We can look at muscle fibers as being on a spectrum with slow twitch on one side and fast twitch on the other side.
Type 1, which are slow twitch, uses oxygen to create energy. The upside is they can work for a longer period of time before fatiguing. The downside is they contract the slowest. When a muscle contracts, these are the first fibers to be recruited. Then, as intensity increases, the fast twitch fibers are then recruited as well.
Type 2A, which are fast twitch, uses both stored energy and new energy created with oxygen just like type 1. The difference is, since they use stored energy, they can contract faster than type 1. They can work for a short period of time before fatiguing.
Type 2X, which are the fastest muscle fibers in humans, uses stored energy only. This allows it to contract the fastest, but it also means it fatigues the fastest. The thing is these fibers are rarely present in the average human. In a study with sprinters they found only 1 out of 1.000 muscle fibers they examined were pure type 2X.