from Mac Cassin, APEX Coaching
Intensity Factor® (IF) and Training Stress Score® (TSS) are great tools for describing training loads for endurance workouts. However, all tools have their limitations. For TSS and IF, one of the limitations is accurately quantifying the training load associated with High-Intensity Interval Training (HIIT).
Addressing this shortcoming is particularly crucial during lower volume, higher intensity training blocks, and for athletes who have real “top-end” power.
Let’s briefly look at what you need to calculate the TSS of a workout.
(Feel free to skip the next section if you are already familiar with how TSS is calculated):
1. Your Functional Threshold Power (FTP). Many view this as the maximal power you can hold for about one hour (we will get into the semantics on this later).
2. The Normalized Power® (NP®) of a workout. This value is a rolling weighted average power for the session, giving more “credit” for efforts above FTP. The idea of NP is to provide a better representation of the physiological cost of a workout beyond average power or total energy output.
3. Total Ride Time. The number of hours from start to finish of a session.
First, you divide NP by FTP to calculate the Intensity Factor (IF).
TSS = (IF)2 x Ride Time (hours) x 100.
To quote Dr Andy Coggan who developed TSS alongside Hunter Allen, “[TSS] allows any endurance athlete the ability to quantify their workouts based on their relative intensity, duration, and frequency of workouts. One single value can now represent how hard, and how long you worked out. 100 points earned by a pro is relatively the same as 100 points earned for a beginner because TSS is relative to each individual’s threshold.” (1)
Complete article here