I have a Polar S150 that registered a max HR of 216 while mountain biking yesterday. Somehow I don't think this is possible. The max I hit on my trainer (Spinervals: Uphill Grind & Suffer-O-Rama) working hard averages 173 and the highest previous reading has been 185. I'm 46 so my calculated max should be around 174 and my resting rate is around 60. Could the heavy jarring of trail riding be affecting the monitor? The trail I was on has 7 ~ 50 to 100 yard steep technical climbs but nothing sustained or above my skill or fitness level. The trail has lots of roots and rocks so it is very rough.