HRM on my 645 usually works very well at rest, running and cycling (indoor and outdoor). However, I'm having a bit of a weird problem when I do cyclocross races that are very intense for 40 minutes as the recorded activity is showing nothing above zone 3, which is def wrong. I suspect it could be to do with gripping the bars so tightly or vibrations in race conditions as every other parameter is the same as usual, ie: position on wrist, clean sensor, no more sweating than a normal hard ride.
Any ideas of how to overcome this? I can see suggestions on this forum of having it higher up the wrist but am reluctant to do this as if it slips down I will struggle to move it back mid race.
Help and suggestions much appreciated in advance!