It's seems that arcgis online is not able to read Extension from a GPX file
some links where people have done it .... or not
Doing More with a GPX File
How to export a GPX file from ArcGIS online?
Re: Import KML/GPX/Shape
thanks for your answer.
But my purpose was to be able to symbolize my track with a color based on my Heart Rate value.
ArcGIS Olnie is very close to doing that. It needs just an upgade of the GPX driver !
Ahhh you should contact tech support and put in a request or post on the
ArcGIS Ideas site if it is a wish list item, or maybe it is even on the list.
Retrieving data ...