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The Pectoralis major is a thick, fan-shaped muscle, situated at the upper front (anterior) of the chest wall. It makes up the bulk of the chest muscles in the male and lies under the breast in the female. Although impressive looking, this muscle is not particularly strong compared to other less noticeable muscles such as those along the shoulder blade. It arises from the anterior surface of the sternal half of the clavicle; from half the breadth of the anterior surface of the sternum, as low down as the attachment of the cartilage of the sixth or seventh rib; from the cartilages of all the true ribs, with the exception, frequently, of the first or seventh, or both, and from the aponeurosis of the Obliquus externus abdominis. From this extensive origin the fibers converge toward their insertion; those arising from the clavicle pass obliquely downward and lateralward, and are usually separated from the rest by a slight interval; those from the lower part of the sternum, and the cartilages of the lower true ribs, run upward and lateralward, while the middle fibers pass horizontally. They all end in a flat tendon, about 5 cm. broad, which is inserted into the crest of the greater tubercle of the humerus.
Laminae This tendon consists of two lamina, placed one in front of the other, and usually blended together below. These deep fibers, and particularly those from the lower costal cartilages, ascend the higher, turning backward successively behind the superficial and upper ones, so that the tendon appears to be twisted. The posterior lamina reaches higher on the humerus than the anterior one, and from it an expansion is given off which covers the intertubercular groove and blends with the capsule of the shoulder-joint. From the deepest fibers of this lamina at its insertion an expansion is given off which lines the intertubercular groove, while from the lower border of the tendon a third expansion passes downward to the fascia of the arm. Variations The more frequent variations are greater or less extent of attachment to the ribs and sternum, varying size of the abdominal part or its absence, greater or less extent of separation of sternocostal and clavicular parts, fusion of clavicular part with deltoid, and decussation in front of the sternum. Deficiency or absence of the sternocostal part is not uncommon. Absence of the clavicular part is less frequent. Rarely, the whole muscle is missing. Having a missing Pectoralis is often a sign of Poland Syndrome. This may accompany an absence of the breast in females. Training The pectoral is often considered one of the most aesthetically-pleasing muscles in the human body, and is a frequent target for bodybuilding. The flat, barbell bench press is the most popular exercise. The pushup is a popular bodyweight exercise targeting the pectoralis major. The muscle is generally worked in compound movements that involve pushing, where the triceps brachii and deltoid muscles are also activated to varying degrees. Exercises that bring the arms together (such as flyes) also work the pectorals, somewhat more selectively. Flat and declining movements generally work the sternal fibers (often called the "lower" or "inner pecs") while inclining movements generally work the clavicular fibers ("upper pecs.") The opposite is true for pushups, where declining (chest below legs) pushups use more scapular fibers; they are also more difficult than flat or incline pushups due to the increased weight on the deltoids and pectorals. Examples of compound exercises used for training the pectoralis major include: Isolation exercises include: | ||||||||
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