I was trying to find out the difference between the calculated Critical Power (CP) and FTP values in GoldenCheetah and came across this explanation
, thought it might be helpful to others:
Critical Power (CP) as calculated by GC is not equivalent to Functional Threshold Power (FTP). The two should not be used interchangeably for training purposes.
If done correctly CP as calculated by GC will be slightly above FTP or the power you can hold for 60 minutes (Mean Maximal Power 60). If you are training by A. Coggan zones, using CP will have you struggling.
Critical Power is a model output taken from fitting curves you your longest effort for given powers. The model tries to define a number (CP) where your power curve flattens out. As peoples power curves generally flatten out before FTP in the 20-30 minute range (but this can very).
I think the confusion comes from two factors. Firstly, that FTP is defined as MMP60, and sometimes people interchange MMP for Maximal Average Power (MAP) or Critical Power (CP), followed by the time unit, to define maximum outputs for given durations. i.e. FTP = MMP60, = MAP 60 = CP60. Critical Power as calculated by Golden Cheetah is NOT CP60, and anecdotal sits in the 20-30 minute range (but varies from athlete to athlete depending on how you are effected fatigue).
The second reason they are confused is that the numbers are often quite similar (~10%). There is a relatively small drop off in power from 20 minutes to an hour (most people assume 5% drop when calculating their FTP from a MMP20). Athletes will know their FTP and when they see GC calculating a number which is within ~10% they assume it is trying (and failing) to calculate FTP (CP60, MAP60, MMP60).
If you are implementing other peoples training plans and are trying to get the intensity right, then use the same measure of intensity that they use (usually % of FTP/CP60). If you are doing your own training plan then you can use whichever value you wish.
The real aim of defining CP is to model your ability to put out efforts above CP and recover below CP. Useful in a road race where the best strategy is to vary power effort to follow attacks or rest in the bunch. Therefore I'm not sure the CP value as calculated by Golden Cheetah is as useful for TTing (unless very hilly).
To which Dr. Andrew Coggan added these comments. The second point is quite illuminating as to the difference in values.
1. FTP is not defined as maximal power for 60 min.
2. If you use tests of sufficient duration, as originally recommended by Monod, as well as by myself and Phil Skiba, then on average there is no difference between CP and FTP. OTOH, if you use tests that are too short (esp. at the short end), as GoldenCheetah tends to automatically select, then CP will be overestimated, and hence be higher than FTP.