Counting Swim laps

[RajRajpal]

currently, I have the sense  2 and am very disappointed that it doesn't display a laps count while swimming. The sence1  displayed lengths and distance, but strangely  the sence 2 has been downgraded to showing time only.

[asiayeah]

Yeah. It’s a well known infamous limitation of Sense 2.

I could only guess the feature was probably not accurate without accessing the cloud, so Fitbit decided to remove it. Or perhaps there is some software licensing issues. Anyone has some better ideas?

[Triletics]

@asiayeah there is a theory (never confirmed by Fitbit) that the gyroscope is necessary to get real-time laps counting. The gyroscope is a part of only a few models of Fitbit and those seem to have on-screen lap couting. Looking at other brands, it may be more true as there are some claims of inability or reduced accuracy of tracking laps when there is a no gyro on other platforms.

[asiayeah]

@Triletics I see. But it can count the laps after syncing with the Fitbit app. So I’m still not quite sure myself. 

[Triletics]

@asiayeah it's just a theory but it may be easier to figure out lengths off-line if only accelerometer data is available. The gyroscope can sense rotation (orientation) which is more useful when it comes to changing the direction in the pool. In other words, a gyroscope can tell you what direction you're heading to. Accelerometer detects acceleration but has no idea about the orientation. If you think about the swim stroke and how the watch moves, you will always get a very similar set of readings regardless of the direction. There will be some subtle differences but to tell whether length ended or not it will be much harder for the watch to do it in real time. However, having all the data it is possible to find repeated patterns when regular accelerometer data is disturbed and make an assumption of turning around (or stopping). The problem is that most people don't swim a proper regular stroke so accelerometer data is quite messed up. See the example below. This is data from elite swimmers with acc sensor mounted on the lower back (not on the wrist):

You can see that the grey dots are strokes and the black dot is a push-off. This looks quite regular and indeed, with good technique, it's possible to tell when the length ends. The first lap (grey dot lower) is breast-stroke and the second lap is freestyle (grey dot higher). Changing direction would not affect grey dot positioning (only changing style does that) which means that it isn't possible to tell which direction the swimmer is going. The only push-off which disturbs the readings pattern is an indicator of turning around (and elite swimmers do rather strong push-off with a long glide). Now, think that the sensor is on the wrist with an arm in constant, not always regular motion and the swimmer doesn't do a proper strong push-off but rather a casual turnaround (this is what I often observe in pools). Most likely there will be not much clear information about the direction. It isn't impossible to detect lengths in real-time just from the acc data but the accuracy will be compromised (yet most watches try doing it).

If Fitbit does that on the server side it still must process some data in the watch, otherwise, it would be a vast amount of readings to store and transfer and this is very unlikely. Maybe smoothing the acceleration every second etc. We won't know that. Anyway, as I said, it is just a theory the community came up with to justify the fact that fewer Fitbit watches support real-time lap counting.