If you're anything like me the specifics are not nearly as important as the big picture, BUT, sometimes you need to know them in order for the big picture to make sense. Believe me the types of muscles and how they work and interact can be a very mind numbing study so I'm going to break down the information as much as possible.
There are 3 types of muscles in your body: cardiac, smooth and skeletal. Cardiac muscles are found in your heart and look similar to skeletal muscles. They work involuntary so you don't have to think about them. Smooth muscles are, as the name implies, smooth and flat. They are generally found around your internal organs and blood vessels and help provide structure and contractions that help the organs function correctly. Like cardiac muscles they work involuntarily. The third type are skeletal muscles. For the rest of this article we will be discussing skeletal muscles.
Skeletal muscles look a lot like a steel cable. Each muscle is made up of individual strands that work together to make up the whole muscle. These muscles work both voluntarily and involuntarily and they are responsible for most all activities including walking, jumping, lifting and standing. They are further divided into 3 types of muscle fibers. (Type I, Type IIa and Type IIb)
Type I muscle fibers or slow twitch muscles are usually longer, leaner muscles that are used whenever an activity requires very little strength but will need a lot of endurance such as walking. The postural muscles that support your spine are another example of slow twitch muscle fibers.
Type II muscle fibers or fast twitch muscles come into play when an activity requires a lot of strength but don't have to continue for very long such as sprinting or weight lifting. Type IIa fibers will perform for about 2 minutes where Type IIb provide small burst of power but only last for a few seconds.
Most skeletal muscles are made up of a combination of all 3 types of muscle fibers and generally when you place stress on the muscle the Type I fibers will engage first followed by type IIa and then IIb as more strength is needed.