Treadmill distance calculation?

[rayphillips]

I recently purchased a treadmill to use indoors when it is raining outside (like now!).  However, I am noticing significant inaccuracy in the distance calculation on my Sense unless I make drastic changes to my stride length in the Exercise settings.  (Set automatically is turned off)

Some background: I walk a lot.  I typically walk everyday and use the Walk exercise with GPS turned on.  My usual walk is about 2 miles in 40 minutes with around 2000 steps per mile.  I have been doing this for many years using different fitbit devices with my current device the Sense after my Ionic stopped working.

So, the first time I used the treadmill I set it up to run at 3.0 mph and started to walk.  I stopped when the miles read 2.00 after 40:02 minutes. I was surprised when my Sense said that I had taken 4072 steps but had only gone 1.34 miles.  The first thing I checked was my walking stride length and it was set to 32" and my running stride length was set to 36".  Fitbit says that distance = stride lenght x # of steps, but for 4072 steps to equal 1.34 miles the stride would have to be: 20.85 inches!

I just ran an experiment:  I set the walking stride length to 48" and the running stride length to 96" and did a walk on the treadmill of 2.00 miles at 3.0 mph.  This also took 40:02 minutes, but this time my Sense said I took 4230 steps and walked 2.06 miles.  (2.06 mi x 5280 ft/mile x 12 in/ft)/4230 steps = 30.85 in for stride length.  This has no relationship to any stride length I have entered in settings.

I have made sure to sync before and after the exercise; I have tried reseting my iPhone and reset my Sense and I seem to get similar results.  Any explanations or suggestions?  I guess it is not a big deal if I keep an artificial stride length - the results are close to my actual distance walked, but it appears to me that the sense is not using the stride length settings entered manually.

[LokeAa]

Hi @rayphillips 

To get an accurate distance on the treadmill, you would have to use a Strava device. Unfortunately, Strava is not compatible with Fitbit devices. The second best would be to know your stride length on the treadmill, which most likely is different from your outdoor stride length, then the app should calculate distance by multiplying steps * stride length. It is supposed to calculate as I described, but it doesn’t. It would be very easy to correct whatever the error in the app is, but since it first was reported way back when, it seems unlikely it will be fixed anytime soon, or ever.

[Christian565]

It would be useful to have a setting in the app to set stride length for the treadmill exercise and continue with automatic stride length on outdoor runs using GPS. Currently you can only have automatic stride length on or off. Because I’m mostly running on the treadmill at the moment I’ve set my own stride length so I’ll have to try and remember to turn this back on if I go for an outdoor run. Surely it can’t been too difficult for the developers to offer a different stride length for treadmill exercise?

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[t.parker]

@LokeAa did you mean Stryd device? If so, then it can be accurate (they in fact released new feature for treadmill training plans) but there is also a good explanation why Stryd and other devices, including wrist trackers are not always accurate on treadmill.

@rayphillips The problem is encountered by the watch sensors as on the treadmill we do not really move forward. There is no forward motion and moment we are off the surface we are rather jumping up than forward. Treadmills confuse most of devices. Let alone a fact that distance on the treadmill often is way off (on motorized treadmill, the motor speed is affected by runner but distance counters just apply simple math based on chosen pace and time that passed). It is in general better to focus on level of effort (or RPE) rather than distance or pace when running on treadmill. Even Stryd footpod requires certain calibration for treadmill (and needs to know incline) and still may be slightly off. It is not as simple as steps x stride problem.

[SunsetRunner]

A solution could be Runn but it's not compatible with Fitbit

[t.parker]

@SunsetRunner Great finding! Runn is a way to make dumb treadmill slightly smarter but requires connecting it to the watch (or at least app). It's pretty much a footpod attached to treadmill rather than foot. The good thing is that the motor inaccuracies and incline are not an issue as it measures belt motion. The distance will be almost certainly different from what treadmill shows but Runn is the most accurate. It detects incline of treadmill automatically and cadence, too (each step slows down slightly the belt). But the best thing about it is... its price. You'd think it costs hundreds of dollars while it's quite budget solution (almost 5x cheaper than Stryd). One thing I don't see is any way to measure running power but if I had treadmill (I find running on treadmill bad for knees and feet as well as it promotes artificialy enforced bad running form) I would certainly buy that!

[rayphillips]

@t.parkerI think that you are right about the Sense senors being part of the problem.  I also think that fitbit does not give us the complete story about how the calculate distance during the day or in a specific exercise.  If I look at the help article "How does my Fitbit device calculate distance traveled?" it states that Steps x Stride Length = Distance Traveled. It also states that your stride length is determined by your height and sex.  It then describes how you can can use GPS to measure distance or measure your own stride length and manually enter stride length in your settings.  The impression from the article is that if you enter a manual stride length it will replace the stride length based on you height and sex.  This may be true for some Fitbit trackers, but I don't think it is true for the Sense - I think that for most of the time the Sense uses the height and sex based stride length and only uses the manually entered stride length for some exercises. 

For example, yesterday my Sense recorded a distance of 5.16 miles and 8,502 steps.  Under settings my walking stride length was set to 48" and running stride length was set to 96".  I did no running yesterday but did to some walking on the treadmill. Doing the calculation for stride length  (distance/steps) I get: (5.16 miles x 5280 ft/mile x 12"/ft)/8502 = 38.47" which is slightly more than my height based stride.  It is definitely not based on any of my manually entered settings other than my height. 

It is interesting that if I turn on the Set Automatically button for Stride Length that my Walking and Running stride lengths go to 29.9" and 32.4" which I think are height or GPS based.  I think I will leave the Set Automatically button On and see what happens when I use the treadmill today.

[t.parker]

@rayphillips it's because Fitbit gives simplifies information. If the distance would be indeed computed by stride x steps then the inaccuracy would be tremendous. Nobody walks or runs with fixed length stride. Especially in running, where you may have hills, many types of surface and other factors affecting stride length. "Stride x Steps" is simple to understand. If Fitbit will tell that non-GPS distance is computed from the accelerometer data applying double integration which has horrible drift so has to be corrected somehow (and there is lot more to it) then who will understand that? In fact, calculating stride length doesn't even need GPS (Fitbit may use GPS to correct the stride length though and as anti-drift adjustment). For example, foot PODs can tell you stride length, yet they never connect to GPS. We know very little about algorithms Fitbit uses and it's certain we won't be told anything else but pedometers exist for quite a while and most of them follow same principles and use similar algorithms (as they do have same sensors on-board).

[rayphillips]

I am beginning to feel that the Sense "smart watch" is maybe trying to be too smart and do too much.  I can remember with my original Charge that I could set my stride length after an exercise and it did a pretty good job of keeping track of my distance over the day. 

I am seriously thinking of looking for a less expensive fitbit - simpler may be better!

I got my Sense in January 2021 and have been disappointed that many of the features did not work then and some still do not work.  Also, new features like Snore Report seem to really drain the batter and need to be turned off to get good battery life.