Firstly you need to understand that muscle is a tissue composed of fibers capable of contracting to effect bodily movement. Having strong, functioning tissue is vital to our health.
The cells in a muscle have two major duties: movement and heat production.
There are three types of muscles: skeletal, smooth, and cardiac (heart). Skeletal is what most people think of as muscle, the type that can be contracted to move the various parts of the body. Skeletal, which are responsible for posture and movement, are attached to bones and arranged in opposing groups around joints.
Skeletal tissues are controlled by the brain and are called voluntary muscles because they operate with a person's awareness. The size and strength of skeletal muscles are maintained or increased by regular exercise.
Smooth tissue contracts without conscious control and is found in the walls of the internal organs, such as the stomach, intestine, bladder, and blood vessels. Cardiac tissue is the specialized striated tissue of the heart.
Heat production, to maintain body temperature, is an important by-product of this system.