I normally don't wear a heart rate monitor when I race, but the other day I tried. I wanted to keep it low for the first half than race the second. Right from the start, my HR was through the roof and I wasn't even running that hard. If I were doing this pace in a training run, I would have been at around 145 -- but that day I was in the 160s. When I went to race pace I was in the 170s. On a normal day, 160s is tempo pace and about fifty seconds faster. 170s is pretty much all out.
Any ideas why this happens?