There are over 600 muscles in the human body.  They overlap each other at many points throughout the body.  They are made of filaments that can stretch and contract, returning to their original shape.  Muscles just below the skin are called superficial, and beneath these are the deep muscles.  There are 3 types of muscle tissue in the body.  They are categorized as smooth, cardiac, and skeletal.

SMOOTH muscle, also known as non-striated, is found in the internal organs such as the intestines, the stomach, and in the walls of the blood vessels.  They are involuntary muscles – muscles that are not under conscious control.  They provide essential power for the functioning of the respiratory, digestive, and cardiovascular systems.  These muscles are composed of short spindle-shaped fibers packed together in layers.

CARDIAC muscle forms the main portion of the heart wall.  It is composed of short, interconnecting fibers.

SKELETAL muscle is the largest group of muscles in the body. They are under conscious control.  This muscle group usually attaches on one end of a bone and stretches across a joint to attach to another bone.  As the muscle contracts, it moves one bone while the other bone remains stable.  The skeletal muscles make up nearly half the total weight of the human body.  They provide the forces that enable the body to make precise movements, lift objects, speak, and to maintain proper posture.  They work in groups as they respond to nerve impulses.  It is composed of bundles of elongated, striated fibers.  The fibers are held together by fibrous connective tissue.  Numerous capillaries penetrate this tissue to keep the muscles supplied with the large quantity of oxygen and glucose required to facilitate muscle contractions.

The muscles of the neck and the large triangular muscles of the upper back stabilize the head and shoulders and permit a range of very complex movement patterns. They provide strength and support. The most powerful muscles in the body are those that run along the spine. They provide the strength for lifting and pushing, and they maintain posture.

Muscles work in harmony with each other and with the bones. When a muscle contracts to produce movement, it is called the agonist. The opposite action that relaxes the muscle is called the antagonist. For example: to raise the upper arm, the anterior and posterior parts of the deltoid muscle balance each other while the fibers in the middle of the muscle contract to pull the arm upwards. The teres major muscle, which connects the humerus to the lower part of the scapula, relaxes to allow the movement. Occasionally, stabilizing muscles also play a role in this coordinated muscle action.