The Power Meter Handbook: A User’s Guide for Cyclists and Triathletes

of time such as a week is an indirect indicator of fitness. It’s indirect in that we aren’t measuring any physiological fitness markers, such as VO 2 max. Nevertheless, this is a good sign that something positive is happening.
    Keep in mind that in the example I used here I was talking only about volume. Intensity, the most important training component, was not factored in. If we can figure out a way to do that, then this whole concept of accumulated stress takes on an even greater level of importance. Let’s take a look at how that that can be done with something called the “Training Stress Score.”
    TSS is merely a way of mathematically combining the duration and intensity of a single workout to produce a number, or “score.” This score is more indicative of the stress experienced in a workout than if we talked only about how long or how hard you rode as separate bits of information.By combining them, we have only one number representing each workout, which allows us to easily compare workouts in terms of how hard or easy they were. Here’s how that’s done.
    Every workout has a TSS. Some power meter head units and analysis software compute this score for you after each ride. The TSS is determined using your ride’s duration in seconds, Normalized Power, Intensity Factor, and Functional Threshold Power. The formula used to determine the TSS of a workout comes once again from Dr. Andrew Coggan’s seminal work in this area:
(workout duration in seconds × NP × IF) ÷ (FTP × 3,600) × 100 = TSS
    The number 3,600 is how many seconds there are in hour, which, you’ll recall, is what FTP is based on and remains a constant in the formula. Also, 100 is a constant and is simply there to give us a two-or three-digit TSS.
    You’ll recognize NP, IF, and FTP as our old friends from previous chapters. They are all measures of intensity—the most important element of training for the experienced athlete. With this concept of TSS, they have even more to do with fitness than as originally explained. In fact, using them in this formula is going to help you measure fitness and fatigue trends while producing peak form on the precise days of your most important races. This is amazing stuff, which I’ll get to shortly. First, however, it’s important to become comfortable with the idea of TSS as an indicator of workload for a workout and a period of time such as a week.
    Let’s dig a little deeper into TSS by using an example. On Tuesday you do a workout that is exactly 2 hours (7,200 seconds) long. On checking your head unit after the ride, you see that your NP was 188 watts. You know your FTP to be 250 watts from previous testing, so the IF was 0.75 (188 ÷ 250 = 0.75). If we plug all of these numbers into the TSS formula above, we get
(7,200 × 188 × 0.75) ÷ (250 × 3,600) × 100 = 112.8
    Your TSS for this ride was 112.8. We don’t know based on this number exactly what type of workout it was and therefore exactly what type of fitness it produced. Your ride could have been a muscular endurance, an anaerobic endurance, or some other ability-type session, as explained in Chapter 4 . But we know it was somewhat hard since you rode at 75 percent of your FTP for 2 hours. That’s a pretty challenging ride. Once you’ve established a history of workout scores, you’ll soon begin to see patterns. So let’s do that for a week.
    Your 112.8 TSS for this ride was on Tuesday, we’ll say. For every ride this week, you also have a workout TSS. By the end of the week, your daily TSS might look like this in your training log:
Monday
0 (day off)
Tuesday
112.8
Wednesday
80.5
Thursday
100.6
Friday
72.8
Saturday
101.2
Sunday
153.9
    Each daily TSS was calculated using the above formula. This gives us some idea of how hard your rides were. We can see, for example, that Tuesday, Thursday, Saturday, and Sunday were the hardest rides, as shown by relatively high TSS. Monday, Wednesday, and Friday were apparently recovery days. The most difficult ride was on Sunday.
    If we add all of these daily scores, we get a weekly workload of 621.8 TSS. So now we have what we never had before—a way of expressing howchallenging a week was by using both volume and intensity instead of volume only. As we said earlier, increases in workload mean an increase in fitness. That is, you become more fit if you train longer and more intensely. Therefore, if 621.8 is greater than what you were capable of doing a month ago, you now have a concrete