How to practice extreme fitness? Do you need to practice often?

Extreme fitness is to do some actions that normal people can't do on the basis of fitness. First of all, on the basis of normal fitness, you can do some corresponding extreme movements when you exercise to a certain extent. For example, if you can do 20 wide grip pull-ups, you have the premise of being a human flag. If you want to do extreme fitness, you must start with normal fitness.