Thoughts on the Stryd Running Power Meter

Stryd FootpodUnless you’ve been living under a rock for the last month or so, you have no doubt heard about the upcoming Stryd running power meter.  As far as I can tell, DC Rainmaker was the first to do a review of the unit.  Since then, more and more publications have been talking about it.  If you haven’t heard of it, it’s a power meter for running and here are my thoughts on it.

What Does it Do?

When you do any kind of movement, you generate power.  Some of that power is functional (e.g. getting you to the finish line faster) and some of it isn’t (e.g. generating heat, wasted body movement, etc).  Today’s generation of power meters all measure functional power.  On a bike, all of our functional power gets transmitted into a single activity– spinning the wheels against resistance.  Thus, a power meter on a bike makes perfect sense as we can look at a single physical movement (rotation of the wheels against resistance) to know our functional power.  And increasing that power is the golden ticket to winning more races.

Using power to guide our training gets more complicated in running because the motion of our legs and feet isn’t transformed as cleanly into horizontal movement.   In running, we fight gravity on every stride so a lot of movements are necessary to absorb the effects of gravity.  Nevertheless, if we look at the movement of the body as a whole, we can get a sense of how energy is being used.  The Stryd promises to do that.  Like a power meter on the bike, it promises to give us one metric– watts– that would tell us how hard we’re running.  Uphill or downhill.  Into a headwind or with a tailwind.  Indoors or out.  Just like a power meter on a bike.  If it lives up to the hype, a running power meter like this would be a great tool to have in the arsenal.

How Does it Work?

The Stryd uses a 3-dimensional accelerometer that is really no different from what’s inside a running footpod.  All you have to do is clip it to the back of your running shorts.  It has special software that translates acceleration data into watts that is then transmitted via ANT+ to your running watch or BLE to a compatible smartphone.  In this sense, it has got to be the easiest darn power meter you can possible use.  So how does the software do that?  Here I can only offer a guess.

First, a bit of background.  Before the Stryd, the way to properly measure a runner’s power is using a force plate treadmill.  This measures how much force a runner’s foot pushes into a treadmill belt while running.  A force plate treadmill is an enormously complicated device, capturing force in three-dimensions– up and down, side to side, and front to back.  It’s an external system measuring how much energy is being applied to it.  In this sense, it is analogous to a power meter on a bike.  Not everyone can afford a $100,000 force plate treadmill, however, and the only suggestions for creating a portable running power meter thus far have involved awkward, expensive, and ungainly pressure-mapping insoles.

A Stryd is fundamentally different– it doesn’t measure energy being applied to it.  It just sits on your hip as you run. I’ve thought long and hard about the Stryd and I don’t believe it’s possible (even in theory) to directly measure power by looking at the movements of the hips alone.  At first, I thought that monitoring the acceleration and deceleration of the hips could give an accurate idea of all the forces acting on the body during the flight phase– and that would give a good idea of the energy need to propel the body.  Unfortunately, it’s way more complicated than that.  The theory would work if the human body were a rigid object (like a steel ball bearing), but instead there are a lot of moving parts– each using different amounts of energy to accelerate and decelerate at different rates.  For instance, when the runner’s foot lands, there is a brief instant in which that foot stops completely.  The knee doesn’t quite stop completely, but slowly arcs over the ground foot.  The hip barely slows at all as it passes over the grounded foot.  When the foot leaves the ground, things become even more complicated.  The lifted foot moves forward quickly– considerably faster than the hips as it swings forward.  The lifted knee also drives forward, but then decelerates well before the foot approaches the ground.  Each of these body parts have mass and require different amounts of energy to speed up and slow down.  Unfortunately, just looking at the hips gives absolutely no information about what’s below it.  The legs could be as long as a giraffe’s or short as a hamster’s.  The feet could be as light as a bird’s or as heavy as an elephant’s.  Each of these combinations would need very different amounts of energy to accelerate and decelerate– yet the motion at the hips could always be the same.  If you need any further proof of why hip motion alone doesn’t work, just go to the gym and watch people running on treadmills.  Now, visually fixate at a point on their hips.  Try to do this discreetly so you don’t look like a stalker.  You’ll notice the hips barely moves at all, even with runners with terrible form.  Moving the whole body by that small amount takes only a little energy but don’t tell that to the person running on the treadmill!  Instead, almost all of the action is happening below the hips– the legs, knees, and feet are moving like crazy.  Obviously, that’s where all the power is going.

Instead, it’s clear that the only way to do measure a runner’s power by observing the hips is to put some runners on some force plate treadmills and (1) collect tons of movement data, (2) compare that data with their actual power measurements, (3) build a model that matches the movement data to the power data, (4) refine that model for different grades and wind conditions, and (5) validate that model over thousands of different runners.  Piece of cake, right?  Instead of measuring power directly, we can only indirectly measure a person’s running power by first measuring his or her body movements and then plugging it into a formula derived from observing other runners.  This pretty much jives with the way that WIRED described how the Stryd works,

Inside, according to company engineers, the device has accelerometers to measure the microsecond rate of change in a runner’s velocity along with a barometer to adjust for the grade of the road. From the limited description the company could give me, the secret sauce is an algorithm that uses acceleration and grade along with an anatomical modeling of the runner’s body to derive power, impact, ground contact time, and cadence.

WIRED’s mention of barometric pressure surprised me– and makes me wonder if it really works with a treadmill.  On this point, only time will tell.  But, as DC Rainmaker points out, Stryd’s algorithms seemed to work pretty well in his non-treadmill testing.  For instance, in his brief experiments with the Stryd, Ray found that watts go up when he was slogging up a hill but watts go down when speeding down a hill.  He also found that running watts correlated fairly well with cycling power in terms of perceived exertion– but that’s hardly an objective measure.

Using Body Movements to Approximate an External System

The problem with indirectly using body movements to approximate power is that its reliability is only as good as its underlying algorithms.  In theory, this should work out fine because, as far as we know, 99.9% of efficient runners share certain common characteristics.  For instance, efficient runners’ ground contact time is usually very short.  Also, they tend not to overstride so they don’t incur massive braking forces (and hence a lot of sudden deceleration).  In addition, their vertical oscillation (how much they bob up and down) is pretty minimal.  All of these things can be approximated with an accelerometer at the hips.  There are probably a ton of other characteristics that Stryd’s engineers looked at– and built into Stryd’s algorithms.  Of course, this means that Stryd’s idea of how “efficiently” you run is based entirely on how closely you match these characteristics and fit their profile of the ideal runner.  This also means that it is theoretically possible that Stryd won’t work for you because you are a super-efficient who has terrible running form.  But I highly doubt that Stryd (or anyone else) has come across any of these folks– and if they did, they would likely be a great research subject.

It Doesn’t Need to Be Perfect, Just Consistent

This kind of modeling might sound like an engineering house of cards but just remember: it doesn’t have to be perfect, just consistent.  If you’ve been using power on the bike on different systems, you’ll know that power across different systems isn’t perfectly consistent.  For instance, my bike has a PowerTap hub and I often ride it on a Computrainer.  Even if I warm the Computrainer up and calibrate it perfectly, there will always be a small difference between the Computrainer and the PowerTap.  That’s fine– as long a the difference is consistent.  For instance, if I always know that the Computrainer is 2% higher than the PowerTap, I can use my test data on the PowerTap or the Computrainer to “translate” performances from one system to the other.  At the end of the day, I don’t care about my exact wattage as long as I know what number I should be shooting for on each system.

Similarly, the exact number of watts I get from examining my data from a $100,000 force plate treadmill doesn’t have to perfectly equal the number of watts I get from my $150 Stryd running pod.  All that is needed is that the difference is consistent.  It could be off by 2% or it could be off by 20%– as long as that difference is consistent.

Why Do I Need It?

Hopefully by now, few of us are relying on speed to give an indication of our workout intensity on the bike.  This makes perfect sense because wind and hills make such a huge difference in cycling speed.  Now that power meters are pretty commonly available, we now generally use power in watts as the yardstick of workout intensity.  At the same time,  most of us are still in the Dark Ages and use running pace as our only metric when it comes to running intensity.  While wind may make a small difference in running intensity, hills obviously make a huge difference in running intensity.  Running “power” would give us a way to normalize that running intensity so that we can objectively compare runs over hilly course with flat courses, windy courses with calm courses.

Assuming that Stryd’s algorithm’s are relatively spot-on, I think a running power meter could be as important as a cycling power meter– but in a very different way.  Watts translate directly into performance better in cycling than in running.  For instance, if you have a very bouncy stride, your watts are going to be quite high because you are fighting gravity unnecessarily.  Alternatively, if you overstride, you are likely to be wasting a lot of energy in braking forces.  In both cases, you could have really high watts but still be slow as a turtle.  On a bike, high watts is high speed– pure and simple.  Looking at it another way, on a bike, watts gives information about intensity; in running watts gives information about intensity as well as efficiency.  This difference is both a limitation and an opportunity; I think this means two practical ways to use running watts day-to-day.

  • Efficiency and Running Economy.  A Stryd power meter could give you an idea of how efficient your running form is– or at least how closely it fits Stryd’s model of the perfect runner.  This efficiency would be indicated by your ability to run quickly while producing relatively few watts.  Just as watts per kilogram is a great metric of cycling prowess, I think watts per unit speed (e.g. watts per MPH) could be a great metric for running prowess.  Actually, in a perfect world, that would be normalized unit speed (which TrainingPeaks currently uses) to factor in uphills and downhills.  That would give a runner an “efficiency index” showing how efficient they are at translating body movement into speed.  Currently, coaches talk about “running economy” and point to pictures of Ryan Hall’s perfect running form or talk about oxygen uptake at different speeds– but these data points are not being very helpful to the everyday athlete.  Being able to see an “efficiency index” in realtime would be much more useful.
  • Safer Recovery and Warming Up.  All of us know what it’s like to run with dead legs– form suffers and speed is in the toilet.  Each footfall feels like an earthquake.  The same thing happens at the beginning of a workout if we’re not warmed up and neurologically primed for the task ahead.  An “efficiency index” like I described above could tell if running form is compromised.  In that case, we may be doing more harm than good by running hard so taking it easy would be the better course until efficiency improves.   And here also watts could be great way to temper easy efforts.  When I’m trashed on the bike, I consciously keep my watts below 150 watts and I always feel better.  Being able to do the same thing on the run would be great help.

Pacing

I’ve blogged before about how to use power for pacing in a cycling race and, assuming Stryd’s algorithms hold true, roughly the same same strategy applies to running.  In fact, the folks over at Stryd envision that this little pod will be able to normalize efforts and really give a much better idea of what is going in inside the body.  For instance, suppose you’re in a race with a decent hill in the middle.  If you muscle your way over the hill to keep your pace, your watts would spike upwards, leaving you trashed and unable to finish as well as you could.  Stryd offers the following chart shows how sticking to a consistent pace can have deleterious effects on later running.

Chart showing a spike in running power on a hill-- and a subsequent drop in running performance

As I discussed in my earlier blog post, average and normalized power give a great insight into pacing on a cycling event.  You’ll recall that normalized power approximates the effect of varying efforts better than average watts– and thus gives a clearer picture of how well you have been pacing and how much you have left in the tank.  It’s unclear how normalized power, however, translates to running– or if it needs to be calculated differently in running.  For instance, easy watts on an steep downhill section in cycling gives the body a real break whereas the same easy watts on a steep downhill in running can utterly trash the quads for later in the race.  Only time will tell how these differences between running and cycling actually play out in practical advice for using a Stryd running power meter as a pacing tool.  In short, pacing with a power meter on the bike is a rock-solid strategy, but I don’t think we know yet if the same holds true for running.

But we also may not have to wait too long to get it all figured out! Stryd’s very low price point (about $150) may mean a very swift adoption in the geeky end of the running community– and there will be a lot of really intelligent people figuring out how to optimize a Stryd for pacing quickly.

How Can I Get One?

Simple.  Head over to the Stryd website and put yourself on the mailing list.  DC Rainmaker reported that the cost will be around $150, which will make it the least expensive athletic power meter by a long shot.  It should also be coming out pretty quickly– certainly by end of 2015 H1 according to reports.

What the Future Holds

Obviously, it’s way too early to tell if the Stryd is worth the investment.  My gut says “heck, yes!!” based on the price point and what I’ve seen so far.  But I tend to be on the early side, even among early adopters.  I like to tinker around with new toys and figure out for myself how to incorporate them into my routine.  Of course that means I’ll be buying one as soon as they become available– then anxiously waiting by the mail box.  $150 isn’t much more than the entry fee in most local races these days.  That doesn’t mean it’s dirt cheap.  While my comparison to race fees probably says more about the ridiculously high cost of races these days, it also says that the Stryd is well within the budget of athletes who are competing in these kind of races on a regular basis.

Thanks for reading and be sure to like the Athletic Time Machine Facebook page and follow us on Twitter @AthTimeMachine.  If you found this post useful, please reblog it on WordPress, share it on Facebook, or retweet it on Twitter to share it with your friends.

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