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Precision and accuracy of Stryd
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Hopefully this will put most of the arguments to rest:

https://www.ncbi.nlm.nih.gov/pubmed/29781934

(Note that power and leg spring stiffness are calculated from the same raw accelerometer measurements used to determine GCT, etc.)
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Re: Precision and accuracy of Stryd [Andrew Coggan] [ In reply to ]
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Do you know of any studies that measure the accuracy of the Garmin Running Dynamics sensors in either the Tri HR Monitor strap or the Running Dynamics pod?

It would be my guess that Garmin isn't too far behind Stryd in figuring out the algorithm to determine run power using GCT, stride length, vertical ratio, and vertical oscillation

USA Triathlon Level 2 Coach
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Head Coach, TriCoach Colorado, LLC
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Re: Precision and accuracy of Stryd [Andrew Coggan] [ In reply to ]
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Notable: The study didn't address or test the accuracy of power...the main thing people care about with Stryd.

It focused on all the other metrics.


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My tiny little slice of the internets: dcrainmaker.com
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Re: Precision and accuracy of Stryd [dcrainmaker] [ In reply to ]
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As I noted, power is calculated from the same raw accelerometer data as everything else. It is hard to imagine how these other metrics could be reasonably precise and accurate, yet power be way off.

Now precisely which power is being calculated might be a different story...
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Re: Precision and accuracy of Stryd [coachjustin] [ In reply to ]
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coachjustin wrote:
Do you know of any studies that measure the accuracy of the Garmin Running Dynamics sensors in either the Tri HR Monitor strap or the Running Dynamics pod?

I have seen a few comparisons against Stryd, but not against a force plate (or mat...we just a runway at work for about $20k that is long enough to collect a handful of strides, thus avoiding possible targeting issues).

coachjustin wrote:
It would be my guess that Garmin isn't too far behind Stryd in figuring out the algorithm to determine run power using GCT, stride length, vertical ratio, and vertical oscillation

Garmin started reporting power 6 months or more ago. Not surprising, really, since calculation of running power is rather trivial, at least if the input data are "clean". A bigger issue, really, is deciding what power is most relevant to calculate.
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Re: Precision and accuracy of Stryd [Andrew Coggan] [ In reply to ]
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Please correct me when (not if) I’m wrong:

the practical usefulness of a watt in cycling is really it’s strong correlation with a Kcal. so, if the “watt” being displayed by Stryd is also just as strongly correlated with a Kcal, how is that not all that really matters?

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Last edited by: milesthedog: May 26, 18 15:43
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Re: Precision and accuracy of Stryd [Andrew Coggan] [ In reply to ]
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Andrew Coggan wrote:
As I noted, power is calculated from the same raw accelerometer data as everything else. It is hard to imagine how these other metrics could be reasonably precise and accurate, yet power be way off.

Now precisely which power is being calculated might be a different story...

I’ve employed plenty of analysts who took perfectly good data and then effed it up in an algorithm. Good inputs do not automatically lead to good outputs.
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Re: Precision and accuracy of Stryd [marklemcd] [ In reply to ]
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I guess I look at it this way: there are more well-trained/highly-qualified PhDs working for/on Stryd than on other "wearable tech", including anything developed/sold by Garmin, Apple, etc. The odds that they would stuff up a relatively simple calculation therefore seems pretty low to me.
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Re: Precision and accuracy of Stryd [Andrew Coggan] [ In reply to ]
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Can you distill this paper down into something more readable for us mere mortals Andy?
Despite having a doc and a masters myself I struggle with this as it is outside my area of expertise. Downloaded the paper and had a good read but struggled.
Is the OptoGait the gold standard by which other systems are normally compared?
Have been tempted to try out the Stryd but not quite sure how it will work for me.
Cheers.
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Re: Precision and accuracy of Stryd [Andrew Coggan] [ In reply to ]
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Posted this to the Stryd Community FB group earlier this week:

"Just an FYI: I am at the American College of Sports Medicine meetings at the moment. I saw a number of posters evaluating Stryd, mostly (IIRC) from Furman University. The executive summary is that Stryd power data are highly reproducible across days/surface/devices, and is closely correlated with VO2. This is of course expected based previous reports and Stryd's own data, but independent confirmation is still nice to see."

To anticipate Ray's commentary: none of the posters I saw seem to have addressed the accuracy of the power data, only the precision/reproducibility and association with VO2. I assume that is because they were from smaller institutions that didn't own a force plate (or better still, a $70k force plate treadmill, like Stryd built in-house). The way that the instrumented mats or walkways are catching on, though, I expect that will soon change.
Last edited by: Andrew Coggan: Jun 3, 18 5:38
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Re: Precision and accuracy of Stryd [Andrew Coggan] [ In reply to ]
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Andrew Coggan wrote:
Posted this to the Stryd Community FB group earlier this week:

"Just an FYI: I am at the American College of Sports Medicine meetings at the moment. I saw a number of posters evaluating Stryd, mostly (IIRC) from Furman University. The executive summary is that Stryd power data are highly reproducible across days/surface/devices, and is closely correlated with VO2. This is of course expected based previous reports and Stryd's own data, but independent confirmation is still nice to see."

To anticipate Ray's commentary: none of the posters I saw seem to have addressed the accuracy of the power data, only the precision/reproducibility and association with VO2. I assume that is because they were from smaller institutions that didn't own a force plate (or better still, a $70k force plate treadmill, like Stryd built in-house). The way that the instrumented mats or walkways are catching on, though, I expect that will soon change.

Let’s assume for sake of argument that it is accurate. How does this tool change run training? In the context of running, where this correlates to vO2, how is this better than just using a Daniels vdot table or another analogue for vO2?
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Re: Precision and accuracy of Stryd [marklemcd] [ In reply to ]
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marklemcd wrote:

Let’s assume for sake of argument that it is accurate. How does this tool change run training? In the context of running, where this correlates to vO2, how is this better than just using a Daniels vdot table or another analogue for vO2?

Purely as a layman triathlete, I really don't understand this argument. An accurate and precise powermeter for running seems to give me the same benefit as one for cycling, i.e., I can gauge my level of effort independently of the grade I'm running on. There's very little 'flat ground' around here - everything I run on is slightly uphill or downhill. Targeting x pace is often a fool's errand, and gauging short intervals by HR is useless, so why wouldn't I want to be pacing by power instead?
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Re: Precision and accuracy of Stryd [el gato] [ In reply to ]
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el gato wrote:
marklemcd wrote:


Let’s assume for sake of argument that it is accurate. How does this tool change run training? In the context of running, where this correlates to vO2, how is this better than just using a Daniels vdot table or another analogue for vO2?


Purely as a layman triathlete, I really don't understand this argument. An accurate and precise powermeter for running seems to give me the same benefit as one for cycling, i.e., I can gauge my level of effort independently of the grade I'm running on. There's very little 'flat ground' around here - everything I run on is slightly uphill or downhill. Targeting x pace is often a fool's errand, and gauging short intervals by HR is useless, so why wouldn't I want to be pacing by power instead?

Because for most people, training by power wouldn't benefit them and could hurt them. For example, your hilly runs - Running downhill at the same wattage as uphill would mean you're running faster. Unlike cycling though where you're clipped in and making similar muscle movements, your quads would take the full force of your weight and if it were a race, you could easily bury yourself with muscle damage due to fast downhill sections.

AFAIK, these power meters also don't take in to account wind differences like cycling PM's do -- something that could make a pretty big impact on pacing as well.

The only real place for them is for trail running and form evaluation / corrections.

In addition, power means nothing in terms of a running race where most people are concentrating on a specific time (and correlated pace). You call pace a "fool's errand", and as an experienced marathoner, I call pace the only metric that matters if you're going for a clock time.
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Re: Precision and accuracy of Stryd [caverunner17] [ In reply to ]
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Given that power correlates to vO2, and in running it’s easy to run by feel or pace depending on course and conditions, what’s the gain in having a power meter? In cycling it’s obvious. What is it in running? I can go to a track and do intervals off pace perfectly. And in a race I am RACING, not trying to hit some power number. If I need to cover a surge, I will cover the surge if I feel I can. Or I can sit on a certain power and lose contact? WTF, no way.

I train completely by power on a bike. It works. In running no one has convinced me there is any benefit, and every single person I know who tried quit it fairly quickly for that reason.
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Re: Precision and accuracy of Stryd [caverunner17] [ In reply to ]
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caverunner17 wrote:
AFAIK, these power meters also don't take in to account wind differences like cycling PM's do -- something that could make a pretty big impact on pacing as well.

It takes nearly gale-force winds to alter the energy requirements of running by more than a few percent.
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Re: Precision and accuracy of Stryd [marklemcd] [ In reply to ]
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marklemcd wrote:
Given that power correlates to vO2, and in running it’s easy to run by feel or pace depending on course and conditions, what’s the gain in having a power meter? In cycling it’s obvious. What is it in running? I can go to a track and do intervals off pace perfectly. And in a race I am RACING, not trying to hit some power number. If I need to cover a surge, I will cover the surge if I feel I can. Or I can sit on a certain power and lose contact? WTF, no way.

The most common benefit people seem have found from running power is using it as a pacing tool in longer events. That is especially true in hilly races, but even in flat ones it can be helpful to have a real time, stable, and accurate indicator of your effort, something that, e.g., ordinary footpods or (worse) GPS often fail to provide.
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Re: Precision and accuracy of Stryd [Andrew Coggan] [ In reply to ]
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Andrew Coggan wrote:
caverunner17 wrote:
AFAIK, these power meters also don't take in to account wind differences like cycling PM's do -- something that could make a pretty big impact on pacing as well.

It takes nearly gale-force winds to alter the energy requirements of running by more than a few percent.

Not even close to true. Look at any track meet. Wind aided times in excess of 2m/s are considered to not be considered for record keeping for sprints. That's about 4.5MPH.
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Re: Precision and accuracy of Stryd [caverunner17] [ In reply to ]
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It absolutely is true.

First, the counter-example you offer is essentially meaningless, since the standard being applied in choosing that cut-off could be a difference of, say, no more than 0.00001%.

Second, a wind speed of 2 m/s *at ground level* is actually a fairly strong wind.

Third, there are plenty of data in the scientific literature going back at least 50 y supporting my statement.
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Re: Precision and accuracy of Stryd [Andrew Coggan] [ In reply to ]
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Andrew Coggan wrote:
It absolutely is true.

First, the counter-example you offer is essentially meaningless, since the standard being applied in choosing that cut-off could be a difference of, say, no more than 0.00001%.

Second, a wind speed of 2 m/s *at ground level* is actually a fairly strong wind.

Third, there are plenty of data in the scientific literature going back at least 50 y supporting my statement.

You said Gale force winds. 4.5MPH isn't Gale force. Having had 10 years of experience running track, it happens fairly often that that limit is exceeded especially in the spring. I'd rather believe the IAAFs rulings on what is wind aided alongside numerous accounts of 100 and 200m wind aided times which are statistically faster than non wind aided times.

It's not hard to believe that a fairly open course running into a 5-10mph headwind would have performance suffer. I don't need 50 years of studies to tell me that wind effects performance. Numerous Boston marathons have shown it does (depending on directions of the wind, either aide or hinder performance). If the stryd power meter doesn't account for wind, then it's not calculating real power usage in the same way a cycling power meter does.
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Re: Precision and accuracy of Stryd [caverunner17] [ In reply to ]
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I said *nearly* gale force, not gale force.

Since you brought up an exact value of 2 m/s, though, let me be a bit more specific:

Even a wind speed of 5 on the Beaufort scale (which on open terrain - an airport taxiway, for example - would correspond to a ground level wind speed of ~2.5 m/s) would only alter the energy requirement of running by a few percent.

This conclusion is based in part on studies in which the oxygen uptake of runners has been measured while they ran on a treadmill placed in a wind tunnel.

"In God we trust. Everyone else must bring data." - W. Edwards Deming
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Re: Precision and accuracy of Stryd [Andrew Coggan] [ In reply to ]
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Andrew Coggan wrote:
I said *nearly* gale force, not gale force.

Since you brought up an exact value of 2 m/s, though, let me be a bit more specific:

Even a wind speed of 5 on the Beaufort scale (which on open terrain - an airport taxiway, for example - would correspond to a ground level wind speed of ~2.5 m/s) would only alter the energy requirement of running by a few percent.

This conclusion is based in part on studies in which the oxygen uptake of runners has been measured while they ran on a treadmill placed in a wind tunnel.

"In God we trust. Everyone else must bring data." - W. Edwards Deming

A few percent is huge though. Let's just go for 2%. 2% of 180 minutes (3 hr marathon) would be roughly 4 minutes. While 4 minutes might not mean much for someone who's less experienced, 4 minutes for someone who has trained specifically for a race at a certain pace means either they need to really be able to run a 256 to break 3 hours or they need to adjust to a 304 to keep the same effort as 3 hours. If it's actually a 3%+ difference, it's even worse.

Look, I'm not disagreeing that the stryd pod can calculate a given power for certain cases. What I'm saying is that the power number for running isn't nearly as useful as a metric as it is for cycling. Things like elevation, wind, muscle fatigue, form breakdown, etc all play factors that a power number from stryd can't account for.

A promise if you had a stryd pod on some elite runners that their power usage at mile 22 is significantly higher than at mile 2 as their form breaks down. That doesn't mean they should slow down though to maintain a certain power number. For them, all that matters is pace and where their competition is. No different than a runner trying to hit a PR
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Re: Precision and accuracy of Stryd [marklemcd] [ In reply to ]
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marklemcd wrote:
Given that power correlates to vO2, and in running it’s easy to run by feel or pace depending on course and conditions, what’s the gain in having a power meter? In cycling it’s obvious. What is it in running? I can go to a track and do intervals off pace perfectly. And in a race I am RACING, not trying to hit some power number. If I need to cover a surge, I will cover the surge if I feel I can. Or I can sit on a certain power and lose contact? WTF, no way.

I train completely by power on a bike. It works. In running no one has convinced me there is any benefit, and every single person I know who tried quit it fairly quickly for that reason.


First off, sure I could go to the track and do intervals off pace. What if I don't want to do my intervals at the track? What if I want to do my mile repeats on the creek trail around here instead? The closest track is the local high school, which I only have access to late evenings (after all the teams have finished their practices), or weekends.

You say that you train with power on the bike and it works, but then you throw out a comletely different scenario for why it wouldn't work when running. Whether you're on the bike or running, you're either racing in a pack (with all that entails) or you're not. If keeping contact with a pack is important, then holding a steady power is not - whether on the bike or the run. If it's a long event, and/or you're an average AG'er, and keeping contact with a pack is not important and you're looking for optimal pacing, then power is equally applicable in either scenario.

All of these arguments for why power will never work for running sound EXACTLY like the arguments that everyone was throwing out for why power would never work on the bike 15-20 years ago.
Last edited by: el gato: Jun 3, 18 17:59
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Re: Precision and accuracy of Stryd [caverunner17] [ In reply to ]
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2% is exactly 2%. Whether that is large or small is in the eyes of the beholder.

Recently I commented in this thread or another that the most frequent use-case for power data has been for pacing purposes, especially in non-flat races. If you are willing/able to dig in a bit deeper, though, there are other applications, including looking for fatigue-induced changes in the ratio of power to pace (i.e., running effectiveness) during longer runs. Simply put, some people show changes but others do not, which *might* provide insight into how to best modify training in the future.

(Related: somebody recently commented on the Stryd FB page that their masters thesis research found a relationship between "form" a.k.a. oscillatory power and running economy.)
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Re: Precision and accuracy of Stryd [Andrew Coggan] [ In reply to ]
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I really don't see why anyone should object to the use of power for running. It isn't as if anyone is saying you should disregard pace and feel.

Sometimes people get set in their ways and don't want change. If you look at indoor rowing where Concept2 machines measure power you see most rowers and coaches ignore the watts and work entirely with pace. When you are working out training plans and looking at performance it's far easier and logical to look at the watts and work on percentages and ratios. A 5% increase in power does not give you a 5% increase in pace rowing etc etc. But because rowers communicate in stroke rate and time / speed most don't understand what a watt is so ignore them.

I suppose if you are a minimalist and want to minimise gadgets and keep your running very zen you would prefer not to measure your running power, but then if you follow that through you would dump the watch too.

I can see a lot of uses for them.
When I did a lot of running there was almost nowhere flat. I used several routes and my average pace was different on all of them so you couldn't compare a runs. Surfaces also affect pace. Even slight undulations affect pace. Then when you move and train on different routes you can't compare your training on the new routes with your old routes and years of training data becomes far less useful.

A question. When running do we see increases in speed requiring greater percentage increases in power, e.g., does a 10% increase in pace on the flat require a more than 10% increase in power?
Last edited by: Trev: Jun 3, 18 23:48
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Re: Precision and accuracy of Stryd [Andrew Coggan] [ In reply to ]
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A 2% increase in power will cause fatigue to occur much sooner. The difference between 20 minute and 60 minute power is only approximately 5%.

In Concept2 rowing the difference in power between 500m and 1000m is only approximately 5%, between 1000m and 2000m approximately 5% although the drop in power from 2000m to 5000m is approximately 14%. NB that was what I worked out from a British Lightweight record holder.

In my opinion 2% is large, but then I get perturbed if one of my watches gains or loses more than 4 seconds a day.

Another use I see is this. I used running for fitness for other sports. My weight was very different for each sport, so it was hard to compare fitness or power output over time as weight affected pace so much, but it would be interesting to know if my power was the same when I weighed 11st 6lbs as when I weighed 14st.

Andrew Coggan wrote:
2% is exactly 2%. Whether that is large or small is in the eyes of the beholder.

Recently I commented in this thread or another that the most frequent use-case for power data has been for pacing purposes, especially in non-flat races. If you are willing/able to dig in a bit deeper, though, there are other applications, including looking for fatigue-induced changes in the ratio of power to pace (i.e., running effectiveness) during longer runs. Simply put, some people show changes but others do not, which *might* provide insight into how to best modify training in the future.

(Related: somebody recently commented on the Stryd FB page that their masters thesis research found a relationship between "form" a.k.a. oscillatory power and running economy.)
Last edited by: Trev: Jun 4, 18 0:41
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