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Excellent question! In Coates says he was a constantly injured runner until he changed his breathing pattern. He writes that research shows,

Impact stress is greatest when your foot strikes the ground at the beginning of an exhalation. This is because when you exhale, your diaphragm and the muscles associated with the diaphragm relax, creating less stability in your core. Less stability at the time of greatest impact makes a perfect storm for injury.

Coates goes on to write that belly breathing, or breathing from your diaphragm, will allow you to take in more oxygen with each breath. Breathing from your chest muscles, he continues, is not ideal because those muscles will fatigue faster than the diaphragm. 

Coates insinuates that switching from a 2-2 pattern of breathing (breathing in for two foot strikes, then out for two foot strikes) to a 3-2 or a 2-1 pattern can help prevent injury and possibly even make you faster.

Here’s the rub: The research Coates cites is from by University of Utah biologist , and according to Bramble, Coates got it all wrong.

“It is true that when your foot lands at the beginning of expiration, the stresses on your leg and trunk are going to be a little bit greater,” Bramble says. But he doesn’t know of “any study that shows a relationship between risk of injury and breathing patterns in runners.” Furthermore, this week Bramble and his colleagues suggesting 2-2 is the most advantageous breathing pattern for runners.

A three-count, or 2-1 pattern, Bramble wrote in an email, “should actually increase the impact stress on the leg that corresponds to exhalation because a shorter exhalation time means higher abdominal and thoracic pressures and thus a stiffer body when landing. The stiffer the body, the higher the impact stresses.”

Using a 2-2 pattern, Bramble says, is ideal because it reduces the amount of work the diaphragm must do. The longer you can keep your diaphragm going strong, the better, Bramble says, because as the diaphragm tires out, the body uses your less efficient chest muscles more.

So Coates was right on that point—that belly breathing is the most efficient way to breathe, filling the lungs with more oxygen than chest-dominant breathing. However, Bramble says it’s not true that a relaxed diaphragm creates less core stability. “The diaphragm is not a muscle that contributes to mechanical stability of the body,” he writes. But the chest and abdominal muscles are, and they contract when you exhale.  “From biomechanical perspective, the runner’s body will be more “stable” at foot impact during the expiratory phase of breathing, not the inspiratory,” Bramble writes.

Additionally, runners must always use their chest to breathe, and chest muscles, Bramble writes, are likely more resistant to fatigue than the diaphragm. Diaphragmatic fatigue, rather than chest fatigue, can be a limiting factor in long-distance running

Here’s how 2-2 may help stave off diaphragmatic fatigue: “Every time your foot hits the ground, your guts want to come out of your crotch,” Bramble says. Your guts are attached to your diaphragm by way of your liver, so every time your foot hits the ground, those guts pull your diaphragm down. “That’s exactly the direction the diaphragm has to go in order to power inspiration, so if you time it right and your foot hits the ground right at the end of expiration,” Bramble says, “you can take advantage of that slogging of your guts to power the first part of inspiration and reduce the workload on the diaphragm.”

THE BOTTOM LINE: Currently, there’s no published study that connects breathing patterns to injury in runners. A new study suggests that, if there is a best breathing pattern, it would be 2-2 (breathing in for two foot strikes, then out for two foot strikes).