In previous posts on Power Meters, we have got to grips with Normalised Power, basically, the power you could have maintained on a workout if conditions were perfect, and your Functional Threshold Power FTP, which is the marker where the body switches from ‘aerobic’ to ‘anaerobic’. Using the two figures, we can gauge what recovery time you might need after a ride by dividing them to find your Intensity Factor.

NP /FTP = Intensity Factor


Intensity Factor

The Intensity Factor value will be between 0.75, the easiest intensity to 1.15 the hardest intensity. If you score 1.0 it means you are spot on your Functional Threshold Power FTP. So equally, if it’s higher than 1.0 you have beaten your Functional Threshold Power FTP amount.


Training Stress Score

To gain a better insight, your head-unit takes the Intensity Factor readings over the period of your workout. The maths here is, 1 hour = 100 points. So the actual Training Stress Score TSS is a composite number estimated from the workout to highlight the overall training load. It follows that if you work at a higher intensity your workout time comes down. Note that if your Intensity Factor is 1.05 for a race of an hour, it suggests that your Functional Threshold Power FTP has changed.

TSS was actually modeled on heart rate-based training impulse (TRIMPS) by Dr. Eric Bannister, where he formulated the amount of glycogen utilized in each workout. Tolerating training intensity though can be different from person to person –  don’t you just hate those natural athletes that never get tired!

As a basic guide the TSS is the amount of stress your body has endured and the recovery time you’ll need, see below, so you can train hard, but not so hard that you’ll ‘burn-out’.

Training Stress Score Guide *

Recovery time

Below 150

Low recovery needed. Cycle again the next day.

150 – 300

Medium recovery needed. Tiredness is gone by 2nd day.

300 – 450

High recovery needed. Tired even after two days.

More than 450

Very high recovery. Tired even after two days +.

* A score of 100 = your FTP


Intensity Factor (IF) and Training Stress Score (TSS) are registered trademarks of Peaksware.