Ride One: It's spring and you do an easy hour on the trainer at a very steady wattage - 150w. Since there wasn't any variability in your power, your Normalized Power will be the same as your Average Power. At this point in the season, your threshold (FTP) is 250w.
Here are the numbers from this ride:
Ride Two: A few days later, the weather warms up so you head out and do some intervals. It's a one hour ride, just like your last ride on the trainer. The difference is that this ride has a lot of variability - some hard efforts followed by easy riding for recovery. Your threshold is still 250w.
Notice the average power is the same as ride one. If you only looked at that, you might think these rides were identical. But if you look at the Normalized Power of ride two, you notice this ride was much tougher than ride one. The NP of ride two tells us this ride had the physiological cost of riding steady for one hour at 200w. Much more intense than ride one.
The IF (Intensity Factor) of ride one and two also shows us the difference in the two rides, .80 for ride two versus .60 for ride one. The VI (Variability Index) tells us ride two had a lot more variability. The KJs are the same for both rides showing you did the same amount of "work," but like Average Power KJ doesn't always tell the whole story. That's where TSS comes in. You'll notice the TSS is higher showing that the physiological stress of ride two was higher than ride one.
Ride Three: It's fall and you decide to head out one day and repeat that interval ride you did early in the spring, Ride Two. You repeat it exactly. The only real difference is that you've had an entire summer of riding and you're fitter than you were in the spring. Your threshold is now 275w. Since you repeated the ride exactly, nearly all of the metrics are the same.
IF: .73 *****
TSS: 53 *****
I put asterisks by the variables that differ between ride two and three. Notice the AP, NP, VI and KJ are exactly the same as ride two. That tells you this ride was indeed the same. You did a great job replicating ride two.
You might remember from Part I that the difference between IF and TSS and the other terms is that IF and TSS are relative to your FTP (they take your current fitness into account). That's why those two variables are different between ride two and ride three. Your threshold was higher when you did ride three so the ride wasn't as intense compared to your current fitness and it didn't create as much physiological stress even though the total workload (KJ) was the same. You were fitter for ride three, and IF and TSS illustrate.
The difference in IF and TSS between ride two and three also highlight how important it is to do periodic threshold testing. If you had never retested your threshold and changed your threshold in whatever software you use (I use WKO+ 3.0 and TrainingPeaks.com), the IF and TSS for ride three would have been incorrect leading you to believe the ride was exactly the same as ride two.
Obviously, this is a simplified example and you're not going to end up with rides quite this easy to compare, but I hope this showed you how the variables change and gave you a better understanding of how they work.
This was originally going to be a two part post, but I decided it needs a little more. In Part III, I'm going to go a little more in depth on Normalized Power. In Part IV, I'm going to dig a little deeper into TSS. There is a lot you can do with TSS and having a better understanding of it (and how it's calculated) can help you take your training and racing to the next level.