Originally Posted by Trsnrtr

HR can be variable from ride to ride which can make it somewhat iffy when training but its a great way to understand your body and its fitness vs. fatigue on any given day.

For example, I often ride with a group of retired racers (really old guys) and we are generally running close or just below our LTHR most of the ride. If my body is going through a period of fatigue, I can tell fairly quickly by my HR that A. it's exceeding its normal values and B. that it's not dropping quickly enough at stop signs, back of the pack or in lulls. Thus, I know it's time to back off for a few days.

Likewise, when you're rested and feeling great, your heart can work at its threshold all day and you'll feel like doing more.

if HR is all you got, it's better than guessing.