training stress score

"I really need to be able to quantify the training dose more accurately than 2 x 20 minute intervals with FTP. I needed a score."
This is what Hunter Allen told Dr. Andrew Coggan and the result of his thoughts was the Training Stress Score (TSS).

Training Stress Score ©

The Training Stress Score (TSS) should serve as a measure of the magnitude of the training stimulus. The higher the value, the greater the stress to which the body is exposed. Training sessions with a high score mean a higher training stimulus, a higher need for regeneration in the short term and a greater increase in fitness in the long term.

History of the TSS

But the desire to quantify the training load existed much earlier. Bannister did important work in his time by developing TRIMP, the idea behind the TSS. TRIMP is the abbreviation of ‘training impulses’ and attempts to “measure” the training stimulus. 

In 1980, of course, the measurement methods and possibilities were different and the method of choice for quantifying intensity was heart rate.

The score is based on the increase in heart rate, which is gradually weighted, and the duration of training. It’s a rather complicated formula, but that’s not what this is about.

Calculation of the TSS

The TSS is calculated according to the same principle as the TRIMP: from the intensity and the training duration.

The intensity can be determined not only by heart rate, but also by power or pace. A reference value is needed for this.

This reference value is the functional threshold power (FTP), – pace (FTPa) or – heart rate (FTHr). This corresponds to an intensity factor (IF) of 1.0. It is now assumed that this power, pace or heart rate can be maintained for a maximum of one hour and is associated with the TSS 100.

Short: IF: 1.0 x duration: 1h ≙ 100 TSS

Depending on how intensively and how long you train, the TSS is calculated. The intensities are weighted step by step as in TRIMP. The time above the threshold is included in the calculation with a higher factor than the time below the threshold, whereby the intensity ranges are further divided.

Dr. Andrew Coggan also establishes a link to energy consumption, explicitly to glycogen consumption. The TSS can be seen as an estimate of the amount of glycogen spent and thus it also correlates with the need for regeneration.

Workouts and the perceived effort

As a general rule, the harder the workout, the higher the TSS. However, harder does not necessarily mean more intensive. 

A higher TSS than 100 can actually only be achieved if the training session lasts longer than 1 hour. Conversely, the IF can only be greater than 1.0 if the unit is shorter than 1 hour. Everything else tells you that your FTP value is not up to date (or you are not exactly the model athlete who can keep his FTP for exactly one hour, which is also possible). 

Because the TSS is calculated in relation to the individual threshold, a training session with 100 TSS theoretically feels the same for every athlete, regardless of level. 

On the other hand, a well-trained athlete can cope with a higher load, so he or she is able to achieve a higher TSS total per training session, per week or per training block. On the one hand because they are fitter, on the other hand because they are able to tolerate more effort and in extreme cases pain, as studies show. 

Training recommendation

It is not possible to say in any general way how high the TSS of a workout should be in the “ideal case”.

Basically, the theoretically “perfect” score of the session depends on three factors: the goal of the workout, the combination with other workouts and your fitness level.

Goal of the workout

Every workout should have a goal, and achieving the highest possible TSS should not be an actual goal. Even a session with little TSS can be of great benefit to your training! Maybe the goal of the session is to actively regenerate, to improve your technique or it is in the top intensity range. Sprint workouts are extremely intense for short periods of time, but because time is so short in these intensity ranges and is followed by long periods of regeneration, the TSS is usually relatively low compared to the perceived effort.

Combination with other workouts and regeneration

The higher the TSS of the workout, the longer you need to recover. The time when the next session follows, how your week and the whole training block is planned, decides how much time is available for regeneration.  A high TSS in one session or in consecutive sessions without adequate time for recovery is not beneficial. 

I personally try to avoid workouts with an extremely high TSS. For me, the benefit of a single, hard training session is usually not in proportion to the exceptionally high recovery time, the risk of overload and the like. From my point of view, consistent, regular training is far more important.

Fitnessstand

The fitter you are, the higher the TSS you can achieve in one session. In some cases it may even need to be higher compared to a time when you were less fit for it to be an effective training stimulus.

Also, how you have trained in the past will affect how high the TSS is that you “can tolerate”. If you are used to high training volumes in the low intensity range and are doing a short, high-intensity workout for the first time, it will put more stress on you than the score suggests. 

Conclusion

There are some recommendations, but ultimately the question in training planning has to be asked not about the TSS, but about the goal. The Training Stress Score is a score to help you quantify your training load, compare training sessions, establish a long-term trend or estimate your recovery needs. The whole thing is a model and does not reflect reality, just as you do not exactly mirror the average athlete.

Don’t get me wrong, the TSS is a great tool, no more, no less. 

Studies

Sandersa D, Heijboer M,  Hesselink MKC, Myers T, Akubat I: Analysing a cycling grand tour: Can we monitor fatigue with intensity or load ratios? J Sports Science. 2018; 36(12):1385-1391.

Cejuela R, Esteve-Lanao J. Training load quantification in triathlon. J of Human Sport and Exercise. 2011; 6(2):218-232

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Josephine Noack
Josephine Noack
I'm the head coach of noack sport support and sports-scientist. As an athlete, I compete in cross-triathlon, normal short-distance triathlon, MTB-Marathon, road crits, and cyclocross during the winter.

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