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Understanding the Stryd Weight Setting
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Understanding Stryd Weight Setting:

How It Impacts Your Metrics

by Steve Palladino

coach and consultant, Palladino Power Project

October 29, 2018

The Stryd running powermeter computes power and other running metrics based on accelerometer data.  From this data, power is natively computed in Watts per kilogram (W/kg).  

Stryd allows the input of a body weight setting.  From the body weight setting, Stryd then reports power as a raw Watts value - converting the natively computed W/kg to W.   For example, a runner that is producing a natively computed 4 W/kg effort, and has a Stryd weight setting of 70kg, would see a reported 280W power value on their watch and in post-hoc data analysis.  

If the 70kg runner above set the Stryd weight setting to 1kg, the runner would see a reported value that is essentially the true W/kg that he/she is producing (assuming watch and post-hoc data analytics can report out to two decimal places).   What the runner would see on the watch would not be true raw W, but instead the true W/kg.  For example, if the 70kg runner ran at the same effort as mentioned above (4W/kg) with a Stryd weight setting of 1kg, they would see 4W on their watch and in post-hoc analysis, yet when converted to W/kg using the set Stryd weight of 1kg, the value would be true: 4W/kg.

If the 70kg runner above set the Stryd weight erroneously to 80kg,and ran at the same 4W/kg effort, the runner would see a reported 320W power value on their watch and in post-hoc data analysis, not 280W.   However, if the runner evaluated the 320W performance in post-hoc analysis using the associated Stryd weight setting (80kg), the runner’s power in terms of W/kg would still be true at 4W/kg.  The raw W value may not be true, but ultimately, W/kg remains true.

In other words, power, in terms of W/kg, will always be true as long as the reported raw W value is divided by the associated Stryd weight setting.

Every time one changes the weight setting in Stryd, the basis for arriving at the raw W value that is reported to the watch, in turn, changes.    (native W/kg)*(Styd weight setting in kg) = raw W reported to the watch

For example, say a runner is running at their FTP of 300W.  On day 1, the runner has the Stryd weight setting at 70kg, and runs on the track for 20 minutes @ 300W.  The runner ran at 4.29 W/kg for 20 minutes.  On day 2, the runner re-set the Stryd weight setting to 72kg.  The runner runs the track again for 20 minutes at the same speed, same effort, same conditions.  The runner’s watch would show 308-309W.  However, the runner would still be running 4.29 W/kg for 20 minutes.

Changing Stryd weight will change reported W.  Changing the weight setting higher translates to higher reported W for same effort.   Changing the weight setting lower translates to lower reported W for same effort.

Now let's revisit the runner that changed their weight setting from 70 to 72kg.  The athlete’s FTP doesn’t change overnight - it was 4.29 W/kg on the last day that the weight setting was 70kg, and it remains at 4.29 W/kg the very next day, when the weight setting was changed to 72kg.  However, what if the athlete does not recognize that raw reported W will change whenever the weight setting is changed?  The runner does the 20 minute run on the track @ 300W on day one.  The next day, the runner erroneously runs the 20 minute track run at 300W, thinking that that is still the target.  The problem is that on the second day, the runner is running at a lower effort - 300W/72kg - only 4.17 W/kg, instead of the prescribed 300W/70kg - 4.29 W/kg.  (likely thinking, dang, I’m killing this 20 minute run today, it feels easier)

We see the question from time to time: My CP dropped from 300W to 295W, but I am faster and in better shape.  Why did my CP go down?   On questioning, we discover that the last CP test was done at a weight setting of 80kg, and the new CP test was done at 76kg.   The first CP was 3.75 W/kg.  The new CP is 3.88 W/kg.  The second CP result is actually improved over the first!

If you never change the Stryd weight setting, then not only are W/kg always true, but raw W reported on the watch can be comparable for that athlete over time.  It is my recommendation to leave the Stryd weight setting unchanged.*  If you wish to change the weight setting, it is best to do so only at a point when comparing old data to new data (in terms of raw W values) is no longer important.  On the other hand, if you do not follow raw W values, and follow only W/kg, then feel free to make changes in the Stryd weight setting.  I follow W/kg in all of my athletes, but nevertheless, to eliminate confusion and for ease of comparison over time, I still advise not changing the Stryd weight setting.

        * I also recommend disabling any outside weight metrics (like Apple Health) from accessing the Stryd weight setting.

But wait, if I leave the Stryd weight setting fixed over time, and I actually do change weight, doesn’t that mean that Stryd will be wrong?   Not in terms of W/kg.   And further, raw reported W will retain the capacity to demonstrate change regarding fitness over time.

Since W/kg are always true as long as you use the associated Stryd weight setting, if your fitness improves while the weight setting is fixed, then W/kg over a given duration will improve.  For example, 20 minute maximal power might improve from 3.95 W/kg to 4.00 W/kg.  You may have actually decreased body weight, or even increased body weight, but if you are faster and fitter, your W/kg will be higher - even if the Stryd weight setting is fixed.  If Stryd weight setting was fixed at 70kg during this period of improvement, raw reported W would have changed from 276.5W to 280W.  If you had lost 2kg during this period and changed weight, your raw W would have been reported as 3.95 W/kg * 70kg = 276.5W, and then later 4.00 W/kg * 68kg = 272W.  And, you would be wondering why you are fitter/faster, yet your raw reported W is lower.  Had you simply kept the weight setting at 70kg, you would have seen the raw reported W improve.  

Keep the weight setting constant.  W/kg will always be true.  The raw reported W may not be ‘true’, but more importantly, will be comparable over time.  If you want to know ‘true’ power, work in terms of W/kg.

Also keep in mind that since power is sensitive to weight and duration, when comparing your power to someone else’s power, it is best to compare W/kg over a similar duration.  For example, compare W/kg @ 20 minutes, or possibly W/kg @ CP or FTP, or even W/kg @ 10K (assuming that duration for 10K is not disparate).  On the other hand, if comparing power for yourself (for example, this year versus last year), comparing raw W is fine - as long as the Stryd weight setting has been constant over the period of comparison (unless, as stated earlier, you convert everything to W/kg).

What if you have changed the Stryd weight setting and wanted to relate the raw W values?  For example what if you record a CP test value of 280W while the weight setting was 70kg, but now just changed the weight setting to 68kg?  Do you need to repeat the CP test?  Not if the test is still recent and legitimate.  You simply convert the old CP @70kg to your new CP at 68kg by this equation:  (new weight setting / old weight setting)*CP at old weight setting = CP at new weight setting.   In this case, the 280W CP converts to 280*(68/70) = 272W.   Of course, this type of math is obviated by leaving the Styrd weight setting fixed.

What about carrying exogenous weight, like a hydration pack, or loaded backpack, or weight vest?

In this scenario, I suggest that the runner modify power targets by applying a correction factor.  For example, a runner with a 70kg Stryd weight setting has an easy aerobic run scheduled to be run at 80% of FTP, which for this runner, is 240W.  The runner is going to do the run while commuting to work with a 2kg backpack.   The power would be corrected as follows:

Corrected target power = “unweighted” target power * (Stryd weight / (Stryd weight + added weight))

In this example, that would be correct target power = 240 * (70 / (70+2)) = 233W.

Some have suggested setting Stryd weight to 100kg and leaving it there.  If this were done, the raw W reported on the watch would actually be the runner’s W/kg *100.  For example, if Stryd weight is set to 100kg, and the runner is seeing 385 W on the watch (or post hoc analysis software), it represents that the runner is/was running at 3.85 W/kg.  This is a very utilitarian application of Stryd weight setting.  In turn, one must be prepared to see numbers like 515W @ FTP if the runner is an elite with a 5.15 W/kg FTP.   Also, one must interpret LSS as LSS/kg (read below)

A last point on power:  If one is using software that allows weight entry (such as WKO4), the user MUST enter the Stryd weight setting in the software, not real weight.  If you want to track real weight, do so elsewhere.  But to the extent that the weight entered into the software is used for calculation of metrics such as W/kg, or LSS/kg, or Running Effectiveness, you must always make sure that Stryd weight setting matches the software weight setting - and make changes to the software weight setting on the same day as any Stryd weight setting change.  (of course, it is best to just set Stryd weight and leave it….and do the same with the analysis software)  

Also realize that power is not the only metric sensitive to weight.  Leg Spring Stiffness (LSS) is another such metric.  The same ‘rules’ apply.  Increasing Stryd weight setting increases reported raw LSS.  Decreasing Stryd weight setting decreases reported raw LSS.  To compare LSS across runners, you cannot use raw LSS values, and you must use LSS/kg values.  Again, if you keep your Stryd weight setting fixed, then you can compare your own raw LSS values over time (but not to other runners).  If you change the Stryd weight setting, then you must calculate LSS/kg to compare your own data over time.

Running Effectiveness (RE) uses W/kg in the denominator of its equation, so it will remain correct, as long as the associated Stryd weight setting is used.  On the other hand, Efficiency Index (EI) uses raw W in its equation.  Whenever the Stryd weight setting is changed, the athlete’s baseline or reference values for EI then change.  EI floats with Stryd weight change - not good.  EI will only work for a given athlete if Stryd weight setting remains fixed.  However, regardless of Stryd weight setting, because EI uses raw W in the denominator, it cannot be compared across athletes.  While the two metrics are similar in trying to reflect a speed:power ratio, RE is clearly superior and EI should be avoided.

Another favorite metric of mine, Horizontal Power Ratio (as well as Form Power Ratio) is automatically normalized to weight.  Thus, whether you change the Stryd weight setting, or leave it fixed, Horizontal Power Ratio and Form Power Ratio will remain unaffected.  Think about it: let’s say that you are concerned that weight is accounted for.   You then do the computation of Horizontal Power Ratio and Form Power Ratio using W/kg instead of raw W.   HPR = (Stryd power - Form power) / Stryd power   FPR = Form power / Stryd power

With kg in both the numerator and denominator, they cancel out.  HPR and FPR are automatically normalized to weight.

Appendix

  1. Read discussion regarding this article on the Palladino Power Project FB group here

  1. A good example of the impact of changing weight setting on CP test result interpretation here

  1. 10/21/2020  Updated thoughts on the Stryd weight setting:

1) Stryd natively calculates power in W/kg, and uses the weight setting as a multiplier to output power in Watts. Regardless of weight setting, W/kg are always relative over time. 2) When you change the weight setting, old data is rendered not comparable in terms of Watts, while remaining comparable by W/kg (using the weight setting specific to each run or run data set being compared. 3) While a non-wind pod user can change weight 10+ kg and never have a compatibility issue, it is subtly different for the wind pod user. Since the airpower component requires height and weight (height being more important to accuracy than weight), having these values *reasonably close* to real is important for appropriate airpower values. However, since a) airpower is a very small percentage of total Stryd power, and b) changes in height and weight make small changes in reported airpower, *reasonably close* takes on the perspective that height and weight only need to be changed for rather large changes. 4) My recommendation with the non-wind pod is to never change the weight setting. Period. 5) My recommendation with the wind pod* is to change the weight only with big changes to "real" height or weight. For example, >3 kg of weight change, or >2cm of growth (for younger Stryd users). 6) That said, the greater factor in determining weight setting change is the loss of relativity - the loss of comparability to older data. When you change the weight setting, old CP estimates lose comparability (unless converted to W/kg), LSS changes, tracking changes in Watts for prior races is lost. Relativity is disturbed. Realize that the Stryd analytics ecosystem is Watts-centric, not w/kg centric. When you change the weight setting, you are exchanging loss of power relativity (in Watts) / loss of analytic capacity (in a Watts-centric environment) for "more true power number in Watts". ***So, the key decision, IMO, on changing the Stryd weight setting is not solely the real weight vs Stryd setting difference, but more importantly, when is best for you to lose analytic relativity - to lose comparability to old data.*** IMO, if one decides that losing analytic relativity is OK, then doing so at the right time is critical. For example, you would not change the weight setting when inside a training period for an A race - you would wait until the A race has passed. You would not necessarily change the weight setting in season, and instead wait until the season is over. 7) if you do change the weight setting, you do so immediately prior to doing a CP test. IOW, upon changing the weight setting, you *must* do a CP test. You cannot rely on a modeled CP/FTP initially after changing the weight setting. 8) * I also condone changing the Stryd weight setting for wind pod users when accounting for the loss of airpower when using an outdoor CP for treadmill running, or in use cases involving a non-motorized TM.

  1. 12/4/2020  Update on thoughts regarding weight settings specific for treadmill use:

When running on a treadmill, typically there are multiple factors that can change the relativity of reported power on the treadmill to outdoor FTP/CP (see figure).  

Factors that change the relativity of reported power on a treadmill to outdoor FTP/CP.

Loss of Airpower

Heat / Humidity Changes

Functional Differences of NMCTM*

Stryd wind version

✔

✔

✔

Stryd non-wind version

✔

✔

 *NMCTM = non-motorized curved treadmill

For power to be meaningful it must be relative.  Power must relate to something….5K power today relative to 5K power 6 months ago....10K power at 6000 feet / 1829 meters of altitude relative to sea level...and so on.   For most Stryd users, the key relativity of their power is to FTP / CP….. this training run is to be at <= 80% of FTP/CP….that 10K was at 101% of FTP/CP...we relate power to FTP/CP.

When one establishes their FTP / CP outside, running overground, we relate training to that value, and indirectly, we relate racing targets to that value.  However, there are those factors (see table) that prevent relating power to one’s outdoor FTP/CP.  

Something has to change.  In short, to account for those factors on the treadmill, one has to change either, a) their FTP/CP, or b) the weight setting on the Stryd device.  Either FTP/CP is lowered or the Stryd weight setting is increased.  There are either indoor and outdoor FTP/CP values that are used, or indoor and outdoor Stryd weight settings that are used.

Fortunately, whichever way decides to go, the percent change is the same.  If one calculates that the loss of airpower and heat/humidity difference between outdoor running and running on the treadmill account for a 5% loss relative to outdoor FTP/CP, then either, a) the outdoor FTP/CP is lowered by 5% to arrive at a treadmill FTP/CP, or b) the Stryd weight setting is increased by 5% for treadmill running.

If a runner elects to lower FTP/CP to account for the treadmill factors in the table above, then:

If a runner elects to increased the Stryd weight setting to account for the treadmill factors in the table above, then:

The advantage of the former is that one does not have to fiddle with the weight setting depending on whether the run is outdoors or on the treadmill.  The disadvantage is that unless one is also changing their Powercenter CP value, treadmill running RSS will be under-reported, and training metrics impacted to a small degree.   Also, one would have to change the FTP/CP on which power targets are based for training - either some maths are involved, or one has to change FTP/CP when building structured workouts.

The advantage of the latter is that there is only one FTP/CP that is used - power targets are relative to one FTP/CP, structured workouts are built on one FTP/CP, and RSS and downline training load metrics are more accurate.  The disadvantage is that one has to remember to change the weight setting, depending on whether the run is outdoors or on the treadmill.

Either way can work - choose what seems best for you.  For the athletes that I coach, I have shifted my preference from the former (two FTP/CP values) to the latter (two weight settings).  I prefer the one FTP/CP, two weight settings approach, and the more accurate training load metrics.

Resources

  1. Video demo: Stryd 'wind' version users: How to obtain a treadmill-specific CP from an outdoor CP test.
  2. Discussion and calculator: Adjust Stryd Weight for Treadmill Usage
  3. Article: Using Running Power on the Treadmill

5) February 20.2025:  Video: Stryd Weight Setting - Fix or Float ?