The body has approximately a quarter-billion skeletal muscle fibers. These fibers can be categorized in two groups. Type I or slow-twitch fibers and type II fast-twitch fibers.

There are two different types of fast-twitch fibers, IIa and IIx. IIx Fibers are the biggest and strongest but they can only be sustained for a short period of time like a few seconds. IIa fibers can last much longer and last up to 3 minutes for very highly trained athletes.

Now, whatever you are doing to use these fibers doesnt have to be slow for the slow-twitched fibers to be the primary fibers engaged or fast for the fast-twitched fibers, it is simple slow-twitch fibers are used for tasks that are edurance orianted while fast-twitch fibers are used for short bursts of maximum strength.

Skeletal muscles which are the muscles that you show off, have a mix of fast and slow-twitch fibers and this mix varies from person to person. Some people are just born with more slow-twich fibers which genetically suites them for endurance sports. Some marathon runners are said to have about 80 of their fibers slow-twitch. Now if you are born with more fast-twitch muscles you are suited for more explosive and quick sports. Olympic sprinters are said to have 70 percent of their fibers fast-twitch.

Have you ever noticed the different muscular builds in these two different types of runners? The long distance runner is thinner and less strong looking than the sprinter. This is because fast-twitch muscles are bigger and have more potential for growth. The sprinter doesn’t look stronger because he trains harder, he targets a different muscle fiber and this type of fiber is the one that grows the most.

Learning about these different types of fibers really shows how muscle development depends on what fibers you are engaging. If you are doing very long workouts that never target these fast-twitch fibers, then these muscles will never develop. If you target these muscles with workouts that are hard, to work the fibers for maximum strength, and are explosive, you will not only see the growth of these fibers, but will see how these fibers are getting stronger and stronger.