The pectoralis major is a very powerful, thick, fan-shaped muscle, situated at the chest (anterior) of the human body. It makes up the bulk of the chest muscles in the male and lies under the breast in the female.
Underneath the pectoralis major is the pectoralis minor — a thin, triangular muscle — and together they are commonly known as the pectorals or “pecs”.
The pectoralis major moves the shoulder forwards and across your chest. It is best known as the muscle that you develop with the bench press exercise.
It attaches to the humerus bone (upper bone of the arm) and is divided into two parts. The upper part is known as the ‘clavicular head’ and the lower part the ‘sternal head’, based on their origins from the clavicle and sternal bones respectively.
The pectoralis major has a number of actions which are primarily responsible for movement of the shoulder joint:
- Transverse flexion of the humerus, as in throwing a ball side-arm.
- Transverse adduction of the humerus, as when flapping the arms.
- Internal rotation of the humerus, as occurs when arm-wrestling.
- Shoulder adduction, moving the upper arm down to the side toward the body.
- Shoulder extension, moving the upper arm down to the rear.
The clavicular head is close to the deltoid muscle and contributes to flexion, horizontal adduction, and internal rotation of the humerus. When at an angle of approximately 110 degrees, it contributes to abduction of the humerus.
The sternal head is antagonistic to the clavicular head, contributing to downward and forward movement of the arm and internal rotation when accompanied by adduction. The sternal fibres can also contribute to extension, but not beyond the anatomical position.
The Role of the Pecs in the Golf Swing
For details of how the pectoralis major should work in the golf swing, see The Role of the Right Arm in the Golf Downswing.
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