Originally Posted by
Doge
A HRM is a better tool for that.
HR only tells you the rate that blood is getting pumped to the muscles, and it's not the most effective way to train. The muscles are what do the work and drives your bike forward or up hill. Power is a direct measure of the work your muscles are doing. The amount of blood flow can't give you that kind of direct measurement.
HRM have been around since at least the late 70s (maybe earlier?) so we've all used them, but for racing or big events I was always so excited and hyped up at the start of a race the HR was always way way too high, so I stopped wearing it for events because it was useless. The power meter never lies and isn't 'wrong' due to race day excitement or a Krispy-Kreme doughnut.