What intensity factor tells you
Intensity relative to your threshold
Intensity factor (IF) is Normalized Power divided by FTP. If Normalized Power is 200 W and FTP is 250 W, IF is 0.80. It describes the session relative to the FTP setting, not a direct measurement of how hard the workout felt.
Move the intensity and duration controls to see why IF has such a large effect on the resulting TSS.
Change the reference, not the ride
Keep the workout the same and change FTP to see how intensity and TSS respond.
Cycling power example: TSS = hours × (Normalized Power ÷ FTP)² × 100. The selected values are assumptions, not a workout prediction.
Intensity relative to your threshold
Intensity factor (IF) is Normalized Power divided by FTP. If Normalized Power is 200 W and FTP is 250 W, IF is 0.80. It describes the session relative to the FTP setting, not a direct measurement of how hard the workout felt.
Move the intensity and duration controls to see why IF has such a large effect on the resulting TSS.
Why FTP changes the answer
The same 200 W Normalized Power gives IF 0.80 at an FTP of 250 W, but about 0.67 at 300 W. The ride did not change; the reference value did. Because power-based TSS squares IF, an incorrect FTP can materially distort the score.
FTP itself depends on how it was estimated and may not describe every effort duration equally well. Keep the estimate appropriate to the athlete and method.
Use it with duration
IF does not tell you how much work you did or how the session was structured. A short effort and a long effort can have the same IF but very different scores and demands.
Normalized Power can be misleading for very short segments. Inspect the trace and duration rather than treating a single IF as a complete description.
Different sports and methods
Running and swimming may use different threshold and load methods. Do not assume an IF or score calculated from heart rate or pace is identical to one calculated from cycling power.
Compare the same method over time and consider terrain, heat, sensor quality and how the session felt.
The same number can hide different rides
A steady hour and an hour with repeated surges can produce different Normalized Power even at the same average watts. IF uses Normalized Power, so its response depends on the shape of the effort as well as the FTP setting.
Open the power trace to see whether the number describes steady riding, repeated accelerations or a recording problem.
A value above one
A value above 1.0 means Normalized Power exceeds the FTP setting for the selected interval. That can happen for a sufficiently short effort, and it can also indicate that the threshold setting or recording needs checking.
The number alone does not prove the FTP is wrong. Consider the duration, how FTP was estimated and whether the selected recording is plausible.
Use the comparison carefully
If the same ride appears with a different IF after an FTP update, inspect which threshold was applied. The underlying power has not changed.
For comparisons across a season, keep a record of the method and threshold used. A change in the denominator can look like a change in training intensity.