Your chest muscle ends in a twisted tendon only five centimetres wide
The pectoralis major fans out across the chest from collarbone, breastbone and ribs, then funnels into a flat tendon about 5 cm wide that twists as it anchors to the upper arm. Nerve recordings suggest the brain can drive at least six separate groups of its fibres independently.
The name comes from the Latin pectus, breast. This thick, triangular muscle forms most of the chest's bulk and lies beneath the breast, with the smaller pectoralis minor underneath it. It starts from the inner half of the collarbone, the front of the breastbone, the cartilages of the true ribs and a sheet of tissue from the abdominal wall, and inserts along a ridge beside the groove at the top of the humerus. Collarbone fibres slope down and out, middle fibres run level, and the lowest ones climb upward, so they all converge on that single tendon.
The tendon has two layers. The lower, deeper fibres slide behind the upper ones and attach higher up the bone, which is what gives the tendon its twisted look. Two nerves supply the muscle: the clavicular head takes signals from the C5 and C6 roots through the lateral pectoral nerve, while the sternal head is fed by C7, C8 and T1 through the medial pectoral nerve.
Its jobs all involve the shoulder. It swings the arm forward, as in an underhand throw or lifting a child; pulls it inward, as in flapping; turns it inward, as in arm-wrestling; and helps hold it against the trunk. The upper and lower portions partly oppose each other, the upper helping raise the arm and the lower drawing it down and forward. Pressing movements across the body activate it most, and heavy loads correlate strongly with how hard it works.
Tears are rare and tend to strike otherwise healthy men, especially powerlifters, usually where muscle meets tendon during a violent lengthening contraction such as a heavy bench press. Women seem less prone, thanks in part to a larger tendon relative to the muscle. Some people are born without it. Poland syndrome typically removes the muscle on one side and can shorten or fuse fingers on the same side. A 1990 report from Seoul described a 22-year-old marine whose missing right pectoral was discovered only during surgery; ordinary life had been fine, but throwing grenades and rope climbing gave him trouble.
Source: Pectoralis major
