What Are Training Stress Scores (TSS) and How Are They Calculated?

When planning and analyzing my training sessions and those of my athletes, one of the most important factors I consider is accurately measuring the training load and the stress level placed on the body. One of the most reliable ways to do this is to understand and apply the Training Stress Score (TSS) system, widely used in platforms like TrainingPeaks.

TSS is a system that quantifies the training load on the body using various metrics depending on the type of workout. Different disciplines like cycling, running, and swimming have specific calculation methods. In this article, I'll explain how training stress Scores are calculated and, based on my own experiences, which situations provide the most accurate insights.

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Training Stress Score (TSS) – Cycling

If you're using a meter, you can calculate your TSS for cycling workouts more precisely. TSS is a scoring system that accounts for both duration and intensity. The key concept to remember here is that riding at your FTP (Functional Threshold Power) for one full hour equals 100 TSS.

This is critical information: You cannot exceed 100 TSS in a single hour, no matter what you do. If you ever see a one-hour workout with more than 100 TSS in your TrainingPeaks data, your training zone settings are incorrect in the system.

Key Metrics in TSS Calculation

One of the most important variables used in TSS calculations is Normalized Power (NP). If you were pedaling with a consistent effort on an indoor trainer, what would the load on your body feel like? That's precisely what NP aims to measure.

Another crucial factor is the Intensity Factor (IF), which helps compare the intensity of a workout relative to your FTP. The formula defines FTP as the highest average power output you can sustain for one hour. The IF value, in turn, tells us what intensity you exercise relative to your FTP.

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TSS Calculation Formula

$$TSS = \frac{t \times NP \times IF}{FTP \times 3600} \times 100$$

Or, since we know that IF = NP / FTP, we can rewrite the formula as:

$$TSS = \frac{t \times IF^2}{3600} \times 100$$

Explanation of Variables

• t (Duration) – The total duration of the workout, usually measured in seconds (e.g., 1 hour = 3600 seconds).
• NP (Normalized Power) – A weighted average power value that accounts for short-term fluctuations in effort, reflecting the actual physiological stress on the athlete more accurately than a simple power average.
• IF (Intensity Factor) – A ratio representing the workout's difference from FTP.
• FTP (Functional Threshold Power) – An athlete's highest average power output for approximately one hour.
• 3600 – A constant that converts the duration into seconds (60 sec × 60 min).

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Example Calculation

Let's say we haet's workout with the following characteristics:

• Duration (t): 5400 seconds (1.5 hours)
  
• Normalized Power (NP): 200 Watts
• FTP: 250 Watts

Step 1: Calculate Intensity Factor (IF)

$$IF = \frac{NP}{FTP} = \frac{200}{250} = 0.8$$

Step 2: Calculate TSS

$$TSS = \frac{5400 \times 200 \times 0.8}{250 \times 3600} \times 100$$

Breaking it down:

• Numerator: 5400 × 200 × 0.8 = 864000
• Denominator: 250 × 3600 = 900000

$$TSS = \frac{864000}{900000} \times 100 \approx 96$$

So, this workout's Training Stress Score (TSS) is approximately 96.

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Running Training Stress Score (TSS)

We use rTSS (Running TSS) instead of TSS for running workouts. The key variable here is pace. We can determine how demanding the workout was by comparing your pace to a specific threshold.

We use the Normalized Graded Pace (NGP) on hilly courses, which adjusts for elevation changes and gives a more accurate measure of actual effort.

In my own training, I rely heavily on rTSS. Even if my pace is the same on a flat road and on a hilly trail, NGP accounts for the elevation factor, helping me better understand my training load.

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Swimming Training Stress Score (sTSS)

Analyzing swimming workouts is more complex. sTSS is calculated using swim pace, total distance, and active swimming time. Rest periods are not factored in, which allows for a more accurate assessment of actual effort.

However, sTSS tends to overestimate training load, often inflating CTL (Chronic Training Load). Because of this, I strongly recommend manually adjusting your swim data after each session to avoid misleading numbers. If you work with a coach, ask them to monitor this regularly.

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Heart Rate-Based Training Stress Score (hrTSS)

When power meters or GPS-based pace data aren't available, hrTSS (Heart Rate TSS) becomes a valuable alternative—especially for steady-state workouts.

However, hrTSS loses accuracy in high-intensity interval training (HIIT), where heart rate lags behind sudden efforts.

From my experience, hrTSS works well for long, steady runs or bike rides but is unreliable for HIIT. When you suddenly start climbing a hill, your power output might spike by 30-40% instantly, whereas your heart rate will take 1-2 minutes to catch up. During that delay, hrTSS will not entirely reflect the actual intensity of your effort.

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TRIMPS-Based Training Stress Score (tTSS)

If no other technical data is available, tTSS (based on TRIMPS) can be calculated using heart rate and training duration. However, this method is the least accurate, especially for workouts with fluctuating intensity.

I always prefer TSS or rTSS for precise training analysis, as they provide more reliable data.

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Conclusion

Understanding the Training Stress Score (TSS) system is key to optimizing your training and accurately assessing your workload.

• If you're using your meter, TSS is the best tool.
• If you analyze your running pace, go with rTSS.
• For swimming, sTSS can work, but be cautious with its accuracy.

While heart rate-based methods (hrTSS and tTSS) are widely used, they aren't as precaren'tf possible and always rely on power-based or pace-based metrics for better accuracy.

I use these methods daily to plan my training and adjust my workload. You can also leverage these metrics to analyze and optimize your own training!

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