Chest

The pectoralis major is a fan-shaped muscle that has 2 heads.

The upper clavicular head arises from the Collarbone and the lower sternal head arises from the breastbone.

The 2 heads pass outward across the chest wall and merge into a single tendon that attaches to the humorous bone in the Upper Arm.

As the Muscle Inserts, the Tendon twists so that the Upper Head attaches beneath the Lower Head.

When the pectoralis muscle contracts, movement takes place at the Shoulder Joint.

The Pectoralis Major adducts, flexes, and internally rotates the arm, thus moving the arm forward and across the chest during movements such as Push-Ups.

Even though the muscle has only two anatomic divisions, functionally it may be considered as having 3 sections Upper, Middle, and Lower depending on the angle through which the arm is moved.

As the position of the shoulder joint changes, certain Fibers of the chest muscle have a better mechanical advantage for creating Motion. Other Fibers of the Chest Muscle are still active but are not able to contract as much because of the shoulder position.

The sidewall of the chest is formed by the Serratus Anterior. This muscle arises from behind the Scapula, and it passes forward around the chest wall to attach to the Upper 8  Ribs. The serrated edge of this muscle emerges from beneath the outer margin of the Pectoralis Muscle.

The Serratus Anterior pulls the scapula forward, stabilizing it against the rib cage. The serratus anterior is active during most chest exercises and works especially hard during the Lockout Phase of a push-up or bench press.

The Pectoralis Minor muscle lies deep beneath the Pectoralis Major. It has only a minor function and does not contribute to the size of the chest.