There are 3 types of muscle fibers in the human body, and they are fast, medium, and slow.
Many elite athletes have significantly higher percentages of one type or the other. For example, sprinters might have much more Type IIb fibers while marathoners might have way more Type I Fibers. Most people can actually adjust the proportions between the different muscle fiber types by the type of training that you do!
And that leads me to my fave mnemonic for these great muscle fibers