Modern endurance training is full of metrics that can provide valuable insight into an athlete’s physiology. These same metrics can also be overwhelming for many who attempt to make heads or tails of these sometimes complex numbers. Knowing which metrics are important, and what they mean for a specific workout or training segment is critical. While there are many performance metrics that are valuable, the following are foundational concepts that every athlete looking for improvements should have a grasp on.
Training Stress Score (TSS)
Training Stress Score is a composite number that takes into account the duration and intensity of a workout to arrive at a single estimate of the overall training load and physiological stress created by that training session. Simply put, it’s a way of expressing the workload from a training session. By taking both intensity and duration into account, TSS allows for a better understanding of the “cost” of every individual effort and workout. TSS is calculated using the following formula.
TSS = (sec x NP x IF) / (FTP x 3600) x 100
Where “sec” is the duration of the workout in seconds, “NP” is Normalized Power, “IF” is Intensity Factor, “FTP” is Functional Threshold Power, and “3600” is the number of seconds in an hour. It’s worth defining and understanding this equation so you have an idea of why TSS is so accurate, and how the final score is derived. The components that comprise TSS are what make it so useful to athletes. Normalized Power provides a better measure of the true physiological demands of a training session. It considers both rapid changes in intensity, as well as critical responses in the body associated with those changes. Unlike average power, Normalized Power is the power your body “thinks” it employed based on the variability of the workout. Intensity Factor (IF) is the ratio of Normalized Power to threshold power (FTP). IF takes into account differences in fitness within or between individuals. It’s a great way to track fitness over time for a given effort, i.e. the same ride with a lower IF indicates increased fitness. Using TSS provides a well-rounded look into both the physiological expenditure of an effort, as well as what that effort means for the fitness and progression of an athlete.
Normalized Power (NP)
Power is inherently variable. To account for this variability Normalized Power is a rather complex algorithm that helps to quantify the physiological “cost” of a given effort lasting longer than 30 seconds. Essentially NP is an estimate of the power that you would have maintained having the same physiological impact if your power would have been completely consistent. As a training tool it’s a great way to track the intensity of a given effort or race. If an effort includes a lot of coasting or drafting, the NP will be lower than a steady state solo effort for example. When looking critically at past workouts NP helps provide a lense through which to examine the session. While TSS may be similar for two seperate workouts due to intensity and duration, NP shows how hard the body had to work to achieve the TSS. This is also one of the biggest benefits to training with power over heart rate based metrics for hyper-variable disciplines like mountain bike racing. Normalized power weights the surges and increased efforts that variable terrain and a dynamic race scenarios provide where average power doesn’t fully capture the level of intensity or stress on an athlete’s body.
Intensity Factor (IF)
Normalized power is a better judge of intensity over average power. However, it doesn’t allow for differences in fitness between individuals. In order to better represent this, IF is the ratio of Normalized Power to threshold power (FTP). Very simply, as your Threshold increases, the same training session or race over time has a lower IF. Intensity Factor is a great way to understand how certain sessions should be positioned within a training plan. With a planned duration and IF a TSS for specific sessions can be calculated. Knowing what IF to expect from key efforts sheds light on important workout factors like intensity, duration, and frequency. IF can also be a good way to track changes in Threshold. If a workout or race close to 1 hour in length produces an IF of over 1.05 there’s a good chance the athlete’s Threshold needs to be adjusted. Here’s a good breakdown of standard IF ranges.
- Less than 0.75 recovery rides
- 0.75-0.85 endurance-paced training rides
- 0.85-0.95 tempo rides, aerobic and anaerobic interval workouts (work and rest periods combined), longer (>2.5 h) road races
- 0.95-1.05 lactate threshold intervals (work period only), shorter (<2.5 h) road races, criteriums, circuit races, longer (e.g., 40 km) TTs
- 1.05-1.15 shorter (e.g., 15 km) TTs, track points race
- Greater than 1.15 prologue TT, track pursuit, track miss-and-out1
While there’s no shortage of metrics that can help to inform an athlete’s training approach, there are a few stand out concepts that help shape everything else. TSS, NP, and IF are metrics that can be dramatically impactful once they’re fully understood. Knowing what they mean and how to utilize them is important for any athlete that’s looking to take an analytical approach to their training.
1 Dr. Coggan, Andrew (2016, February) Normalized Power, Intensity Factor and Training Stress Score. https://www.trainingpeaks.com/blog/normalized-power-intensity-factor-training-stress/
