So you can spend a LOT of time learning about this if you read up on coach @chad 's suggested reading list, (there is a reference to this in the podcast from right before @Nate did leadville as well, I will link it if i find it…) pretty much anything by Coggin or Friel. They all establish that you can determine an HrTSS using heart rate, and it will be SOMEWHAT accurate, but unlike power, its never an absolute. HR is affected by so many environmental concerns (Sleep, Fatigue, Hydration, etc) that its hard to treat as a hard metric. Not that it isn’t useful, just that it isn’t an absolute number like power. When you make 250 watts, its 250 watts. Like Henry Rollins says the Iron never lies, 200lbs is 200lbs.
From the TR Blog –