Sternocostal HeadSternocostal Head of Pectoralis Major

chestPars sternocostalis musculi pectoralis majoris

The sternocostal head is the largest part of the pectoralis major, running from the breastbone and ribs across to the arm. It is what a flat bench press trains, and it is most of what you see when you look at a chest.

Sternocostal Head highlighted on an anatomical figure, from the front.
Where it sits, from the front.

This is the bulk of the muscle — the wide sheet running from the length of the breastbone and the cartilage of the upper six ribs, horizontally out to the upper arm. It is bigger than the other two parts of the pectoralis major put together, and when people talk about chest size they are almost entirely talking about this.

Because its fibres run more or less straight across the body, it is at its best when the arm travels horizontally: out to the side and back in toward the midline at roughly chest height. That is a flat bench press, a flat dumbbell flye, and a cable crossover at chest level. It also brings the arm back down from in front of you, which is the same fibres working in the other direction.

The three parts of the pectoralis major share one tendon, and the fibres cross over on the way to it — the sternocostal fibres end up attaching higher on the arm bone than the clavicular fibres that started higher on the body. The twist is a real anatomical feature rather than a diagram artefact, and it is part of why the muscle can pull the arm in usefully from so many different starting positions.

Training notes

This head takes care of itself in almost any program that includes pressing, which makes it the wrong thing to add volume to when the chest is not responding. If flat bench has been the main movement for months, the sternocostal fibres are not the gap; the upper and lower fibres are.

What it does benefit from is range. The sternocostal fibres are longest when the arms are wide and back, and shortest when the hands are together in front of the chest. A dumbbell press or a flye lets the arms travel further apart at the bottom and further together at the top than a barbell allows, and training both ends of that is worth more than adding load in the middle of it.

Grip width on a bench press changes how much of the work belongs here. A wider grip shortens the distance the bar travels and puts the arms into more horizontal adduction, which suits these fibres; a very close grip turns the movement into a triceps exercise the chest happens to assist.

Anatomy

Origin
  • Anterior surface of the sternum
  • Costal cartilages of the first to sixth ribs
Insertion
  • Crest of the greater tubercle of the humerus, deep to the clavicular fibres and attaching above them
Actions
  • Horizontally adducts the arm across the front of the chest
  • Adducts the arm toward the midline of the body
  • Internally rotates the humerus
  • Extends the arm back down from a flexed position
Innervation
Lateral and medial pectoral nerves (C5–T1)

Exercises

Antagonists

Synergists