TSS, or Training Stress Score, is the most widely-used way of quantifying workout stress in cycling. It’s also one of most commonly misunderstood metrics in the sport, frequently interpreted to mean things it doesn’t. So what is TSS, and what is it good for?

Key Takeaways

  • TSS uses Time and Intensity to quantify the relative stress of a workout.
  • TSS is most effective as a way of tracking long-term trends.
  • Comparing rides by TSS can be misleading, as effort type and workout structure are not factored into the calculation.
  • No metric is a goal in and of itself. High TSS does not necessarily mean high fitness.
  • TrainerRoad’s Workout Levels are a new way to compare workout difficulty.
  • TrainerRoad shows your TSS, but our training plans take many more factors into account so you don’t have to.

A Common Confusion

When I first started training with a power meter several years ago, I spent a few seasons self-coaching. After each ride I’d study my power data in an attempt to glean some insight. I usually just confused myself, but the one metric that seemed mercifully straightforward was TSS.

One day I noticed that an easy 4 hour ride resulted in the same TSS as a much shorter and more intense workout from the previous week. These two rides didn’t leave me feeling the same, and it seemed obvious they did not have the same effect on my body. I thought I’d discovered a flaw in cycling data analysis, but I’d actually discovered a flaw in my thinking. Like many other cyclists, I thought all TSS was equal, and that this number was a universal way to compare one workout to another. I was wrong.

So What Is TSS, and How Is It Used?

TSS, or Training Stress Score, is a description of how much relative physical stress a workout places on the body. TSS is calculated from workout intensity and duration. Most people use software to find their ride’s TSS, but the specific calculation is:

TSS = (# of seconds of the workout x Normalized Power x Intensity Factor) / (FTP x 3600) x 100

This formula means that riding for an hour at your FTP results in 100 TSS. It also means a short intense workout can theoretically result in the same TSS as a long easy ride, since time and intensity both factor into the equation.

In TrainerRoad, each workout is quantified by TSS, among other metrics. This is used to organize training plans, as each week is designed to impart a progressively higher TSS than the week that preceded it. Occasional rest weeks of much lower stress allow recovery.

TrainerRoad’s Training Stress graph, displaying TSS/week over time

Weekly TSS is shown on the TrainerRoad calendar. Also, our Training Stress graph displays a 6-week rolling average of TSS/week as a handy visualization of your training load over time. In both instances, TSS is used as a convenient marker of training, but it is only one data point to consider. To be useful, Training Stress Score must be considered in the context of other metrics such as Intensity Factor (IF), interval duration, and time spent in power zones. These variables are all considered in the design of every TrainerRoad training plan and are displayed in the workout analysis for each ride.

How TSS is Useful

TSS is most effective as a tool for tracking long-term training load. Comparing total TSS/week over time is a helpful way to monitor Ramp Rate and ensure you are not doing too much, too fast. As a way of comparing individual workouts, it is important to remember that because of how the TSS calculation includes handles duration and intensity, it can be misleading.

A similar TSS value might seem to equate the stress of two dramatically different efforts, but this is not an accurate way of quantifying how your body responds to training. For this reason, TSS is best used for direct comparison only for workouts that stress similar energy systems- sweet spot and threshold rides, for instance. However, as we will consider next, even comparing similar rides by Training Stress Score can be dubious.

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How TSS Can Be Misleading

By definition, TSS is an approximation and a simplification, but problems arise when riders treat it as a definitive measurement without considering other factors. For instance, consider two workouts within a sweet spot training plan. A workout with 2x 20-minute intervals can result in the same TSS as a workout with a single 40-minute effort, but anyone who’s ever ridden at sweet spot knows it gets dramatically harder as efforts get longer. In this case, the TSS calculation of time and intensity doesn’t tell the entire picture, as it ignores the context within which that time and intensity is generated.

A similar situation can arise during a hard training block or over a short period such as during a training camp. The first workout of the camp will almost always feel easier than those that follow due to the fatigue that builds up over time, even if these workouts are identical. The nominal Training Stress of these rides would also be the same, but the physical toll and necessary recovery would not. 

Finally, TSS from rides with repeated surges often seems lower than expected. Many riders experience this in racing, as the stochastic rhythm of hard efforts feels nearly unsustainable. However, the overall time spent at high power in a race is usually fairly low, and the TSS calculation makes no allowance for nuance. The low TSS recorded from a race might seem to imply low stress, while your legs undeniably insist otherwise.

A New Way To See the Difficulty of a Workout

Workout Levels are a new metric in TrainerRoad that compares the relative difficulty of workouts within each training zone. They range from 1 to 10, with 1 being easiest and 10 being the hardest. Thus, a Level 6 Threshold workout is significantly harder than a Level 3 Threshold workout, but not as challenging as a Level 9 threshold workout. A very few particularly challenging workouts even exceed Level 10. 

Workout Levels go beyond metrics that only tell part of the story, like TSS. Instead, they utilize Adaptive Training’s data-driven insights and incorporate nuanced factors such as repetition, interval length, and recovery time. The end result is a ranking of workout difficulty that is both easily understandable and based on real-world experience.

Because Workout Levels go beyond TSS, you can not only compare workouts with greater clarity, but you can now easily see your training progressing, over time, through each training zone. For instance, a training block may start with a level 4 Threshold workout, but by the end, you’ll be completing ones at a much higher level.

Putting TSS in Context

TSS is just one of several metrics used to measure workout stress, and these metrics must be considered holistically to be useful. As with any other measurement, TSS is meant as a tool to help quantify and enable training; it is not a goal or target in and of itself. While TSS is can be one way to help measure progress within a training plan, it should never be the singular benchmark by which workouts are designed. Properly structured periodized training challenges specific energy systems in an intentional and progressively challenging way; TSS helps express this progression but comes far from capturing the total picture. 

Athletes often ask why their Training Stress Score doesn’t always increase from week-to-week within TrainerRoad plans, or why it decreases between training phases. These questions reflect a misunderstanding of what TSS represents and an oversimplification of how training works. After an FTP increase, your recorded TSS may actually trend downward as workouts become harder at higher power targets. In this case, a decrease in TSS does not indicate an actual decline in training stress, but is just a numerical reflection of how TSS is calculated and a demonstration of its limitations in showing progress.

As shown earlier in the case of interval length, workout structure is more significant and impactful than the workout’s training stress score would indicate. By varying workout design within a training plan, we can stress the body in progressively specific and useful ways. In many cases, TSS does not reflect or express these variations, but it’s the body’s response to training that matters.

In Conclusion

We all feel tempted to oversimplify our training, and imagine that one number or bit of data offers the key to perfect fitness. In reality, training stress and how the body responds is incredibly complicated and subjective, and dependent on a multitude of factors. TSS is one tool for quantifying workouts, but good training plan design is far more nuanced than a single number can describe, and TrainerRoad takes care of this complexity for you. Trust the process, listen to your body, and let the results speak for themselves; TSS is just a number to help you get there. 

For more cycling training knowledge, listen to Ask a Cycling Coach — the only podcast dedicated to making you a faster cyclist. New episodes are released weekly.

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Sean Hurley

Sean Hurley is a bike racer, baker of sourdough bread, and former art professor. He is a connoisseur of cycling socks and a certified USAC level 3 coach. Rumor has it he also runs a famous cycling instagram account, but don't tell anyone about that.