Muscle Physiology

Muscle Types
Type I Muscle Fibers: These are also called "slow-twitch" muscles because they have a slow maximal shortening speed, but they are also slow to tire. They use aerobic (oxygen) enery. These muscles are used for low to moderate intensity exercise.
Type IIa Muscle Fibers: These are a subset of "fast-twitch" muscles that are fatigue-resistant. They use both aerobic and anaerobic energy sources. They are used for low-moderate to high intensity exercise.
Type IIb Muscle Fibers: These are another subset of "fast-twitch" muscles that use anaerobic (glycogen) energy. These muscles are used for moderate to high intensity exercise. Type IIb fibers contract faster and develop higher peak tension that the other two types of muscles.

Types of Muscle Contraction
In order to move a joint or to stabilize a joint, the muscle must contract when stimulated by a nerve.
Concentric contraction: Tension is produced causing the muscle to shorten. If the load is heavy, the muscle contracts slower than if the load is light. In other words, a person can lift the weight of a book more quickly than a 50-pound bag of sand. If a muscle shortens but the force remains constant, it is called an isotonic contraction.
Eccentric contraction: Tension is produced during a time when the muscle is lengthening. This can be understood by thinking of setting a 10-pound object onto a table; the muscle produces tension to control the motion of the arm and the object, but the muscle (in this case the biceps muscle) becomes longer. Exercises using eccentric contractions actually are more effective in building muscle strength and mass compared to concentric contractions.
Isometric contraction: Tension is produced in a muscle but the length of the muscle does not change. An example of this is holding a weight at a fixed position, so the muscle does not lengthen or shorten but maintains a steady contraction.

The next section will discuss different methods of conditioning the muscles.