Musculoskeletal System
Pectoralis major: where it lies and what it does
Pectoralis major is a fan-shaped muscle on the front of the chest, behind the breast, that adducts (pulls the arm in) and medially rotates (turns it inwards) the humerus.
Remember
Pectoralis major pulls the humerus in towards the trunk and rotates it inwards; when the arm is fixed, as in climbing, it works in reverse and pulls the trunk up towards the limb.
Pectoralis major is a fan-shaped muscle on the front of the chest. Of the muscles in the pectoral (front of the chest) region, it is the most superficial, lying in front of pectoralis minor (a deeper pectoral muscle) and behind the breast.
It starts along the clavicle, sternum and costal cartilages (the strips of cartilage that join the upper ribs to the sternum) and ends on a groove on the upper humerus.
At the shoulder joint its main action is adduction (pulling the arm in towards the body) together with medial rotation (turning the arm inwards). The upper, clavicular part also helps to flex the shoulder, which means swinging the arm forwards.
The muscle forms the front wall of the axilla (the armpit) and gives rise to the anterior axillary fold, the ridge at the front of the armpit. Thinning of that fold is a recognised sign of pectoralis major injury. Like pectoralis major, latissimus dorsi adducts and medially rotates the arm, but it is a broad muscle of the back, so injury to it would not flatten the front of the upper chest.
Muscles can work in reverse. When the hand grips a fixed hold, as in climbing, the humerus cannot move, so the sternocostal part (the section arising from the sternum and costal cartilages) draws the trunk upwards towards the arm. This is a reversed action.
Memory aid
think of a hug, in which the arm wraps in towards the chest and turns inwards
Picture a hug to recall that this muscle pulls the arm in and turns it inwards.
Key facts
- Position
- superficial muscle of the front of the chest, behind the breast and in front of pectoralis minor.
- Attachments
- starts on the clavicle, sternum and upper costal cartilages (strips of cartilage joining the ribs to the sternum) and ends on a groove on the upper humerus.
- Main actions
- adduction and medial rotation of the humerus.
- Extra action
- the upper, clavicular part also flexes the shoulder, swinging the arm forwards.
- Fixed limb
- with the arm held still, as in climbing, it draws the trunk up towards the limb.
- Armpit
- forms the front wall of the axilla (the armpit) and gives rise to the anterior axillary fold.
Exam traps
- Latissimus dorsi is not the muscle when the front of the upper chest has lost bulk, because it also adducts and medially rotates the arm but lies on the back.
- Pectoralis minor is not the main muscle behind the breast, because it is smaller and lies underneath pectoralis major, which separates it from the breast.
- The front part of the deltoid assists medial rotation, but the deltoid does not adduct the arm, so it does not account for weak adduction.
- Elevation of the scapula is not an action of pectoralis major, because shrugging the shoulder blade upwards is done by the upper part of trapezius and by levator scapulae (a second muscle that elevates the scapula), while pectoralis major draws the scapula forwards and downwards.
- Lateral rotation is not an action of pectoralis major, because it turns the front of the arm outwards, the opposite of medial rotation, and is done by infraspinatus and teres minor, muscles on the back of the scapula.
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