Stryd Power off - how to adjust rFTP? | 80/20 Endurance

# Stryd Power off – how to adjust rFTP?

Viewing 10 posts - 16 through 25 (of 25 total)
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• #15397
winoria
Moderator

that’s my point. I know it is simple physics. Here is what makes me struggle:
Stryd recommends to not change the weight. Well, lets assume some person runs a 8min mile at 240 Watts at 155lbs of weight. That very same person loses 5lbs and is now down to 150lbs but as per Stryd’s recommendation is not changes the weight setting. That would mean that all the displayed running power is simply not correct as that very same person would now need a lower power output to propel the now only 150lbs at a pace of 8min/mile.
Or, lets assume an athlete races an Ironman. It can be assumed that by the time the athlete starts the running portion of the race, some weight was lost (water). The running power is therefore not correct.
As per my understanding, running power (whether it is Garmin, Polar or Stryd) is a simple or for that matter not so simple calculation that includes weight, pace, force, elevation changes, wind, etc. and comes up with a number. If one of the factors changes, the number changes.
Looking at a power meter for cycling, this is not quite the case. On a bike, a watt is a watt is a watt. No matter if your weight changes, or if it is windy. If you ride at 200 Watts you ride at 200 Watts. If you go up a hill, you will be slower than down hill, if you gain weight, the output is the same but your speed will change. If you have headwinds, you still ride at 200 watts but you go slower. If you lose weight while riding, you still push 200 Watts. What changes is the power per weight ratio or W/Kg. The way Stryd and others tackle(d) it is never changing the W/Kg but actually artificially show a too high number if weight is lost or a too low number if weight was gained.
The running power devices tackle everything from the backend and are applying a formula to come up with a theoretical power output.
If a running training plan is based on power and the weight of the athlete is updated as it changes, the reading and therefore zones are suddenly wrong or could be wrong.

I’m not saying there is a better way to measure power for runners but I still struggle with it.
And yes, I run with a Stryd and also have a HRM Pro, so I can look at Garmin’s and Stryd’s power numbers. The issue mentioned above remains the same.

#15400
Bstarr
Participant

I would totally agree. I don’t know how one would claim to have an accurate power number if not adjusted for weight.

#15408
David Warden
Keymaster

winoria, I can’t argue with any of your points, and I don’t have a perfect answer. For me it’s academic:

– An accelerometer requires a mass value for an accurate power calculation.
– Unless the accelerometer adjusts for a change in mass, it will continue to drift away from accuracy during a long activity.

I can’t explain why Stryd recommends to not change your weight. Makes no sense to me, but there must be a reason.

However, I would not stress to much about this. If I were to make a list of all the advantages and disadvantages of using Pace, Power or HR as a primary measure of run intensity, Power would still come out on top, even with this issue. Remember, this issue applies to Pace just as much as does to Power. The intensity required to run 7:00 per mile is different at 160 pounds than 165 pounds. Over a long event, you have the same problem with Pace as you do with Power in that the mass change means less effort to maintain a given Pace.

I think the best-balanced approach is to change your weight in Stryd every 5 pounds you gain/lose when training, and just accept the 3% drift in power accuracy (5 pounds out of 160) over the duration of a long event as a necessary margin of error.

David

#15409
David Warden
Keymaster

Just had a thought… anyone willing to change their weight in Stryd to, say 50 pounds for a few runs, then switch it to say, 200 pounds for a few runs? How does that change the reported Power output for a given weight/pace?

#15469
curtis.dublanko
Participant

David,

Independent of this post, I already did this a couple of weeks ago funny enough. Did two RF12 workouts a week apart, only thing that changed was my weight (75kg to 79kg). Results below (I took screenshots from TP but can’t for the life of me figure out how to attach them…so forgive the formatting). Time for both workouts was 1hr 30m.

75kg
Pace = 6:00 min/km
AVG Power: 211W
Calculated W/kg = 2.813

79kg
Pace = 6:01 min/km
AVG Power: 225W
Calculated W/KG = 2.848

I have some other advanced stats from Stryd just not sure what would be useful. Some of these metrics are pretty arbitrary in my opinion.

#15472
David Warden
Keymaster

Curtis, thank you! Somehow, our Forums has recently lost the ability to post HTML content and links, I’ll have to get our IT department (me) to look into that.

Well, I have to say those looks numbers look accurate, so Stryd is definitely taking weight into account.

So why does Stryd recommend not to change your weight? I suspect because you’d have then change your rFTP every time you changed your weight. BUT all Stryd would have to do is adjust FTP automatically when you change your weight and keep the W/KG the same for the FTP.

David

#15474
winoria
Moderator

Ha, i was just planning on trying different weight settings on the treadmill as I can maintain pace perfectly. I guess I don’t need to go this anymore…
Running power from Stryd is 100% related to the athletes weight.

I kind of don’t like that…but it’s the best option available I guess.
Don’t you hae close contact to Stryd, David, and could ask why they recommend to not change the weight setting?

#15476
curtis.dublanko
Participant

David and winoria,

I would recommend you do NOT adjust weight, as I did and then after several runs I could not get used to the new power numbers I was receiving. While they might be more absolutely “correct”, my mind was not calibrated to the new numbers and it was a huge mind game to see bigger power numbers and not feel the effort.

Not adjusting weight and just living with the offset over long runs is likely what I will do going forward, as it gets a solid frame of reference for when adaptation happens.

My 2 cents and all…

#15530
glyder
Participant

I was doing 1km splits around 3:30-3:40 min/km and toping out 330CP. if I pushed 3:15 close to 370CP. On easy it was around 240-260CP. First 2 shoes both fine seemed accurate.

Today, My power on my 3rd pair of shoes on 3rd time use power shot up 60+ today. My pace was around 3:30-3:40 I was fatigued and it was pushing 400CP and fade to 380CP at 3:45min/km.

I’m totally confused but I believe the weight and height might be because you can’t update settings through the WEB <— very frustrating.

Drum roll…I installed it on my partners IOS cause mine won’t upgrade past IOS12 (IPHONE 6s). So I have a feeling her weight and heigh leaked from her iphone apple health. I’ll let you know if that’s the issue. I believe it is. Could she hurry up home so I can check the settings! lol

#15539
David Warden
Keymaster

winoria, I do have good contacts at Stryd, but I’m not comfortable pressing this issue with them. That inquiry (“why do you recommend to not change the weight?”) would best be made by an athlete instead.

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