There are three types of muscle tissue found in your body: cardiac, smooth, and skeletal. Skeletal Muscle is further divided into 3 types of muscle fibers (Type I, Type IIa, Type IIb) The different types of muscles are determined based on certain functional and structural characteristics.

Cardiac Muscle - is obviously found in the heart. It looks very similar to skeletal muscle as it is made up of individual "cable" looking strands but the big difference is each strand can work individually as well as in a group. Cardiac muscle is mostly an involuntary muscle which means you don't have to think about it to make it work.

Smooth Muscle - is found around internal organs and blood vessels. Smooth muscle tissue is flat and smooth and looks somewhat like a bed sheet. Its main function is to provide support to the internal organs and to help move things along as in the lining of the stomach and intestines. Like cardiac muscles it is largely and involuntary muscle.

Skeletal Muscle - is found all over your body and is the focus of most of your workout routines (Biceps, Quads, Abs, etc.) Skeletal muscles look like steel cables with lots of individual fibers working together to form each muscle group. They are both voluntary and involuntary allowing us to move at will but also helping us to react when something happens to us. Skeletal muscles are responsible for walking, standing, lifting, jumping and pretty much every type of movement you can think of.

Skeletal muscle is further divided into 3 Types Of Muscle Fibers (Type I, Type IIa, Type IIb).

Type I - are often referred to as Slow Twitch Muscles. Type I fibers are used for activities that don't require much strength but do require lots of endurance. Your postural muscles, the muscles that keep your back erect, are a good example of Type I muscle fibers. The leg muscles that you use for walking are another Type I muscle fiber.

Type II - are also called Fast Twitch Muscles. Type II fibers enable you to perform powerful movements that don't require very much endurance. They usually kick in when an activity requires more than 25% of your total strength. They are commonly more abundant in shorter more compact muscles such as Biceps and Triceps. Type II muscle fibers are further divided into Type IIa and IIb.

Type IIa fibers are mostly used for activities that use a lot of strength that lasts for up to a couple of minutes.

Type IIb fibers are used when you need a burst of power but only for a few seconds. These fibers are the largest of all the fibers and should be the focus of your lifting if you want to build large, bulky muscles. (Lifting as much weight as possible for 3-5 reps)

Most skeletal muscles are made up of a combination of all 3 types of muscle fibers. The percentage of each muscle fiber plays a large part in your body structure. People with a large portion of Type I muscle fibers tend to be long and lean and good at endurance activities such as marathon running. People with a bigger portion of type II muscle fibers tend to have short, bulky, powerful muscles and are better at activities like sprinting and weight lifting. In general when you perform an activity the Type I fibers kick in first and then if more strength is needed the Type IIa then the Type IIb fibers will contribute as needed.

There is a debate as to whether muscle can change from one type of muscle fiber to the other but it is certain that with proper training you can get them to behave more like a different type. Example: (If you do nothing but run long distances the Type IIa and IIb will start to behave like a Type I fiber having more endurance than before the training started.)

There is another debate as to whether or not your body can actually make new muscle fibers or if you are born with the same number as you will always have when you die. To me this really doesn't matter the fact is the more you work the different types of muscle fibers the better they will work so get out there and use them.