What do you use to determine the intensity or pace in races or training on trails?
For road runners pace is a good a metric as any to work from but pace on trails varies too much for it to be of any value.
What about heart rate? Heart rate training has been around for a while and has some merits but also some limitations. It is subject to many variables – mental fatigue, physical fatigue, caffeine, adrenaline, stress, weather and cardiac drift to name a few. There is also a lag between your hearts response to an increase in effort.
Heart rate is a measure of the hearts response to the work being done by the body. Why not measure the work directly and use that instead of heart rate? Power meters allow us to do just that.