A primary key to effective training is the appropriate application of stress to the body, followed by a recovery period for the body to adapt and/or recover for the next workout. Too much stress back-to-back and the body cannot reach the required intensities to gain new fitness in subsequent workouts. Too little stress and the result is no new increase in fitness.

But how can we quantify the intensity of a workout? Using volume alone removes the critical element of intensity from the calculation. Using intensity or speed without a benchmark risks over or under-training. Additionally, stress is always relative to the athlete’s abilities. One athlete’s maximum sustainable output will be quite different than another, and yet the relative stress could be identical for each athlete. My 18-minute 5K run is just as stressful to my body as PowerTri Pro Heath Thurston’s 16-minute 5K is to his body. Any determination of how stressful a workout was on an athlete must a combination of 3 inputs: duration, intensity, and relative intensity.
 

TSS (Training Stress Score) is an incredibly useful tool for a coach or an athlete. It allows us to quantify how much stress a particular workout was to the body. We can then use that information to structure the next set of workouts. It is a combination of a workout’s duration, intensity, and relative intensity. The duration is, of course, the length of the workout, the intensity is the raw output (power in watts or run pace). The relative intensity is measured using an Intensity Factor (IF),which represents how intense a ride was relative to your FTP (Functional Threshold Power/Pace, or the average power an athlete can maintain for approximately 30 minutes). True TSS and IF can only be measured with a power device or pace device, but I’ll show you how can get TSS with heart rate instead later as well.

Let me give you an example of how to calculate a TSS based on these inputs: Let us look at a cyclist with a ride of 1:31:27 in duration. The Normalized Power (NP, or power excluding zeros) for this ride was 181 watts. This athlete has an FTP of 240 watts.  Therefore, the IF for this ride is 0.754 or Normalized Power/FTP (181 watts divided by 240 = 0.754). We now have everything we need to calculate TSS for this workout.
 

The TSS calculation is (workout in seconds x NP x IF)/(FTP x 3600) x 100. Or, in this case, (5497 seconds x 181 watts x 0.754)/(240watts x 3600) x 100 = a score of 86.7.

What does this 86.7 mean? A TSS of 100 means the stress that riding 1 hour at your FTP would introduce to your body. Therefore, this ride was 86.7% as stressful to you as a 1 hour time trial.
Another ride for this same athlete may have been 3 hours long with power at 176 watts, or an IF of 0.733. The TSS score would be a 162.8 for this athlete. Meaning, it was 1.63 times more stressful to his body than a 1 hour time trial.

How can we use this data? In several ways, but the way I use it is:
a) to determine how much rest needed between workouts. I start with the guidelines from Dr. Andrew Coggan and adjust based on what I know about the athlete:
•    TSS less than 150 – low (recovery generally complete by following day)
•    150-300 – medium (some residual fatigue may be present the next day, but gone by 2nd day)
•    300-450 – high (some residual fatigue may be present even after 2 days)
•    Greater than 450 – very high (residual fatigue lasting several days likely)

b) to simulate the stress of a race without having to use race-day intensity. For example, let’s say that an athlete’s FTP is 255 watts, and I want them to race with an average of 210 watts (mid-Zone 3) for 2.75 hours. That would be an estimated TSS score of (9900 seconds x 210 x 0.823)/(255×3600) = 186 for the bike portion. As a coach, I have the option of introducing workouts that mimic the stress of racing in several ways. I could have him do a 2.75 hours ride at 210 watts for a score of 186, or a 3.75 hour ride at 180 watts, which would also equal a TSS of 186 points. This second option is a great option for athletes who have Ironman aspirations as well, we can get in a nearly 4 hour ride to increase IM endurance, while also mimicking the stress of a 70.3 ride.

c) to map long-term stress and chronic load, and ensure that there is a constant rise in stress throughout the season. This only works if the athlete uses a power meter for every single ride, and a GPS outside for every single run. Otherwise, the true stress cannot be calculated. Although it is possible to calculate the stress and TSS score based on HR, and there is quite a bit of literature and calculations on that, I think it is not worth the calculation if using HR to calculate chronic load. HR is significantly sensitive to time of day, temperature, indoors or outdoors, food intake, external stress, etc. The thought of making long-term decisions on HR data scares me. For an individual workout, yes, I can see using HR to calculate TSS, but not for chronic load analysis.

Calculating TSS for the run is almost identical to the bike. The difference is that you use graded pace instead of power. Graded pace is the pace that would have been maintained had the run been performed on a flat surface. It can only be calculated through a tricky manual calculation, or through software that can calculate it for you, like WKO+ or Golden Cheetah. You must have graded pace in order to do the calculation. The only differences are that a) the intensity is miles per hour instead of watts and b) a multiplier of 112 instead of 100 is used, since running does add more stress to the body. The calculation would therefore be (workout in seconds x MPH (graded pace miles per hour) x IF)/(FTP x 3600) x 112. For my European readers, kilometers per hour would work just as well.

For example, an athlete with a run FTP of 8.78 mph does a 1:44:01 run at graded pace of 7.627 mph. The calculation is therefore (6421 x 7.627 x 0.869)/(8.78 x 3600) x 112 = 146.5. Voila!

The calculation for heart rate also requires just a little bit of tweaking. See Joe Friel’s explaination of how to do this at http://home.trainingpeaks.com.

David Warden is a 3-time USAT All American and Elite Coach with Joe Friel's TrainingBible coaching. His work has been published in Triathlete and USA Triathlon Life magazines. He is the former Vice-Chair of the USAT Rocky Mountain Region, and the host and producer of the #1 triathlon podcast, Tri Talk and part owner of  www.powertri.com.