I thought I'd just put this out there in case anyone has experienced the same and can help me. When I train mostly this happens on the bike my hr continually drops throughout the session despite increasing power. As an example today I did a 90min ride the first 30mins was average of 245watts my hr during this first 30mins was mostly between 134-139 for the next hour I increased power to 255 watts my hr steadily dropped into the 120 range and then In the last 20mins or so it dropped to the 117-119 sorta range. So my hr dropped by 20bpm over a 90 min session and this is not a one off it happens all the time. After the first 30mins or so of riding it will slowly keep dropping? Is this normal or what is wrong ?