Knowledge Level: Advanced
Training Stress Score is a measure of training workload that is found on most power head units and on TrainingPeaks.com and TrainingPeaks WKO+ software. It’s 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 Training Stress Score (TSS). Most power meter head units and TrainingPeaks and WKO+ compute this score for you after each ride. The formula used to determine the TSS of a workout comes from Dr. Andrew Coggan’s seminal work in this area:
(workout duration in seconds × NP × IF) ÷ (FTP × 3600) × 100 = TSS
Here is what the components of the formula mean. NP, or Normalized Power, is average power that has been adjusted to give more emphasis to the surges that may have occurred during the workout. The more surges, the higher the NP is relative to average power. IF, or Intensity Factor, is a comparison of your workout NP and your FTP. It’s found by dividing NP by FTP. FTP, or Functional Threshold Power, is the highest average power you can sustain for an hour. NP, IF and FTP are all measures of intensity—the most important element of training for the experienced athlete.
In the formula, the number 3600 is how many seconds there are in hour, which is what FTP is based on, and remains a constant in the formula. Also, 100 is a constant as well and simply there to give us a two- or three-digit TSS.
Let’s dig a little deeper into TSS by using an example. On Tuesday you do a workout that is exactly two hours (7200 seconds) long. On checking your head unit after the ride you see your NP was 188 watts. You know your FTP to be 250 watts from previous testing, so the IF of the workout was 0.75 (188 ÷ 250 = 0.75). If we plug all of these numbers into the TSS formula above we get:
(7200 × 188 × 0.75) ÷ (250 × 3600) × 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. It could have been a muscular endurance, anaerobic endurance, or other ability-type session. But we know it was somewhat hard since you rode at 75 percent of your FTP for two hours. That’s a fairly 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)
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 since the TSS for each is somewhat low. 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 how challenging a week was by using both volume and intensity instead of volume only. Increases in workload ultimately 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 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.
Using TSS, the athlete or coach can design a long-term training plan by assigning weekly, total TSS values for a given period. Those weekly TSS can then be further broken down into daily TSS. All that remains then is to determine how the unique TSS will be attained each day. This would involve the normal variations in training such as intervals, hill work, group rides, tempo rides, races and recovery days. Every day’s TSS would result from the interplay of duration and intensity for a given workout. Some of the rides would closely simulate the durations and intensities of upcoming races so the TSS of these rides would be similar to or even greater than the race’s anticipated TSS. That way, the athlete will be well prepared come race day.
As with any periodization method, the training load over time could be adjusted in such a way that planned TSS increases at an appropriate rate with low TSS blocks allowing for recovery.
Using such a system, the rider and coach can feel confident that they are addressing both the duration and intensity components of training.
You can use TrainingPeaks.com to track your daily TSS and see your fitness improve over time on the Performance Management Chart. British Cycling members receive discounts of up to 40% on TrainingPeaks Premium, based on BC certification level. Or, just sign up for a free Basic TrainingPeaks account.
Joe Friel is a coach, co-founder of TrainingPeaks.com and the author of several
books on training including The Cyclist’s Training Bible.
© Joe Friel 2013