Power 101

A place where you can find explanations to all the TrainerRoad and Power acronyms and terminology...

FTP - Functional Threshold Power; Functional Threshold Power, or FTP is the maximum average power you can hold for one hour.  So if you can hold 300 watts for one hour, that means your FTP is 300. FTP is the best single marker for fitness in cycling.  The higher your FTP is, the stronger you are. 

LTHR - Lactate Threshold Heart Rate; LTHR is the maximum average heart rate you can hold for an hour. So if your average HR for an hour ends up being 180bpm, that means your LTHR is 180.  It's very similar to FTP. LTHR is not a good fitness marker, but it can be used to scale workouts.  

TSS - Training Stress Score; TSS 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.  It is conceptually modeled after the heart rate-based training impulse (TRIMP).  By definition, one hour spent at Functional Threshold Power (FTP) is equal to 100 points. 

Normalized Power (NP); An estimate of the power that you could have maintained for the same physiological "cost" if your power had been perfectly constant, such as on an ergometer, instead of variable power output.  NP is used to calculate TSS.

Intensity Factor (IF) for TSS: For any workout or part of a workout, the ratio of the Normalized Power to the rider's functional threshold power, which gives the user a relative intensity in relation to their threshold power. IF is used to calculate TSS.

How do TSS and IF relate to each other? Some rides can be intense, some can add a lot of fitness, some do both.  A workout that is just 30 minutes long can be really intense and have an IF score of over 1.0, but that doesn't mean you'll earn a lot of TSS from it.  By contrast, you can have a 2 hour ride that's not very intense, but will earn you a lot of TSS.