deapee

So here's a ride I did this morning on a stationary bike that is fairly new and gives similar numbers to any other bike I've ever been on...

I rode at 125 Watts for 25 minutes, then at 25 minutes I went to walk on the treadmill for about 5 minutes. My goal was to keep my heart rate under 130...Clearly there's some points where the sensor being able to read drops off for whatever reason, but ignore those few points...We can see that I can consistently push 125 watts in a zone 2 ride for an extended period of time.



And here is a ride I did yesterday afternoon -- where strava credits me with 88 watt average. I averaged 15 mph on this ride, it's a rail-trail so the incline is slight, but exists -- and on the way back i fought wind. On incline with wind at my back I was at 14.1 average, and once I was back to my vehicle the average was 15, so I assume I averaged 16 on the way back. There are stops that interfere with average speed, etc. Those two blips I accredit to eating a bug and nearly puking...The end is probably some fluke, I don't think my heart rate reached 184 as the chart indicates.



My point is -- how can I ride at 125 watts for 25 minutes and barely be sweating...I can literally go forever at 125 if I wanted to...but yet according to Strava I'm really pushing and averaging 88 watts...Strava did credit 0 elevation gained...and no i'm not too concerned with strava or anything, but I am a numbers guy...so I'd like to kind of determine what is going on here. I shouldn't be pushing zone 4 at 88 watts if I can maintain 125 watts indefinitely in the upper end of zone 2.

Thanks.

dksix

If I'm understanding you correctly you are saying you believe Strava under rated your output. From what I read Strava's calculations are off in the other direction, being the estimated Strava power outputs are higher than you are actually putting out. The same with calories burned, Strava my esimate 1500 where 900 is more accurate. I'm just relaying what I've read, no power meter to compare my rides.

deapee

I guess that would be a good clarification.

Either way, it just doesn't make sense to me that while outputting 88 watts my heart would be up at 155-160...and pushing 125 watts on a machine I trust for a recovery ride that my heart would be steady at under 130 bpm.

All conditions, grades, wind, out the window, producing 125 watts of power is producing 125 watts of power, right? If you're pushing 125 watts going down a hill, you'll be going a lot faster than pushing 125 watts going up a hill. That is to say that to keep speed consistent (air resistance withholding), you need to vary your wattage while going up or down a grade by a specified amount, which should theoretically be easy to calculate...by determining how much power is required to propel a certain amount of weight (again air resistance withholding) at a certain speed on a certain grade.

It just doesn't make sense to me that pushing 88 watts of power creates a zone 4 heart rate -- and 125 watts of power in a different environment produces a zone 2 heart rate.

--

I've been riding quite a while, mtb, cross, etc...but am relatively new to paved road, and just got into heart rate and power, etc...and find the numbers very intriguing.

gl98115

Strava power guesses are wildly inaccurate.

CliffordK

Make sure Strava has the proper weight defined. 88W sounds like a pretty low average, but I don't really pay attention to ride average, just watts on a few hills or segments.

Perhaps Strava is calculating watts at the rear tire, while your exercise machine calculates it at the cranks. It wouldn't account for the whole difference, but it would make some difference.

deapee

Yeah makes sense.

My main thing is looking back the more metrics you have available to compare and track progress the better. Thanks.

andr0id

2 things...

Indoors on a trainer will usually be require more effort for similar watts. Unless you have a really big fan that keeps you cooled to outdoor speeds.

How does your trainer measure power? Is it a real power meter with a torque/strain gauge or is it just guess also?

100/125 watts is going to be an easy 13-15mph on flattish ground with no wind unless you're huge.

DrIsotope

The more rides you log, the more accurate Strava's Power Guesstimator 5000® becomes. After ~300 rides logged, the power guesser was fairly close to my power meter measured riding, within 10%, Strava generally under-guessing. My LTHR on-road test of a few months ago Strava guesstimated 297W average, and my recent FTP with the PM gave me 319W (95% of the measured 336W) so Strava under by right around 10%. It would throw est. averages of 230-250W average on long rides, and that's been my results so far on the PM-- 254W on a 79 miler during the week, and 245W on a 53 mile ride today.

Strava's calorie guesses are directly tied to it's power guesses so if it's over-estimating your power (work done) it's gonna over-estimate your calories burned as well. I did a pre-PM 76 miler with a suffer score of 179, Strava estimated 4,467 calories. The above mentioned 79 miler (with Stages) had a suffer score of 226 (a thousand feet more climbing than the 76) and came in at 4,531 calories. Once "trained," non-PM power/calorie numbers are good enough to get you in the ballpark.

Lastly, I've found no correlation between power output and heart rate. It was cold today, so I was doing +350W for ~2 minute bursts, with my HR well below LTHR, like 20 beats below. HR is impacted by every factor you can think of-- wind, grade, temperature, hydration level, etc, while power is just power.

Carbonfiberboy 

IME Strava is pretty darn close, however wind obviously throws it way, way off. On climbs, it only calculates power between GPS points, so short climbs are frequently way off. Also, garbage in, garbage out. If your position isn't the normal road position, the power on the flat won't be particularly close. If your personal and bike inputs aren't exact, it won't be accurate. As noted by others, HR is frequently not a good gauge of power. In particular, output outdoors is normally lower at a particular HR than it is indoors. After you get more experience, well, you'll have more experience.

chaadster

I think the main issue is that Strava includes zero-watt time in the average-- so time spent coasting, as in slowing-- which is generally a substantial time in urban riding, and also that on the spin bike, you probably never coast.

For example, on April 20th I did a 1:01hr ride on the trainer for a 146bpm avg. heart rate an 241w avg. This morning, on a ride, With the same average HR over 1hr 40mins, Strava estimated 165w. The ride detail shows 28mins at zero watts. Additionally, it recorded another 9:17 at 0-50w, making it roughly 37 mins of the ride I was essentially doing nothing. The trainer ride showed 1min of that 1:01 at 100-150w, and no time spent below that.

So you can see that the type of riding and how power is counted matters immensly, and can make it look like Strava is wildly underreporting power.

jsk

Not really. As long as you understand it's limitations, it's actually fairly accurate (but certainly not as good as actually having a power meter).
You have to make sure your athlete weight and bike weight are entered correctly in your profile. If you do that, Strava's "virtual power" will give you a pretty reasonable estimate for solo rides. Drafting in a paceline will throw it off, of course, since Strava can't know if you're sitting in the draft which requires about 30% less power. Strava's virtual power will also be off on windy days since it doesn't account for wind; so if you ride hard into a strong headwind you'll be putting out quite a bit more power than Strava estimates.

Jarrett2

I found Strava power estimates to be low after installing and using a power meter.

Oddly, Strava's estimated calorie numbers were within 100-200 calories of accurate, just a fuzz low compared to the power guesses.

caloso

Why do you assume a gym spin bike to be accurate?

deapee

I guess based on the following:
- The bikes cost about $3,000 each.
- The bikes yield consistent results with one another, when cycling and perceiving the exertion rate.
- The bikes yield consistent results based on perceived exertion between upright and recumbent.
- The results of the bikes are consistent with what my Kurt Kinetic fluid trainer smart measures.

All four of those reasons are taken together and kind of give some validity to the numbers presented in my head (so please don't just quote "The bikes cost about $3,000 each" and then proceed to tell me how wrong I am).

I've never gotten too into heart rate or power output in the past...I've always just rode to ride. I guess after suffering an ankle fracture and two severely torn ligaments, I've put a lot of pressure on myself to not only get back into shape but in better shape than I've ever been...and there's only so much you can determine from feel alone. I mean I can barely walk around the block still, but I can put out 524 watts in a 30 sec sprint or sustain 200 for a fairly long duration keeping my heart under 160...so I just question how riding at 88 watts is a zone 4 effort...when I can ride for an hour in zone 2 at the gym or on the trainer at 125 watts on a recovery day.

Heck, even after an all-out sprint (180+ heart rate), I'll slow down to 100 watts to recover and my heart rate will go back down to 130.

deapee

I think you're onto something...In fact I'd be willing to bet that's exactly what it is.

caloso

Okay. Makes sense to me. I ask because there are a lot of examples of gym equipment where power seems to be for novelty purposes. Like the Lifecycle in my gym, for example. Carry on!

chaadster

Almost certainly...that, and Strava's estimates are just way-to-wildly off. I once recording a ride using the Strava app on my phone while simultaneously recording with my Garmin Edge500 and Powertap G3 hub. I uploaded both to Strava for comparison, and the estimated power came back as 177w average, while the Powertap reported 273w average. It confirmed in a more complete way that which I had experienced while using a wired Powertap hub wheel before replacing it with the ANT+ G3, namely that Strava greatly underestimated my power. And it continues to do so, despite accurate rider weight and three years worth of data from both stationary and road Powertap rides in addition to frequent "simple" recordings using an iPhone app with only HR, speed, and cadence. If Strava's power estimating "engine" learns, that's news to me.

EDIT: Here are links to those two "estimated" and "with Powertap" files from the same ride I was talking about above, if you're interested:

[Strava estimated]
https://www.strava.com/activities/287825958/overview

[Powertap]
https://www.strava.com/activities/287810052/overview

deapee

@chaadster -- you got me really putting a lot of thought into what you said...and I'm sure you're right.

I did about 44 minutes that day...I crossed 6 roads, 2 wooden bridges, 3 groups of recreational cyclists with children (I'll go fairly fast by others, but definitely not when a child is there, I literally go by children at probably 5 mph, if that)...so if you figure at road crossings, 30 sec to coast/slow down, 15-30 seconds to cross, then I was in no hurry to get back up to speed, saying I lost 6 minutes would be fairly accurate, plus the other hazards I encountered...we'll round it off at 8 minutes of time spend slowing down or casually getting back up to speed, which is 1/5th of the time not only producing zero watts but also included in that time is purposely taking away energy that's already in play (hitting the brakes)...yeah...that makes total sense.

I guess that's why an hour on the trainer is a lot harder than an hour out and about (if you're really gunning for a power number on the trainer, of course, which Zwift helps a lot with).

It all makes sense now.
@caloso -- thanks for the input sir.

bbbean 

One anecdotal data point: When I compare Strava estimated power with my PM=measured power, I find that Strava is pretty close for the overall ride, but wildly inaccurate on any given segment.

Ultimately, though, if you're not using a calibrated power meter, you shold treat ANY power estimate (including that at the gym) as a rough estimate at best. Note that many pieces of gym equipment tend to estimate power and calorie burn high because those numbers help sell gym equipment. "My new brand X burns 1000 calories an hour!"

If you want to get better numbers, get a power meter. If that isn't an option, ask a cyclist with a power meter who has a well established baseline to try your gym machine and compare his estimate with the numbers it gets.

BB

Flinstone

There's one place where anyone can get a good power estimate, that's on a steep hill with no wind. 200 watts for 185 kg total weight is about 5mph on a 10% grade. There's no wind resistance at that speed. You can get within about 6% of any of the online calculators just by taking mg dh/dt. In other words, total mass (you and bike) in kg times 9.8 times height of the hill divided by time in seconds to get up it. You have watts. Run any online calculator to get a slightly more accurate estimate that does correct for air speed and such, but since they become small corrections at low speed, they don't have to be done very well. At 5mph you can skip the correction altogether and do pretty well, so adding the small correction only makes it that much better. Even at 10 mph we're talking about something like 45 watts going into non-gravitational resistance. If that's only estimated with 30% accuracy you're still within 15 watts. Not so bad.

I don't have a power meter and the gym bikes don't fit well enough to compare, but from hills, I know what I'm capable of. That's good enough for me. Gravity is universal.

Heathpack 

FWIW, my impression is that Strava majorly overestimates power. The reason I say this is that a lot of the TT courses I race are set up as Strava segments. Prior to a race, I'll sometimes go to the Strava leader board to see if I can find times for any of my competitors. When I look at the leader board, women with estimated Strava power frequently have ave power that is 50-100 watts greater than women with power data from a power meter. The estimated power is typically implausible for the woman's age & performance.

jsk

This makes sense for time-trial bikes if you think about it though. When calculating virtual power, Strava has to make some assumptions rider position and aerodynamics, that are not going to hold true when riding a TT bike in the aero position. The better your position/fit on the TT bike, the less power you need; so Strava's estimates will be high.

Heathpack 

Doh! Excellent point. Yes of course that's it.

Its the only time I ever look at Strava estimated power, when planning for a TT, but I never really fully processed that. Just in my head concluded "Strava power estimates are way off".