What are the IF (Intensity Factor) and TSS (Training Stress Score) parameters?

You have probably seen these parameters on occasion when examining the detail of a training session or indicated in a predefined workout.

They were proposed by Hunter Allen and Andrew Coggan in 2010 as a way to quantify power-based training.

Let's see what they mean.

IF. Intensity factor

The ratio between the normalised power (NP) of a session and your current FTP level.

For example, if your FTP value is 270 W and in a one hour session you have performed 270 W of normalised power, the Intensity Factor is 1.0.

TSS. Training Stress Score

This parameter provides an estimate of the exhaustion in the training session. It takes the previous Intensity Factor and the session duration into account.

It is obtained through the recorded power data and is calculated using the following formula:

TSS = (time in seconds x NP x IF) / (FTP x 3600) x 100

There is no need to learn it, since this data is normally calculated automatically and is shown in the session detail.

The TSS can also be used as an indicator of the recovery time required before the next session. It is therefore useful to schedule and plan training based on this parameter.

Here is an approximate relationship between the TSS of a session and the necessary recovery time:

Keep in mind that for this data to be reliable, your FTP value must be correct, hence the importance of updating it regularly.

Note: Normalised Power® (NP), Intensity Factor® (IF) and Training Stress Score® (TSS) are registered trademarks of Peaksware, LLC.