The term "pectoralis muscle" is used to describe any muscle attaching the front chest wall to the shoulder and primarily refers to the pectoralis major and pectoralis minor. The pectoralis major muscle is large, attached at the clavicle, sternum and upper ribs and ending in a tendon that inserts into the bicipital groove of the humerus, the upper arm bone. This pectoralis muscle moves the shoulder joint, enabling the arms to move forward, and assists in breathing deeply. The pectoralis minor muscle is much smaller and beneath the pectoralis major, attaches to the third, fourth and fifth ribs and ends in a tendon that inserts into the coracoid process of the shoulder blade. This smaller pectoralis muscle moves the shoulder forward and downward, allowing a downward pushing motion.

The pectoralis major muscles are known as the “pecs,” the large chest muscles that weightlifters try to develop. The pecs are more important cosmetically to men as a sign of upper body strength. In females, the pecs are much less noticeable and mostly underneath the breast.

Ruptures of the pectoralis major, while rare, can occur in weightlifting activities. Surgical repair is recommended, with nonsurgical treatment recommended only for older patients or for tears of the muscle's belly. The pectoralis minor, when tight, can impinge on the brachial nerve, causing numbness and tingling in the hand.

The pectoralis muscles control posture, which affects many other health issues, including respiration and circulation. These muscles can become tight when one is leaning over a computer or desk, because the muscles are in a constant state of flexion. When tight, these muscle pull the shoulders into a hunched position, with the head protruding forward. The pectoralis major muscle also pulls the shoulders forward into a hunched position as a result of excessive workouts that don’t also work the corresponding back and neck muscles.

Incline and decline bench presses, push-ups, flat dumbbell flyes and pullovers are all exercises to strengthen the pectoralis muscles. These muscles flex, laterally rotate and help to adduct — or move toward the middle of the body — the upper arms. Most trainers recommend working the corresponding back muscles, the lattisimus dorsi, as hard as the pectoralis muscles in order to balance their effect and to avoid injuries.

Breast implants for cosmetic or reconstructive purposes can be placed over or beneath the pectoralis major. There are doctors who prefer one way over the other. The most recommendations seem to prefer placing implants below the muscle, but there might be some loss of strength in a pectoralis muscle that must be split to allow the implant.