Threshold power or Functional Threshold Power (FTP) forms the basis of power based training. How hard can you go for one hour, i.e. what is the maximum average power you can produce for one hour, when well rested?
Andy Coggan has suggested energy system zones related to FTP, allowing us to train different energy systems effectively based on understanding our FTP.
How do you find out your FTP? There is no magic formula for calculating it based on age, gender, weight, star sign, etc. You have to ride and ride hard for a sustained period to be able to estimate FTP with any degree of certainty.
It is key that you use the same method to test your FTP each time. There’s no point comparing your FTP based on one method to your FTP based on another. You want to track your fitness level in a meaningful manner.