Sweat Science

Why Fitness Trackers Should Measure Your Breath Rate

Scientists make the case for a new real-time measure of endurance effort

Breathing rate offers a surprisingly accurate estimate of how hard you’re working. (Davide Illini/Stocksy)

If you could design the wearable tech of your dreams for endurance training and performance, what primary data would you choose to display on the face of the device? Would it be heart rate? Heart rate variability? Cadence and stride length? Real-time pace? Mechanical power? Blood lactate level? VO2?

The possible choices are pretty much endless, and each of those options has pros and cons that provide fodder for spirited debate. But there’s another, less heralded pick that doesn’t get much attention—and according to a recent article in Frontiers in Physiology, it may trump them all. Well-known cycling expert Louis Passfield of the University of Kent’s Endurance Research Group, along with Italian researchers Andrea Nicolò and Carlo Massaroni, make the case for “respiratory frequency,” or breathing rate, which is simply how fast you’re panting.

The gist of the argument is that breathing rate offers a surprisingly accurate estimate of how hard you’re working—something more typically quantified by asking an athlete to subjectively rate their effort on a scale of 1 to 10 (or 6 to 20, an effort scale that is more often used for historical reasons). Unlike VO2 (the amount of oxygen you’re using, which requires fairly cumbersome lab equipment to measure), breathing rate is easy to measure with a chest strap, although most current wearables don’t bother to record or display it. And unlike heart rate, your breathing rate responds rapidly to sudden changes in effort like you’d see during an interval workout.

Normally, respiratory physiologists focus on the total amount of air you’re breathing in and out, which depends on both your breathing rate and the depth of each breath. But those two factors have distinct behaviors during exercise. The size of each breath increases primarily in response to the metabolic disturbances in your muscles and blood, like the rising levels of lactate produced by hard exercise. Your breathing rate, on the other hand, seems to depend more on the outgoing signals from your brain to your muscles. And that’s important, because perception of effort—according to some but definitely not all scientists—is also determined by the magnitude of these outgoing signals from the brain.

The scientific debate about the neuroscience of “effort” is interesting, but from a practical perspective, what’s more relevant is that, empirically, effort and breathing rate seem to be very tightly linked. This remains true, the authors of the new paper argue, even when you change the conditions in various ways, such as raising body temperature, prefatiguing your muscles, and so on.

And breathing rate responds so rapidly that it’s a good marker even during sprint interval training. Here’s some data showing the relationship between breathing rate and perceived effort during 30 minutes of cycling continuously or during sprint intervals of 40, 30, or 20 seconds. The breathing frequency is presented as a percentage of maximal, all-out breathing frequency:

(Frontiers in Physiology)

In addition to real-time monitoring of breathing rate, there’s a lot of sophisticated post-workout analysis you can do with it, as the paper (which is free to read online) goes on to demonstrate. You can analyze how much of a workout was spent in different breathing zones. You can get an overall picture of how hard a workout was by calculating your average breathing rate for the entire session. Since breathing rate responds so much more quickly than heart rate, this average will give you a more accurate view of your session’s difficulty, especially if you had lots of pace changes.

There are, however, some challenges to using breathing rate as a training indicator. While the research is conflicting, sports like running and rowing tend to have somewhat different breathing responses, because some athletes unconsciously match their breathing rate to their stride rate—a phenomenon known as entrainment. In swimming, you have to sync your breathing to your strokes to avoid drowning. Obviously that’s not the case in running, but the jarring impacts of each step do shake your breathing muscles and make it more likely that you’ll synchronize your breathing to some multiple of your step rate, though there’s a lot of variability from person to person. That means breathing rate may be pulled up or down by your stride rate, rather than being perfectly proportional to your sense of effort. For those reasons, analysis of breathing rate may be best suited for cycling.

There’s also what I think of as the “I’m aware of my tongue!” problem, in reference to a 1963 Peanuts cartoon in which Linus suddenly becomes uncomfortably aware of the presence of a strange object in his mouth. Breathing rate is something you can choose to control, if you want. Unless you’re really maxed out, you can choose to slow your breathing down (and your body will automatically compensate by breathing more deeply) or speed it up (resulting in shallower breaths). My general opinion is that you’re better off letting your body dictate when and how you breathe, but once you start tracking your breathing rate and perhaps even choosing to accelerate or decelerate based on its value, I suspect it will be hard to avoid becoming uncomfortably aware of your ability to alter it.

So, to return to the question I posed at the top, if I were designing my dream wearable tech, it probably wouldn’t have breathing rate as the primary display screen. Still, Passfield and his colleagues make a strong case that the metric should be in the conversation. If it’s easy to build the measurement into future wearables, it seems like an interesting extra source of data to slice and dice after a workout.

The authors note one other point in passing: “Anecdotally, athletes report monitoring the breathing sounds of their opponents as a gauge of their physical effort during endurance competitions.” If, as the research seems to suggest, breathing rate really is a subtle gauge of effort, this means you can also monitor the breathing of your competitors, listen for changes, and be confident that you’re getting some real insight into how much longer they can hold on. Unless, of course, they’ve read this article, too.

My new book, Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance, is now available! For more, join me on Twitter and Facebook, and sign up for the Sweat Science email newsletter.

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