Stryd, the company that first introduced the idea of performance meters for running, recently published a scientific white paper entitled “Definition of mileage and utility”.”This may be a strange topic for a company that has been selling electricity meters since 2015. You would think that you should already know what a running force is and why it is useful.
But these questions are much more complicated than you think, and Stride has always been open enough to admit it. In the initial coverage of their launch, one of the co-founders said that their main problem was “lack of knowledge”, and expressed the hope that the first users will help the company understand what its product is good for. In the years that have passed since then, Stryde has received an excellent word of mouth radio. The users I spoke to found this helpful. But there is a troublesome discrepancy between the positive user ratings and the general consensus of scientists who actually learn to walk, which is that “the power of running” is a fundamentally meaningless concept.
In this light, the new White Paper looks more interesting because it is (at least in my reading) an attempt to reconcile the true utility of the device with the science underlying it. This requires rejecting some deep-rooted assumptions about what power means. But even if you’re already a believer, dealing with the dirty details of what’s under the hood of Stride’s device can convince you that it’s even more useful than you thought.
Two types of power
Force is the speed at which you use energy. You can imagine a runner as a machine that takes energy from food and turns it into useful forces that propel you on the road. However, there is a Problem: No machine is perfect. You don’t get as much energy as you invest. Cars, for example, are about 25% efficient: if you burn enough gas to get 100 joules of energy, only about 25 joules will turn the wheels, and most of the other 75 joules will be released as heat.
Under normal conditions, muscles are also about 25% effective, but they vary greatly depending on the specific circumstances. This means that there is a big difference between your entrance, which is known as metabolic force and reflects the calories of the food you burn, and your exit, which is known as mechanical force and reflects how hard you hit your foot on the road, how vigorously you swing your arms and so on.
I deepened this difference and the debate in an article in 2018, and I took it for granted that we all agree that runners and other endurance athletes are most interested in metabolic power, which is essentially a real-time estimate of how quickly they burn calories. It turns out that not everyone agrees: “We don’t think most serious runners are that interested in calories,” an engineer from Garmin, who has his own Running Power app, told me when I wrote another article about running Power.
I agree that runners say little about calories. But I think that’s basically a matter of terminology. When you go to a lab and use a number of complex devices to measure your VO2max, you’re basically measuring calories. You are interested in oxygen intake just because it is a good indicator of how quickly you burn aerobic energy. And if you use these fancy lab data to determine your heart rate, which allows you to work with the lactate threshold, use your heart rate again as a proxy for energy consumption, i.e. calories. And I would even say that if you give up all technologies and just run by touch and try to estimate your pace as quickly as possible to cover the prescribed distance, you rely on your perception of effort as a proxy for how quickly you burn calories.
Cycling and Running
In the world of cycling, no one connects with knots. Force is force, and it is considered the gold standard of an effective rhythm. The reason for this is that the mechanical and metabolic forces in the cycle are almost perfectly correlated. If your power meter detects that you pedal 15 percent more, it means that you burn calories 15 percent faster. The figure on display is a mechanical force, but people are concerned that it tells them what happens to their metabolic power.
Running is, unfortunately, quite different. The Stryde White Paper, written by associate scientist Christine Snyder with the participation of external scientific advisors Shalai Kipp and Wouter Hugkamer, identifies three reasons why mechanical and metabolic forces in running do not have a consistent relationship. One of them is that the movement of your limbs is much more variable than when cycling, which means that the muscle performance also changes more. Secondly, every footfall requires that you absorb forces instead of producing them, but you are still wasting metabolic energy by amortizing those touchdowns. And thirdly, they collect and recycle energy in their spring-loaded tendons with each step and increase their mechanical power without metabolic costs.
All this would be out of place if you ever just walked on a smooth, flat treadmill. The ratio between mechanical and metabolic energy is difficult to calculate, but no one cares about the exact ratio as long as these two forces are in sync. The problem is, once you get off the treadmill into the real world, relationships will change. For example, if you go uphill, your step becomes less elastic and as a result, you get less free energy from the tendons.
Snyder e-mailed me some illustrative figures based on a recent journal article by a well-known group of biomechanists in Italy. If you walk uphill from the ground to a 10 percent gradient, your efficiency drops from about 60 to 50 percent. With a steeper gradient of 20%, the efficiency decreases even more-up to 40%. (Don’t get stuck with exact numbers that depend on which body parts you include in the calculation.)
In practice, this means that trying to maintain a constant mechanical performance while climbing hills would be a ridiculous step approach. If you move at a speed of 200 mechanical watts, an efficiency of 60 percent means that you burn 333 metabolic watts. Once you increase 10 percent, you now need 400 metabolic watts to maintain the same 200 mechanical watts. They work about 20 percent heavier, although the counter shows that their mechanical performance is constant! In this sense, I do not understand how one of the few companies that offer performance meters or applications can claim that mechanical performance itself is a useful metric.
What Runners Really Want
This is a reality that Strade officially acknowledges. Your device displays the readings that look like the mechanical power, calculates the fret acceleration sensors, gyroscopes, barometers, Probe wind and other sensors in FT capsule. But the algorithm is clearly designed to maintain a constant connection between the number on the screen and its metabolic power. In the example above, if you support 200 watts on a Stryd device, you actually produce 166 mechanical watts, which is equivalent to 333 metabolic watts. Maintaining a constant performance on a stryd is equivalent to maintaining a constant metabolic performance and changing mechanical performance.
In the White Paper, Snyder and her colleagues introduce a finer terminology. What Stride actually wants to deliver, they explain, is a measure of current metabolic demand and not metabolic performance.
In comparison, one of the most important problems with heart rate is that it does not immediately respond to changes in metabolic demand. When you start to climb a hill, your muscles immediately start to consume more energy, but your heart rate increases more slowly as the body’s control systems respond to the change. This means that your muscles temporarily do not get enough oxygen to meet your aerobic energy needs, so you fill the gap with anaerobic energy. If you run up the hill and try to keep your heart rate constant, run the first section and slow down only when your delayed pulse finally meets the new requirements.
Even in a trendy laboratory that measures your metabolic performance with a VO2 device, you will encounter the same problem. Your oxygen intake does not immediately respond to changes such as a steep hill. So Stryd is trying to work better than a VO2 machine: it estimates how much metabolic energy your muscles consume in real time (metabolic need), rather than how much energy your aerobic system provides, including both aerobic and anaerobic energy contribution. In this sense, Stride does not only mimic what you could do in the laboratory; it does something new and unusual-and if you believe the data, it’s better.
This opens up some fascinating possibilities, even beyond the ability to trust the power to kick as you climb the hill. Earlier this month, I wrote an article about an incessant controversy over what we mean by the term “threshold.””The conclusion was that the most relevant threshold definition for endurance athletes is a so-called critical force, which outlines the boundary between metabolically resistant and unstable efforts. Critical performance is a surprisingly accurate performance predictor in endurance races: the best athletes, for example, tend to run marathons at about 96% of critical performance.
You do not necessarily need a power meter to determine your critical threshold. The study, published earlier this year, used Strava training data to evaluate the critical speed—that is, the speed that corresponds to the critical performance under normal conditions—for 25,000 runners. But “under normal conditions” – that’s the catch. This approach works best when all the training data is collected on windless days on a smooth, smooth road and your race destination takes place under the same conditions. If these conditions are not met (and in fact are never met), then they prefer a metric that makes adjustments to things like wind, surface, and gradient. Stryd does this and automatically evaluates the critical performance for you based on your training data.
What’s on the screen
That really leaves a question unanswered. The number on the Stride screen is not really a mechanical force. There is also no metabolic demand, although it is proportional to it. So does it have an internal meaning, except as an intermediary for metabolic demand? Snyder and I discussed this several times, and each time she had to consult with Stride’s team to avoid disclosing confidential information.
The closest I came to what I suspect is the real answer: “The scaling factor used is clearly not arbitrary. It was selected to ensure consistency between the output power values in different activities.”I read it as a desire to have a power meter with a number that makes sense for cyclists who already have a strong intuition about how much power they can expect to get through an hour, for example. If you were to sell a device that only shows metabolic watts, it would create all sorts of cognitive dissonances for people who knew they could endure 250 watts during an hour of cycling, but suddenly tried to endure 1000 watts during an hour of running.
I don’t necessarily think the Stryd team was sitting and having this conversation when they designed the device. As an article from 2015 shows, which I mentioned above, you have found things out in the course of things. The number on the screen probably really corresponds to a certain part of the mechanical performance, which is calculated in a certain way under certain conditions. It takes some courage to essentially tell the company, ” Forget the number. The number does not matter. What is important is what it represents.”But I think it’s the right choice.
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