The difference can also play a role for marathon runners.

The study uses a technique called Computational Fluid Dynamics to simulate the design approaches of Ethiopian star Kenenisa Bekele when he ran the Berlin Marathon 2019 2:01:41, just two seconds before Kipchoge's marathon world record. Bekele had three pacemakers side by side up to the 25 km mark. Based on a video of the race, the researchers found that Bekele spent most of the race about 1.3 meters back in one of three positions: behind the central pacemaker; behind one of the side pacemakers; or between two of the pacemakers.

These are the four positions:

(Photo: Journal of Biomechanics)

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

Drafting(Photo: Journal of Biomechanics)

What is important for a runner is the difference between the print on the front and the print on the back. Compared to running alone, running behind pacemakers reduces frontal pressure (less red) and increases pressure behind you (less blue). Interestingly, this means that the pacemakers themselves have a slight advantage if someone pulls behind them because the pressure behind them doesn't decrease as much. This is known to cyclists, but perhaps more surprising for runners: everyone benefits in a speed line, although the greatest benefits by far benefit the trailer.

The best of Bekele's three formations was when he was behind the central pacemaker, but with a tiny margin. These results were almost indistinguishable from running behind the side pacemaker. So you're wondering what the results would be for running behind a single pacemaker.

But running between two pacemakers wasn't nearly as good. According to the researchers' calculations, you feel a resistance of 7.8 Newtons that runs in calm air at a marathon pace of just over two hours (4:35 per mile). (In context, a medium-sized apple weighs about 1N, so imagine that the weight of a bag would pull apples straight back.) When walking between two pacemakers, the resistance drops to 4.8 N; If you walk directly behind a pacemaker, you will reach between 3.3 and 3.5 N.

What we really want to know, of course, is how much faster Bekele drove thanks to dropping these 3 or 4 Newtons. While Beaumont and his colleagues do not estimate the time, they do some calculations about how much energy he has saved. To do this, some assumptions must be made about how efficiently runners convert energy into mechanical force – a topic that remains controversial even among biomechanics.

I asked Wouter Hoogkamer, a biomechanic at the Massachusetts University Integrative Transportation Laboratory, about his thoughts. To answer the question: "How much time does it save?" If the question is correct, he proposes a slightly different three-stage approach that bypasses the debate on mechanical force:

  1. Calculate how much force is pushing you back. This study has done this using computational fluid dynamics, and the results of drag (approximately 4 N when pulling, 8 N without) are consistent with other estimates of drag when running.
  2. Find out how much additional energy runners need to overcome this force. This is the difficult part.
  3. Determine how much you need to slow down due to the extra energy you burn. This was the subject of a paper by researcher (and former Olympic steeplechase) Shalaya Kipp from the University of British Columbia (in which Hoogkamer and biomechanics Rodger Kram from the University of Colorado were involved). So it's a solved problem. If you know how much extra energy you use due to air resistance or how much you save due to the draft, you can calculate how much slower or faster you are driving at a given pace.

So the second step is the hard part. Imagine that you have attached a rubber band to your back that pulls you back very gently with a force of a few Newtons. How much extra energy do you have to spend to keep up your pace? Since running is such a complex movement, there is no obvious and easily calculable answer. Instead, Hoogkamer says, the most practical way to measure the relationship directly is to attach pulleys and rubber bands to a treadmill in the laboratory.

That's exactly what he and his colleagues did, but the results have yet to be published. An interesting preview detail: It turns out that some people can do this "better" than others. In other words, if you exert an increasing force with the elastic band, the energy consumption (estimated by the oxygen consumption) increases only slightly. Others have much larger increases. This suggests that, just like the controversial benefits of Vaporfly shoes, some people benefit far more from drawing than others.

Without this missing piece, the current study in my opinion cannot fully answer how much time Bekele has saved or lost through drawing. Nevertheless, it offers some useful comparisons between different design positions. Above all, walking back, but between pacemakers – as elite marathon runners often do, even if they set world records – is measurably worse than hiding right behind it. Of course, it is also less pleasant to be right behind you because your view is obstructed and you run the risk of getting tangled with the runner's recoil in front of you. However, if you want to get the greatest aerodynamic advantage, you have to get used to it.

To learn more about Sweat Science, visit me on Twitter and Facebook, subscribe to the email newsletter and read my book Endure: Mind, Body and the Strange Elastic Limits of Human Performance.

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