Forget FTP – Critical Power (CP) is the threshold concept being used to win Olympic medals.
Your CP is the maximal power output you can sustain through aerobic pathways. This is important because when you exercise above CP you start to use your anaerobic reserve (W’) and that is limited. CP represents the maximum power output at which your muscles can achieve homeostasis. Ride above CP and substances start to build up within the muscle cells that will eventually limit the muscles cells ability to contract and therefore mean you are unable to sustain that power output.
CP is therefore linked to the underlying physiology of power production, not simply how much power you can put out (as is the case with FTP). Because of this, by manipulating if you train below, or above CP, and by how much, we can target specific physiological changes and therefore specific adaptations to training.
CP has a companion component called W'. W' is a reserve of power producing capacity - produced through anaerobic pathways - that your body can call one for those short sharp bursts.
The substances produced when riding above CP limit your ability to produce power. You body has a certain tolerance to those substances - the larger your tolerance to those substances the more power you can produce above CP before you fatigue!
Because the tolerance to those substance is always the same it doesn't matter if you produce them slowly over a long time or very quickly over a short period of time - once you hit your limit you can no longer produce that power output.
Alongside measuring your CP we can measure your W'. This allows us to monitor and train the very important component for short hard efforts up to ~20mins in duration
W' can be thought of as an energy battery. Its there you you really need it, however it is limited but can also be recharged when it is not in use. If your power output drops below CP you have some excess energy left to process all those substances that have built up and were stopping you from continuing that hard effort. Essentially you are re-charging your W'.