Getting better at what you are doing!

People say practice makes perfect.  Well, that may sounds true only if the practice itself is near perfect.  Bodies tend to adapt to the exercise conditions they are given.  Run slowly, and you improve at running slowly.  Specificity of training means that improvements in training are specific to what your are doing and includes things like the speed of movement, the load lifted, the intensity of the exercise, the angle of position of the limbs, the environment ..... etc you get the idea.   Improvement in training is very difficult to please.

Energy systems are simply a combination of chemical reactions.  hey start with a molecule and use enzymes to help the reactions take place.  The more enzymes you have, the faster you make ATP, and the way to get more enzymes is to train in ways that use particular enzymes.