TSS or Training Stress Score is a metric used to define the training load put on the body by an athletes training. It is a numerical scale that starts at 0.
The metric is used to measure individual workouts and can also be used to understand training load over a period of time.
In the case of the Hub App, it is used to define the training load obtained by completing individual workouts on your indoor smart trainer.
How is TSS Used?
TSS is worked out by considering both the intensity and duration of the effort.
A TSS of 100 is assigned to a workout where the individual working out spends one hour riding at their FTP** (Functional Threshold Power). A TSS of 50 will be obtained by riding at FTP for 30 minutes.
It is important to understand that TSS, difficulty and intensity are different things.
For example; A 30 minute HIIT ride might give a TSS of 25 but be feel difficult or hard to complete whereas a user completing an endurance ride, during which they ride at 50% of their FTP for 45 minutes, will achieve a higher FTP without feeling the workout is as difficult.
For TSS to be used accurately, an individuals FTP must be set in the App
Functional Threshold Power
An athletes FTP represents their ability to sustain the highest possible power output over 45 to 60 minutes, depending on whether you're a trained athlete or not. As a result 95% of the 20 minute average power is used to determine FTP.
Further reading is available here.
IF, or Intensity Factor is the ratio of normalized power** above the riders Threshold Power.
It defines a level of intensity that can be applied to a workout to help a user understand how difficult the session is.
How is IF Used
IF is used in the Hub App to understand the intensity of the various Hub training sessions as follows:
Less than 0.75 - Rides that have a very low intensity
0.75-0.85 - Rides that have a low intensity
0.85-0.95 - Rides that have medium intensity
0.95-1.05 - Rides that have a high intensity
1.05-1.15 - Rides that have a very high intensity
Greater than 1.15 - Rides that have an extremely high intensity
An up to date FTP is important for this to be accurate.
Normalized Power is another metric used to quantify training intensity.
Essentially, Normalized Power is an estimate level of the average power you would be able to create if the level of effort was constant.
It is complicated to isolate and it is not necessary to understand the process.
Further reading is available here