What does 100 TSS mean?

OK, so much for a simple definition of TSS. How about a simple explanation of what the number means. In other words, what does it mean to say 100 TSS?

Basically (Training Stress Score) TSS is a measure of physiological strain, or training load. By comparing TSS scores from different workouts you get a really good measure of how much each contributed to your overall training load.

In order to quantify TSS in a relative sense it is set against a standard, which is to say TSS is normalized. The easiest and most readily understandable way to do that is to set 100 TSS points to be the equivalent of your best effort for an hour.

Make your best power over 60 minutes, you get 100 TSS

TSS is power based. There isn't any reliable way to translate it into heart rate or distance covered or speed. If you don't have a bicycle power meter such as an SRM or PowerTap, you can only estimate what 100 TSS feels like. Think back to your hardest race lasting an hour or more. You know what a hard hour's effort feels like under grueling race conditions. That's 100 TSS, or a very good approximation of it.

The thing is, you don't get TSS points in direct proportion to effort and duration. The way it's calcualted, TSS varies by the square of intensity. That means if you're only going at 90% for an hour you're only accumulating 81 TSS per hour (90% squared is 0.81, then you multiply by 100 to get TSS)

Here are a couple of examples:

Train at 75% of your best hour power for 120 minutes: 112 TSS
(0.75 * 0.75 * 2 *100 = 112.5 TSS)

Train at 92% of your best hour power for 45 minutes: 63.5 TSS
(0.92 * 0.92 * 0.75 * 100 = 63.48 TSS)

Because TSS varies with the square of intensity it's really hard to estimate how much TSS you're accumulating on any given ride. In fact, even if you have a power meter on your bike it's hard to tell, especially if your output level varies much such as in a group ride or race. In fact, this reality - that estimating intensity over the course of a ride is so impossibly difficult - is what makes TSS such a valuable training metric.