Carmichael, who is best know as being Lance Armstrong's coach, also authored another book on cycling training a few years ago called The Ultimate Ride in which he describes what he believes to be the best way to train. Simply put, it's what every pro endurance athlete does: periodization consisting of tons of long slow base-building, followed by shorter and more intense periods of fine-tuning and tapering.
The thesis of the Time Crunched Cyclist is that while the above method is best - it's just not feasible for amateur athletes with families and jobs and other priorities. If you can't put in the 30+ hour weeks that a pro cyclist does to build up an endurance base, you need to substitute intensity.
In this context "stress" (or load as Carmichael calls it) is something you do to your body to tear it down. Specifically it is the intensity of your workout multiplied by the length of your workout. So, for example, a long slow bike ride might have a similar load on your body as a shorter but faster bike ride. Sounds great right? You can cut your training in half - all you have to do is make each workout twice as intense! Not so fast. All "loads" are not the same, and long slow rides build on different systems in your body than short fast ones. Read the book for lots of details as to why, but the gist of it is that you can get away with shorter, high-intensity training and get you to your peak performance - but with one huge caveat: you can't stay there for long. You will hit the wall after your peak and probably take weeks to recover. Anyway, read the book if you train less than 12 hours a week - it's pretty interesting.
Theoretically this all sounds straightforward - just track your intensity and duration of your workouts, and you will know the "load" you've put on your body. But in practice, what is the intensity of a workout? Hard? Breezy? 4.7? That's where the Intensity Factor (IF) comes in. An IF of 1.0 means your threshold effort - the point at which you start breathing heavy. You can get a full description of how to estimate IF here. Or, if you are lucky enough to have the equipment, WKO+ will calculate IF for running workouts using data from your speed/distance wristwatch and for bike workouts from your power meter. Now, take your IF for each workout, multiply by the duration of the workout (in hours) and you have your Training Stress Score (TSS). Perfect, but what good is that? TSS is a good measure of your volume - in fact it's a little better than using total hours like most of us do. Though, to be honest, the two metrics track pretty closely - below is my volume for the last few months: green line is TSS, orange is hours.
Next time, I'll talk about what benefits you get from using this method of tracking your workouts - specifically how you can use them to predict your performance, and to help you peak on race day.