What Is “Running Power,” Anyway?


An extended-running debate flared up once more a couple of months in the past on Twitter, when tech reviewer Ray Maker posted a comparability of 3 units—from Stryd, Garmin, and RunScribe—that promise to measure how a lot “power” you’re producing throughout a run. “Anybody got any insights into why people think estimating mechanical power during running is useful?” asked Max Donelan, a neuromechanics professor at Simon Fraser University in Canada. “Unlike cycling, there is nearly no net external mechanical work.”

The reply, from someone at Stryd, one of many energy meter makers, got here swiftly and a bit peevishly: “Because it solves real problems.”

I’ve been following the rise of working energy meters since 2014, after I first noticed a Stryd prototype in motion. It’s a neat thought, and one of many few genuinely new coaching aids lately. But I’ve had a very powerful time getting my head round what working energy truly means. Spurred by Donelan’s tweet, I’ve spent a while studying up on the idea and speaking to individuals with broadly diverging opinions on its sensible usefulness. I don’t have an final last reply on whether or not you can purchase 1, however I do have some ideas on what an influence meter is for runners, and what it’s not.

The Bike Analogy

To perceive the potential enchantment of working energy meters, the most effective place to start out is biking. Bike-mounted energy meters have been round for the reason that 1980s. They measure the mechanical work you do in spinning the pedals by detecting the delicate bending of the cranks (or different components of the bike, relying on the design of the meter you’re utilizing). If you’re biking at 250 watts, meaning you’re doing 250 joules of labor (about 0.06 energy) per 2nd on the pedals, which primarily goes to overcoming air resistance (on flats and downhills) or gravity (on uphills).

Why is this handy to know? The golden promise made by energy meters is that they offer you a real-time measure of how laborious your physique is working. If you realize that you simply’re able to sustaining 250 watts for an hour, then you need to use the facility meter to maintain your effort at that degree whilst your tempo and coronary heart price rise and fall with hills and headwinds.

The implicit assumption right here is that the watts you ship to the pedals (the mechanical energy) correspond to how laborious your physique is working total and, extra particularly, to the speed at which you’re burning meals vitality (the metabolic energy). The metabolic energy, in spite of everything, is what finally determines how shortly you’ll fatigue and whether or not you’ll exceed your lactate threshold or bonk. Normally, the one approach to calculate metabolic energy is to fastidiously measure the oxygen and carbon dioxide that you simply breathe out and in. That’s what the cumbersome masks and different tools used throughout a VO2max take a look at are for: to calculate metabolic energy.

Humans are sometimes about 25 p.c environment friendly at finest, which means that lower than 1 / 4 of our metabolic energy will get transformed into mechanical energy, with the remainder dissipated as warmth. The exact relationship is dependent upon precisely what your muscle groups are doing. Purely concentric contractions (a typical motion like lifting a weight or urgent down on a pedal) are probably the most environment friendly, at about 25 p.c. Isometric contractions (the place you exert drive however your muscle doesn’t transfer, like pushing in opposition to a wall) eat metabolic energy however don’t ship any mechanical energy. Eccentric contractions (braking motions like reducing a weight, the place you’re pushing a method however transferring in the other way) even have a adverse effectivity of about -120 p.c. There are additionally contributions out of your tendons and different connective tissues, which may retailer vitality like an elastic, then ship mechanical energy with out consuming any metabolic energy.

In different phrases, it’s difficult. If you’re biking at 250 watts, it’s laborious to know precisely how a lot metabolic energy you’re consuming (or, to place it one other manner, what number of energy you’re burning). It’s in all probability someplace between 1,000 and 1,200 watts, however your pedal-mounted energy meter can’t let you know the precise quantity. There’s an important saving grace for biking energy meters, although, which is that your effectivity stays roughly fixed below all circumstances, as a result of pedaling a motorbike requires basically the identical movement on hills and flats, gravel and asphalt, in headwinds, and so forth. Whatever your metabolic energy is when the biking meter says 250 watts, that relationship will maintain true below all these circumstances. The backside line: In biking, energy is an efficient proxy for a way a lot vitality you’re burning.

The Trouble with Running

An influence meter for working is engaging for precisely the identical causes as in biking: as a real-time calorie estimator. So why has it taken so lengthy for energy to return to working?

Most of the discussions about working energy give attention to how a lot tougher it’s to measure in comparison with biking. And that is true. In working, you have got inside work (your legs and arms pumping backwards and forwards) and exterior work (your middle of mass bouncing up and down); you have got optimistic work (pushing off with every stride) and adverse work (braking as you land); and you’ve got a major contribution from elastic vitality saved in your Achilles tendon and different tissues as they stretch upon touchdown after which snap again that can assist you push off. According to some (though not all) estimates, the vitality saved and launched on this manner might contribute as much as half of the facility required for every stride.

There’s no single machine that may measure all these contributions. But the large technical advance of the previous few years has been the arrival of wearable accelerometers and different sensors that may measure your stride and center-of-mass movement with ample element to estimate a big fraction of this energy. You can examine the readouts from these energy meters to the gold-standard energy measurements from laboratory-grade force-sensing treadmills that value a whole bunch of 1000’s of , and the settlement is fairly good. So, drawback solved, proper?

But the objection leveled by Donelan and different biomechanists isn’t actually about measuring energy; it’s about what energy means. The underlying assumption remains to be that mechanical energy throughout working is proportional to complete vitality consumption, as it’s in biking. But as apparent as that appears, it’s in no way clear that that is true. As one of the many papers on the topic argues, “measurements of whole body mechanical work cannot explain how the rates of metabolic energy consumption change with speed or animal size.” Or, as Donelan put it in a subsequent tweet: “The criticism of running mech[anical] power is not about how well current devices estimate it, but that even perfect measures are not closely related to effort in running.”

Stryd’s algorithm, as the corporate defined in a recent white paper, makes use of solely the optimistic portion of mechanical energy—how a lot you’ve generated to push off the bottom. This appears affordable on the floor, and the info (Figure 6 within the white paper) exhibits that this part has a pleasant tight correlation at numerous speeds with complete vitality consumption, which Stryd measures with VO2 tools in a lab. In its dataset, Stryd’s energy metric has a 96 p.c correlation with complete metabolic vitality expenditure at completely different speeds—on a degree treadmill.

Unlike in biking, although, the connection between mechanical energy and complete metabolic vitality consumption doesn’t keep fixed when circumstances change. As you go up steeper and steeper hills, the contribution of elastic vitality turns into more and more negligible. At an incline of about 30 p.c, you now not have any bounce in your step. On downhills, your muscle groups do much less optimistic work, and past a couple of 15 p.c grade solely do adverse work, braking as you descend. Stryd’s conceptual shortcut, measuring solely the optimistic mechanical energy of your middle of mass, can’t deal with these adjustments and now not has the identical relationship with complete vitality consumption.

You would possibly anticipate, then, that Stryd customers would report loopy information every time they enterprise into the hills. That doesn’t appear to be case—and the reason being easy. When I chatted with Stryd co-founders Kun Li and Wyatt Mohrman not too long ago, they defined how their algorithm works. On degree floor, as defined above, they estimate mechanical energy from the movement of physique’s middle of mass, similar to a force-sensing treadmill. But on hills, the connection between mechanical energy and vitality consumption breaks down—in order that they reverse engineer it.

During the machine’s improvement, they examined runners on inclined treadmills whereas sporting Stryd models, measured their oxygen and carbon dioxide consumption to calculate vitality expenditure, and used that information to regulate their algorithm to offer the “right” solutions. So, on flat floor, they measured energy (200 watts, say) and famous that this corresponded to a given price of metabolic vitality consumption (1,000 watts, say). On the inclined treadmill, they cranked up the velocity and angle till the metabolic vitality consumption was 1,000 watts, after which, for consistency, programmed the algorithm to name that energy 200 watts.

From a scientific perspective, this implies the quantity your working energy meter spits out is actually meaningless. Even on flat floor, “positive external mechanical power” is an indeterminate mishmash of contributions from muscle groups and springy tendons. It doesn’t precisely mirror the underlying processes that decide vitality consumption. And on hills, you’re not an precise energy measurement in any respect—you’re “the positive external mechanical power I would be generating on level ground if I burned energy at the same rate I’m burning it on this hill.” I don’t know the inside workings of the Garmin or RunScribe algorithms, however they face precisely the identical concern: Either you’re measuring mechanical energy otherwise you’re estimating metabolic vitality, however you possibly can’t do each without delay.

From a sensible perspective, alternatively, it’s completely truthful to ask whether or not any of this actually issues. Fundamentally, what Stryd and its opponents have constructed is a real-time metabolic vitality consumption estimator—or, to be much less fancy about it, a calorie counter optimized for working. It’s the equal of with the ability to carry round a full VO2 testing lab all over the place you run. Unlike coronary heart price, it responds immediately if you velocity up or decelerate. Unlike the tempo in your GPS watch, it displays the change in effort from going up and down hills. Unlike your inside sense of effort, it doesn’t (in idea) lie.

The Case Study

But does the disconnect between mechanical energy and vitality have any sensible implications? Under sure circumstances, sure.

Consider, for instance, some data collected by Hans van Dijk, a Dutch scientist who co-wrote The Secret of Running, on working with an influence meter and optimizing working kind. For cyclists, the facility meter supplies a useful gizmo for taking part in round with their driving place to search for effectivity enhancements, so van Dijk and his colleagues tried the same factor with working, asking a bunch of runners to extend or lower their cadence by about ten steps per minute from their naturally chosen cadence. They measured effectivity in 2 alternative ways: as soon as by measuring oxygen and carbon dioxide to calculate vitality consumption, and as soon as by measuring energy utilizing Stryd.

Here’s what that information seemed like in skilled runners:


In this graph, the next worth means the runners needed to burn extra vitality to run at a given tempo. The blue curve, calculated from gold-standard oxygen and carbon dioxide measurements, exhibits the anticipated sample. The runners had been most effective at their self-chosen cadence; rising or reducing cadence by about ten steps per minute pressured them to eat extra vitality on the identical tempo—a nasty factor.

But the pink curve, calculated from Stryd’s energy information, exhibits a special sample. In this view, rising cadence was a seemingly good factor, decreasing energy consumption and saving vitality. When I requested Li and Mohrman about this information, they defined that it’s possible an artifact of the shortcoming of the facility meter to account for elastic vitality saved and launched by tendons. An ultrarapid cadence reduces up-and-down bounce in your stride, so the facility meter thinks you’re getting extra environment friendly. But it doesn’t understand that you simply’re shedding out on among the “free” vitality return out of your tendons.

The Bottom Line

At this level, you possibly can in all probability see why I didn’t promise an “ultimate final answer” on the utility of energy meters. Personally, I feel tuning in to your inside effort sensor is each highly effective and actually helpful for racing to your limits. When I run, I don’t have any issues discerning whether or not I’m going up or down a hill. But I may also see the enchantment of goal information that may aid you tempo your self in uneven terrain and examine efforts on completely different programs the place tempo and coronary heart price aren’t dependable gauges.

For scientists, it would stay annoying that they’re calling this “power” in any respect, fairly than one thing generic that doesn’t carry preexisting which means (like, say, “NikeFuel”) and makes clear that it’s actually supposed as a real-time calorie tracker. But if, because the Stryd tweeter I discussed on the high promised, the machine finally ends up fixing actual issues for athletes, most individuals received’t lose a lot sleep in regards to the nomenclature. Time will inform.

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(Editor references)

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