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

indication that you are more fit. In putting together the TSS number, we’re simply using the training components known to produce a biological phenomenon called fitness to create a mathematical model.
    Now it’s time to take a look at how you can use TSS to monitor fitness in order to be ready on race day. That has to do with periodization.
POWER AND PERIODIZATION
    If you’ve been around your sport for any time at all, you’ve undoubtedly come across the concept of periodization. “Periodization” is merely a fancy term for how you organize training relative to time. You can schedule hard and easy days and hard and easy weeks. That’s a basic way of periodizing in order to stress your body and yet also allow time for recovery so that you do not wind up overtrained. The TSS calculation sharpens this concept. In fact, through the use of TSS, you can precisely manage the level of stress to increase your fitness, limit fatigue, and produce peak performance on race day.
Fitness
    What is fitness? As athletes we throw that word around a lot, yet seldom do we stop to consider what it means. In terms of athletic performance, it’s simply the state of being ready to race at a high level of performance. Training is therefore specific to a given race. Fitness for a cyclist doing a 1-hour criterium is nothing like the fitness required to race an Ironman triathlon.So the training for these events will be significantly different even though they both involve riding a bike. The differences will be evident in terms of how long (duration) and how hard (intensity) the workouts are. The TSS scores for a crit rider and an Iron-athlete may be the same for some of their key workouts, but the ways they get to those scores are completely different. One emphasizes high duration and moderate intensity (Ironman); the other focuses on high intensity and moderate duration (criterium).
    Both athletes will watch their daily and weekly TSS to make sure they are increasing their fitness. As explained earlier, they’ll know if they are making progress because both daily and weekly TSS will increase over time. What was once a hard TSS will eventually become an easy TSS, and so the workouts are gradually made harder to apply stress and therefore make the athlete more race ready.
    You can observe that TSS progression using software and a chart called “Performance Management” (WKO+ and TrainingPeaks.com provide this feature). The software assumes, with good reason, that changes in fitness are slow and occur over many days—probably over several weeks, in fact. It may take 6 weeks to actually see measurable indications of improved fitness, such as an increase in VO 2 max. The improvement doesn’t happen overnight.
    With this in mind, what the software does is produce a daily rolling average of your TSS for 6 weeks—42 days. (That’s the default setting; you can manually change that setting in the software if you believe you are an exceptionally fast or slow “responder.”) The software takes your TSS for today’s workout, adds it to the previous 41 daily TSS, divides by 42, and places that average TSS-per-day data point on a chart. You can see such a Performance Management Chart™ in Figure 7.1 .
    On this chart the X axis is time in days for one entire season, with the first day on the left end being October 21, when the training season started,and the last on the right end being September 1, when it ended. The Y axis is TSS per day (TSS/d). Each daily data point is the 42-day average up to and including that day. The data points have been connected to show how TSS, or fitness, has changed over time. As the line rises, fitness can be assumed to be increasing, also. A dropping line suggests a loss of fitness.

    Now realize, again, that I’m using the word “fitness” in a nonspecific way. The line doesn’t say anything about “fit for what”—crit or Ironman. It only shows that the athlete is capable of handling more training workload or less, which, as explained earlier, can be used as a proxy for fitness. The software avoids this issue by calling the connected data points the “Chronic Training Load”™ (CTL).
    Call it what you want, but what we are seeing here is an increased (or decreased) capacity for training stress management. But having a high CTL, such as 93 in Figure 7.1 , doesn’t mean that you can outrace your training partner whose CTL at the same point in time is only 76. This number is not reflective of actual