The Science Behind Stryd, the World’s First Running Power Meter
It's nothing like a cycling power meter and it aims to change not only runners' physiology but their form. Which, in the end, matters much more.
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When a Boulder, Colorado-based startup called Stryd launched its Kickstarter campaign to pre-sell almost 1,600 of its prototype running power meters, it met its goal in just 12 days.
That’s high demand for a $150 product the running community doesn’t yet know how to use. But here’s the thing: neither does Stryd.
Our “fundamental challenge right now is lack of knowledge,” co-founder Li Shang says. Stryd relies on data for two essential needs: to calibrate the software that powers it; and to determine the value of a power number to runners. “We can’t do that ourselves. We need the running community to help us,” he says.
That's a bit like a bike maker creating a brand-new prototype and asking its customers both how to ride it and whether it's any good. The running community is tasked with answering the questions: What exactly is Styrd for? And do we need it?
Cyclists use power because it’s an objective measure of workout intensity that doesn't drift or vary. Heart rate is the response to exercise—it’s a valuable metric, but can change based on everything from how much training you’ve done to whether you had a beer the night before. So far, people have compared Styrd to a cycling power meter, but that's an ultimately flawed assumption. The two metrics are very different.
“I didn’t know anything about it at first, but I thought we’d use it like a cycling power meter. It’s not,” says Mike Ricci, founder of D3 Multisport coaching, and USA Triathlon’s 2014 Coach of the Year.
Ricci has used Stryd in limited testing with several athletes. He believes its biggest effect won't be on physiology, but on form and efficiency—two stats that until now have been hard to quantify.
“Think of a runner like a rubber ball,” says Lim. Throw the ball down the road and it’ll bounce efficiently, like a runner with great form. But throw the ball at a sharp angle, and you’ll need to throw a lot harder to get that ball to travel the same speed as the first ball.
To understand that, you have to understand what Stryd is and what it’s designed to measure. Cycling power meters operate on the same basic technology: a strain gauge measures the deflection in a metal part, like a crankarm or wheel hub or pedal axle, in a mechanical system. In this case, we're talking about the drivetrain. By knowing the properties of the metal part and the system, you can correlate the amount of deflection with a level of force required to produce it. And velocity is cadence.
Then you plug those numbers into a simple equation:
force x velocity = power
Because cycling involves pushing pedals in a rigidly defined circular motion, metabolic efficiency differences between athletes are narrow—on the order of low single digit percentages. If you can do 300 watts on the bike, changing your pedal stroke to become more efficient won't make you ride any faster on the same amount of power.
Running is a whole different animal. Some athletes are far more efficent than others; they're able to run faster on less power. “The difficult part [about measuring running power is] that there’s so much variability in how runners run,” says Ewen North, who operates Revolution Running, a 500-odd member club of mostly recreational athletes in Boulder, with his wife, Heather. “There are different stride lengths, and body makeup, and form, and they have to take all of that into account.”
That's where Stryd comes in. It doesn’t measure force directly, says Shang. Instead, the small pod clips to the back of a runner’s waistband. From there, a collection of sensors (mostly accelerometers) and a powerful processor measure acceleration and deceleration on a variety of axes. Stryd converts those forces of a runner’s forward motion—and her vertical motion—into a similar, but subtly different, value they call running power, or center of mass power. And this is the number that Stryd will allow you to train.
There’s no power associated with forward motion in running, says Lim. In running, all power used comes in the energy we expend moving our limbs.
“Think of a runner like a rubber bounce ball,” Allen Lim, an advisor to Stryd, who helped develop the Powertap hub-based powermeter for cycling. If you throw a ball down the road, it’ll bounce along fast and efficiently, like a runner with great form. But if you throw the ball at the ground at a sharper angle—one that produces a higher arc—then you’ll need to throw a lot harder (with more power) to get that ball to travel the same speed as the first ball.
The sensors in Stryd measure the force of that bounce. “The measurement is units of power, but it’s a bounce meter,” says Lim. That’s both the device’s strength and weakness: measuring that bounce gives runners instant feedback on changes they make to their form. But because it’s measuring the runner’s various movements that make up that bounce versus a traditional power value you’d get from a lab treadmill with a force plate, Stryd’s accuracy depends first on making sure it is accounting for all the movements that make up that bounce.
“We don’t even have a product yet,” Shang says. “We have a technology. We want that to be beneficial and help people to be better runners.
It’s easy to point out that runners and coaches already know that form is important. But traditional gait analysis and coaching only go so far in tweaking it. There’s no way other than pace, subjective perceived effort, and the laggy, fickle heart rate number to measure the effect, and there’s no constant feedback loop to ensure that changes in form are consistently applied.
But a single number that instantly changes when you drop your shoulders, or change your cadence, or lean forward a bit, closes that feedback loop. It’s a constant and—crucially—consistent gauge of whether what you’re doing is helping—and how much. Combined with expert knowledge and coaching of the sort that North possesses, and even a recreational runner could make, well, big strides.
How big? A well-known concept in running circles is running economy, which is essentially the amount of fuel a runner burns at a sub-maximal effort. A number of studies, dating back 30 years, find a 20 to 30 percent difference in running economy between runners with similar Vo2 max measurements. That’s so large that some physiologists argue running economy is a better predictor of performance among elite athletes than traditional measures, like VO2 max. Running economy is down to a number of variables, from biomechanical factors like limb length to, yes, VO2 Max.
But a significant component is based in efficiency: things like cadence, vertical motion, and excessive upper body movement. Change those to become more efficient and, with time, your running economy can improve. There’s a linear relationship between power and running economy, says Shang. If you can measure one, you can determine the other.
The net result, Ricci said, could be that running becomes more like swimming, another sport where strength and power take a backseat to form and efficiency. “Every swim workout my athletes do has some kind of skill-set or technique component to it,” he says. “And that’s not true in running. This may change that.”
Stryd will almost certainly need refinement of its algorithms to improve accuracy. Shang says two of the areas the company is focused on are the effect of hills and headwinds, although the software currently does factor in both. And the flow of data will also help to more finely correlate center of mass power with running economy, since the relationship varies slightly depending on the runner.
But that’s what phase 2.0, when coaches start to get their Stryd units and test them, is designed to produce. With permission, Stryd plans to anonymize and publish all its data openly to invite more feedback and refinement. More important, it may start to realize, and quantify, some of the potential gains that can be made.
“We still don’t know what will be the most important metrics,” says Shang. Will they include TSS, the proprietary measure of workout intensity that training software company Training Peaks developed? Some as-yet unnamed efficiency score? Something else no one can yet foresee?
If Shang has an idea, he won’t say. “We don’t even have a product yet,” he says. “We have a technology. We want that to be beneficial and help people to be better runners. But I still don’t know that we have an answer.”