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Thursday, November 21, 2019

Stryd running power - validation with VO2 testing

I am always looking for interesting wearable devices but since I don't run, have neglected that particular sport.  I recently reviewed some impressive Stryd data metrics and wanted to share them.  First, what is Stryd?  It is essentially a power meter for runners.  Using various motion sensor data fields and rules of physics it is able to compute effective running power.  The hardware includes both 3-axis gyroscope, 3-axis accelerometer and now wind sensing in the newest version.

Is this tech new?  No, in fact many years ago I used something called an "Ibike", now rebranded as the "Powerpod".  This device used similar principles of physics to estimate cycling power:

Unfortunately, it was a pain to calibrate and was not very accurate compared to conventional power meters.

Given my prior poor experience with the "Powerpod" and my personal avoidance of running (bad knees), I never paid much attention to Stryd.  However, recently my friend the cross country skier did his VO2 (near) max test and was wearing the Stryd.  This gives us an ideal opportunity to see how well the Stryd tracks with VO2.  Why is this important?  With cycling power, we have the simple ability to test an established strain gauge related power meter (like the Powertap, Assomia) against something like the Powerpod that uses environmental sensors (speed, wind, acceleration).  The Stryd has no easy direct comparison device.  But, since work rate (power) is linearly related to VO2 (oxygen consumption), especially up to the second lactate threshold, we can benchmark the Stryd to an alternate reliable benchmark.

Are there any validation studies? 
I searched through Pubmed and found very little published work on the Stryd.  The company claims to have VO2 vs Stryd power testing results on their website, but of course that is open to skepticism.

We do have this comparison study:

Which does not look good at all.  A very weak relation of Stryd power to VO2 exits, hence their conclusion:
The main issue here is that the test was done with an early device, the Stryd Pioneer which is a chest mounted triaxial accelerometer:

I almost wonder if they would have been better served not even producing this device.  

In response to the comment from the Stryd developers (see below), here is the reply from the authors of the above paper:

A different study looked at the newer Stryd from a different point of view.  They simply wanted to see how consistent the measure of power was on a treadmill at a constant velocity.  Theoretically, with no change in wind, incline or running dynamics, there should be a steady power output as in cycling on an indoor trainer at constant cadence.

From the study:
To the best of the authors’ knowledge,
just one study has examined its validity and reliability (in this case,
to measure spatiotemporal gait characteristics [9]), with no data
to demonstrate the validity and reliability of this device for measuring
power and related variables
. For this study, only two out of
twelve metrics were used (running velocity and power output).
The agreement between measured intervals (watts) at the same treadmill speed was good.
For example the last graph showed near identical power over 0-120 vs 0-180 seconds:

With the conclusion:
The results show that
power data during running, as measured through the Stryd™ system,
is a stable metric with negligible differences, in practical
terms, between shorter (i. e., 10, 20, 30, 60 or 120 s) and longer
recording intervals (i. e., 180 s). 
In summary, these results show that power output during running,
measured through the Stryd™ system, is stable over time
when velocity is constant and under controlled conditions, with no
differences between different time intervals recorded during a
3-min run. Nevertheless, it is worth noting that the analysis conducted
shows that longer recording intervals yield smaller systematic
bias and narrower limits of agreement.
Therefore, if maximum
accuracy is required (e. g., scientific approach), longer recording
periods must be used (i. e., 2–3 min).
  • So one still needs to be careful with the shorter time recordings, but they are still reasonable.

We still don't have a published VO2 vs power study of the newer, foot mounted unit.  Thanks to my friend the cross country skier we at least have data in one indivisual.  

To start with, let's look at what VO2 vs cycling power looks like from my data.  This was using the Assomia Duo pedals (1% accuracy) and VO2 done at the University of Florida Sports Performance Center.

Here is an example from my VO2 max testing - cycling power vs VO2 (oxygen usage):
(VO2 average from last 60 sec of interval, cycling power average from each 3 minute stage)

Note the last value falls off (it's an error in the gas exchange measurement), we won't count that.  Otherwise, the curve is quite linear, with only a few watts deviation off the fit.

How did the cross country skier do in regards to VO2 vs Stryd power?
Here is his treadmill test, plotting power over time with each stage encompassing 5 minutes, followed by a brief time between stages to do lactate testing, then resumption of running.  The trend of higher power per stage is very evident.

A close up of the last 2 stages shows no major lag from zero power to full treadmill speed. 

Therefore, "heart rate like lag" is avoided (from Kubios):

Finally, the all important VO2 vs power:
(VO2 average from last 60 sec, Stryd power from the entire 5 minute interval)

  • This looks every bit as close as my cycling ramp shown above!

Why do we need power when the heart rate to VO2 curve is just as good if not better?
After all the heart rate to VO2 relation is very tight and predicable.
  • In this case, if he knew his heart rate, the VO2 would be easily extrapolated.
Why do we need power?
  • One problem with simple heart rate is measuring high intensity work rates during HIT.  For instance, if you do a 30 second burst interval at high power, the heart rate does not fully stabilize until the interval is over.  
  • In addition, if you train by pace/speed, knowing the speed is problematic given the state of GPS tracking accuracy.  
  • Many studies point to the importance of intensities beyond LT2/MLSS as critical for optimal training.  Those higher power zones can't be properly monitored by heart rate, especially in the early part of the stage.  
  • If one is an advocate of the "fast start" strategy, power is an essential way of achieving that type of session.  Careful power modulation is the only way to perform a fast start interval.
Where we don't necessarily need power monitoring is during the mind numbing, long, slow runs that should be kept below VT1.  In that case, heart rate stability kinetics are very acceptable to keeping effort down.  However, in situations like hill and trail running, having running power may be of additional benefit to staying in the proper zone 1 location.

  • The current Stryd device appears to measure power at stable levels at constant treadmill speeds.  The older Pioneer device is not recommended.
  • According to a plot of VO2 vs power in one individual, the Stryd relative power is linear.  In other words, with a given percent boost in power, the percent oxygen consumption rises the same relative amount.  This indicates there is relative accuracy of the device.  Absolute accuracy is not easily testable with this method.
  • Usage with HIT and running intensities above MLSS would make sense since heart rate will lag well behind the effort.  Studies have shown the downsides to intervals at "threshold" intensity (MLSS).  Stryd usage may be a way to avoid excessive zone 2 activity.
  • Usage in recovery and zone 1 training is potentially useful especially with less than stable running conditions.  As discussed in previous posts, high volumes of zone 1 are essential to proper performance development.
Many thanks to my friend for sharing his data!

See also:
How to get your training zones from HRV and muscle O2 data
VT1 correlation to HRV indexes - revisited  


  1. Hi,

    Angus from Stryd here.

    For the study on the Pioneer noted in this article, please see our three part response here:
    Part 1:
    Part 2:
    Part 3:

    In a nut shell, the data indicates that there *is* significant correlation between the Stryd Pioneer & metabolic energy expenditure, but the methodological flaws in the paper led to false conclusions.

  2. According to the reply from the authors, the VO2/speed vs Power/speed relationship was done to handle the diverse subject population and terrain. In their reply, they state that the weak correlation was still present. I have included the reply above. From my perspective, this early device should not be excessively dwelled upon. The latest incarnation seems sound at least from the above data.