The pectoralis major constitutes the bulk of the chest muscles, lying underneath the breast. It is thick, fan-shaped, and originates from multiple places along the medial and superior chest: anterior sternum, clavicular head, sternal end of rib 6, superior six costal cartilages, and from the aponeurosis of the external oblique.
The pectoralis minor
The pectoralis minor is much thinner and triangular in shape and resides below the major. It originates from the margins of the third to fifth ribs adjacent to the costochondral junction.
How do tight pectorals occur? People often complain of tight pectoral muscles if they do a large amount of upper limb exercises in the gym. These include chest and bench presses which are not counterbalanced with other exercises. This is known as a muscle imbalance injury.