If you don't have a Power Meter or train with Power Meters..you already know that HR training/monitoring is the next best thing. (If you don't, read some of my prior blogs on why I think so.) And, if you're one of the fortunate athletes that I coach, you'll know that TSS, or Training Stress Score, is my underlying coaching metric/principle. It's all about stimulus/dose and response. (If you want more info on stimulus/dose and response...do a Google Search on: "stimulus response coggan".)
What you need to calculate your TSS for a given exercise, is a Heart Rate monitor that is capable of recording "Time in Zone" TIZ. Take those numbers and enter them into my handy-dandy calculator (that I'll email you if you request so) and you'll get your TSS.
Note: Calculating TSS from Heart Rate is NOT as accurate as it is compared to Power..but it's close (in the ballpark) and that's all I'm looking for when I'm exercing away from home at a gym or hotel fitness room.