The skeletal muscle make up the bulk of the human body. They are the muscle masses that  attach to bones for the purpose of movement and posture. Muscles vary greatly in size and shape, ranging from the tiny stapedius of the middle ear to the bulky gluteus maximus of the buttock. The typical muscle has a main part (called the body) and two extremities ( called the body), both of which are attached to bone or cartilage. When such an attachment terminates as a strong white cord, we call it a tendon.

By convention, the origin is the extremity attached to the stationary bone (during contraction), and the insertion is  the extremity at the movable end. In not a few instances, however, the origin and insertion are interchangeable, depending on the particular movement. The muscle whose contraction actually produces the desired movement is called  the agonist, whereas the muscle that concomitantly relaxes ( to permit the movement) is referred to as the antagonist. Only rarely is a single muscle responsible fo a given movement, and for the sake of smoothness the agonist is aided by neighboring muscle called synergist. For instance, in extending the leg four large muscles are called into play.

Skeletal mucle are customarily characterized according to their principal action. Flexors decrease the angle of a joint; extensors increase the angle of  joint ; abductors move a part away from a median line; adductors move a part toward a median line; supinators turn the palm upward; pronators tun the palm downwars; levator raise, or lift, a part upward; depressors lower a part; rotators cause a part to pivot on its axis; tensor stiffen, or tense, a part; sphincters reduce the size of an opening; dorsiflexors pull the foot backward; and plantar flexors pull the foot downward.