IMPORTANT UPDATE (June the 14, 2024):
More or less 10 years ago I used to use a Garmin Etrex for MTB and Hiking, and a Polar watch GPS for running and other sports. I feel confortable using Garmin connect, so I imported all my trainnings from Polar to Garmin.
Several years ago Garmin changed their internal import functionality and I implemented this polar2garmin conversor created as a TCL standalone application. The problem was that on any kind of update I had to update it in all the computers where I used it. That was the reason why I have created this polar2garmin online conversor.
Some months ago, Garmin changed their code again, making the file conversion of polar2garmin not working anymore. Nowadays, I use my Polar device for tracking all my trainnings, even MTB and Hiking, so importing the trainnings from Polar to Garmin was just due I prefered Garmin connect. I kept this polar2garmin project just for fun, but I don't want to spend more time dealing with changes that does not depends on me. I haven't tried to guess what they have changed now, but it makes no sense continue investing more time on this topic having other services like strava that synchronizes both applications automatically (at least for now).
Also, since a while, I had in mind implementing a kind of "Garmin Connect" and "Polar flow" application for tracking the trainning and having some options that I do not have in these systems, so I prefer invest time on that.