[acwan93]

Don't file formats like .FIT or .GPX handle this case? I had this problem when migrating nearly 10 years of running data from Nike Running --> Strava. It wasn't perfect but eventually I got "good enough" data after weeks of wrestling with unknown aggregators who either scraped sites or used an unofficial API. Apple Health is the closest thing I've seen where there's somewhat of an open standard despite the walled garden in Apple's ecosystem.

Without some overarching body, like a government regulator, I'm not optimistic there will be widespread adoption of interoperability.

[nradov]

The FIT file format isn't truly open because it's controlled by Garmin. But anyone can download the SDK for free, and most fitness trackers will export data in that format. With Garmin devices you can connect to a USB port and copy the FIT files directly to do whatever you like.

https://developer.garmin.com/fit/overview/

[NoboruWataya]

Yes, the three file formats I have seen used are FIT, TCX and GPX. FIT and TCX are controlled by Garmin but the documentation (and, in the case of TCX, the schema) is publicly available. I'm not sure what licence it is available under.

GPX is described on its website as being an open standard and is very widely used, both for fitness data and other applications. The purpose of GPX is to describe GPS data; by itself it is not as suitable for fitness data as FIT or TCX as things like heart rate and cadence data are not part of the standard. However, GPX can be extended.

From an open web perspective, GPX with standard extensions for other fitness-related data would probably be best, but FIT and TCX are getting ever more popular. For now, the situation is fine because it's fairly easy to convert between them. The risk is that Garmin tries to close off the TCX and FIT standards in the future.