Hey all posted this to help someone understand and thought it could help a lot if it were its own thread... Hope you all enjoy.

Your body has approximately a 1/4 billion skeletal muscle fibers, all of which can be categorized as one of two main types. Type 1 fibers, also called slow-twitch fibers, are used for endurance activities, or tasks that don't require maximum strength. Type 2, or fast-twitch fibers, come into play when a task utilizes more than 25% of your maximum strength.

There are two subcategories of fast-twitch fibers, IIa and IIx. IIx fibers are the biggest and strongest, but they're incapable of sustaining effort for more than a few seconds. IIa fibers not only are used for strength and power activities but also keep longer, for up to three minutes. (That being in the most highly trained athletes.)

It's easy to remember the types if you see them as part of a continuum. Type 1 are the smallest and most endurance-oriented, IIa are bigger and have limited endurance, and IIx are the biggest, with almost no endurance beyond what it takes to perform a single maximum effort. There are other subtypes of fibers, but its simplest to focus on the big Three.

Your muscles operate on two basic physiological principles, The all or nothing principle and the size principle. The all or nothing principle states that a muscle fiber either gets into the action or it doesn't. If it does it performs at an all out effort. The size principle states that in any task, the first muscle fibers pulled into action are the smallest ones. Since the smallest fibers on your body are most likely your slow-twitch fibers, they go in first. When your body realizes that the effort needed exceeds about 25% of your total strength, it activates your IIa fibers. And when it sees that the effort requires more than about 40% of your strength, it calls up the IIx fibers.
When you were a baby your body already had all the muscle fibers you're ever going to posses. Those fibers became bigger as you grow and they can become even bigger still of you give them sufficient exercise. Your percentages of fast-twitch and slow-twitch are predetermined. A fast-twitch fiber can't turn into a slow-twitch fiber and a slow-twitch fiber can't become a fast-twitch fiber. Because you're born with all the fibers you're ever going to have, you're also born with a predisposition toward either endurance- type activities or strength and power sports, depending on which type of fiber is predominant