Originally Posted by
Carbonfiberboy
....HR is really good at seeing when your blood sugar is dropping (HR drops at same power, apparent effort increases) ...
this is really valuable for me; my main metric of "how i'm doing" is actually the relationship of heart rate to power. 99% of my riding exists in a range between 90 and 130BPM, and if the pretty tight correlation between power and HR (for a steady state over a few minutes) is off i know that something is "wrong," so to speak - or on occasion extra right. on the really wrong days i know to just throttle back and have an easy cruise, and the "extra right" days where an extra 20 watts came from the sky at the same heart rate, breathing is good, the bike is quiet, it's time to go for some PRs.