TSS® stands for Training Stress Score and is a way to objectively quantify how hard or easy a workout is via a points-based system.

Dr. Andy Coggan and Hunter Allen pioneered TSS with the original seed concept being developed by Dr. Eric Bannister’s heart rate-based training impulse (TRIMPS).

TrainingPeaks

TSS Points Breakdown

TSS points are accumulated by a workout’s relative duration, intensity, and frequency. Super simple example:

  • 1 hour at 100% FTP would equal 100 TSS.
  • 1 hour at 50% FTP would equal 50 TSS.

You get the idea! The actual TSS formula =

TSS = (sec x NP® x IF®)/(FTP x 3600) x 100

Joe Friel

Where NP = Normalized Power, IF = Intensity Factor, and FTP = Functional Threshold Power.

Now that we understand what TSS is and how it’s calculated, how can we apply it to training?

Enter The PMC

The Performance Management Chart® (PMC) is something most of us have seen before in one form or another and is based on Bannister’s TRIMPS method as well, with it being able to use power data in addition to heart rate data. The PMC is fed TSS data which results in all the pretty lines we see below:

Screenshot taken from TrainingPeaks

Crash Course For The PMC

If you have no idea what the above is, it’s fairly simple to understand:

  • The blue line = CTL (Chronic Training Load) = Fitness
    • “An exponentially weighted average of the last 42 days of training”. If you see the blue line trending upward, that’s a good sign the athlete is doing enough training to create a positive fitness response and vice versa.
  • The pink line = ATL (Acute Training Load) = Fatigue
    • An “exponentially weighted average of training stress from the past 7 days”. Fatigue must happen to create an overload and subsequent response, so imagine the pink line “pulling” the blue line upwards. More training fatigue = higher fitness response.
  • The yellow line = TSB (Training Stress Balance) = Form
    • Simply CTL minus ATL. A positive TSB is a good indicator the athlete will be fresh and have a good performance, and vice versa.
  • You can also see the natural ebb and flow of a season, when the athlete had a taper period, “peak” period, illness/injury, etc.

So, What’s My Problem?

If we remember, 100% FTP for 1 hour = 100 TSS. This statement makes a rather large assumption that everyone can hold their FTP for 1 hour, which isn’t true. This means the TSS accumulated will be skewed for each and every athlete that doesn’t have an exact time to exhaustion (TTE) at FTP of 1 hour. For example:

Athlete A

Athlete B

In looking at the 2 athlete examples above, we’re just concerned with MFTP (modeled FTP from WKO) and TTE, with TTE differing by almost 20 minutes. This should mean that Athlete A will accumulate 100 TSS if they maintained 336w for 32:14, versus Athlete B would if they maintained 299w for 51:54, which isn’t substantiated by the TSS equation.

In understanding that, another can of worms is the relative inaccuracy of the PMC chart, which is based purely on TSS data…

In Conclusion

TSS and the PMC are still useful to track trends in training, but using the data as an absolute can be misleading due to individual differences in an athlete’s FTP and TTE. I recommend looking at the PMC chart loosely to plan your training, but always listen to your body when push comes to shove in providing it with a training load as well as rest – e.g. if your “Form” is a +15, but you don’t feel ready to hammer it on the day, don’t!

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