By The Numbers: What is TSS?

How do you quantify the quality or exertion of a workout? When it comes to training metrics, most programs will calculate this using TSS or the Training Stress Score. There are many things TSS represents in a workout and many misconceptions within the endurance community about this metric. So what does it all mean? 

What is TSS?

TSS is an estimate of the training load created by a workout based on intensity and duration.1 TSS is calculated by using both IF (intensity factor) and time, to determine how “hard” a training session was based on the magnitude of the physiologic adaptations to training. In simplest terms, TSS determines the “cost” a workout has had on your body. Typically the higher the calculated TSS score, the more fatigued you will feel. 

How is TSS Calculated?

Metrics Breakdown:

For a more in-depth explanation of cycling, metrics visit The Metrics that Matter. For a quick review, check out the following factors that are involved in TSS calculation below:

TSS is calculated by the following equation:

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

sec: the total number of seconds in the session

NP: Normalized Power (more on this in a bit)

IF: Intensity Factor (more on this in a bit)

FTP: Functional Threshold Power (power you can hold for the 60-minute duration)

3600: total number of seconds in an hour

For example, a 60-minute ride at FTP (functional threshold power) would yield a TSS score of 100.

Keeping this in mind, it is also important to distinguish that not all TSS scores are the same. What do we mean by this? Since duration and intensity are both used in calculating TSS, you can go for a 3-hour easy ride and have your TSS equal to a 90 minute VO2 max interval session, but one of these sessions leaves you more physically exhausted than the other. Why is that? It is not that the metric is wrong, but the way we approach TSS is skewed. Not all TSS numbers are created the same, and we cannot always view the same number as meaning the same type of intensity effort. In your easy long ride, you were taxing a different energy system than you were when you did your VO2 max repeats. One session allowed you to remain in Zones 1 and 2 for your heart rate, while the other had you drifting into Zone 5 throughout multiple intervals. So while each workout was taxing in its own right, the same score did not represent the same amount of recovery needed post-sessions. 

Power and Heart Rate Based TSS

Power-based TSS training is the default method of calculation for cycling. In order for power-based TSS to be accurate, this requires you to have an FTP (functional threshold power) calculated and then power zones set based on this. If you do not have power-based metrics and use heart rate-based training, this can still be used to calculate TSS.

Heart rate TSS or hrTSS is a less accurate model for determining your TSS of a workout. While hrTSS is accurate when performing steady-state efforts, or endurance rides, it is far less accurate when intervals come into play. hrTSS is based on time spent in each heart rate zone based on your Threshold Heart Rate. When there are large or frequent fluctuations in your heart rate data, such as data post-interval session, it is hard for an accurate TSS to be quantified from this as it takes time for your HR to stabilize pre, during, and post effort. 

If you do not have a power meter, or for example are trying to determine TSS for an activity that is not heart rate or power-based, then using RPE is a good estimate to gauge your TSS for an activity. You will need to gauge the perceived exertion of the activity and then factor in the duration of the activity as well.

TSS and 4DP

How does TSS relate to your 4DP power profile? With 4DP, the workouts are individually tailored based on your ability to produce power above FTP. For shorter harder efforts, the power targets can be drastically different even if those two riders have the same FTP.

For example, 2 riders both with an FTP of 200W.  Rider 1 can hold 500W for 1 minute, Rider 2 can hold 350W for 1 minute.

Having both riders do 5 x 1-minute repeats at 150% of FTP (300W)  would give identical IF/TSS values since their FTPs are the same.

However, Rider 1 will find this workout much easier than Rider 2.

Having the riders do 5 x 1-minute repeats at 90% of 1-minute power (450W and 315W respectively) will effectively make the session equally “hard” for both riders, though Rider 1 will have a much higher IF and TSS for this session.

TSS and your Training

TSS is a good tool to use for analyzing trends in your training. Whether this is weekly, monthly, or within a training block. Following and analyzing your TSS scores can help get a glimpse into the fatigue levels your body is going to experience. Being able to recognize when your fatigue levels are accumulating and staying at a sustained high level for weeks at a time can help you avoid overtraining.

If you continue to train with increased fatigue for long periods of time, your body will be begging for rest. This is why it can seem easy to perform a difficult interval workout during the first week back post-recovery, versus performing the same session at the end of a three-week training break – accumulated fatigue has set in. 

When you are able to track and recognize these patterns, you can then recognize when it is time to take your foot off the gas pedal and take some rest. 

While metrics can oftentimes be confusing and make us stress over “hitting the right number,” it is important to remember that there are pros and cons to always relying on metrics. TSS is a great tool to get a gauge of how hard or easy an effort should be, but it is not concrete or set in stone that this is in fact, how your body perceives it. Having the knowledge of understanding metrics like TSS can help us as athletes make better-informed decisions when it comes to our training than relying on the data alone. TSS can help us track trends that ultimately help us learn when our body needs rest more than anything else. Pushing past our body’s limits is a good way to gain fitness, but too much for too long is when we get ourselves into trouble. 


References:

  1. https://help.trainingpeaks.com/hc/en-us/articles/204071744-TSS-Training-Stress-Score-
Mac Cassin is the Chief Cycling Physiologist at Wahoo Sports Science. He holds a degree in Integrative Physiology from the University of Colorado-Boulder and has won multiple National Championships. The experience of juggling athletic goals with collegiate and career responsibilities has taught Mac that peak performance is achievable even for those who cannot focus exclusively on training. While concentrating on exercise physiology in an academic setting, Mac competed at the World Championships, Pan American Championships and World Cups on both the road and track.
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