My very fit and competitive 12-year old runner experiences a consistent issue where her Garmin 935's HR data show a dramatic bump up at around 7 to 10 minutes into what is an otherwise easy (5/10 effort) long run, and then stays high. It does not correspond to her perceived effort at all. Any ideas? This is happening both when worn by itself as well as when using a chest strap.
At the same time, the Performance data doesn't kick in until right after this big bump. Is that by design?