What Is a Pec Muscle?

A pec muscle is one of a group of muscles that forms your chest, which is active during shoulder-girdle movements. Activities that flex your shoulders and spread your shoulder blades apart, including bench pressing, activate most of your pec muscles. The pec muscles are common indicators of muscular development, particularly in men. The ideal circumference of a man's body around the pecs is 1.48 times the circumference around his pelvis, according to a formula developed by bodybuilder Steve Reeves.

Sternal

The sternal head of the pectoralis major is the most prominent pec muscle. The sternal pectoralis major is a collection of muscle fibers that extend from points on the breastbone between five of your upper ribs to the front of the humerus, which is your long upper-arm bone. Activities that move your upper arms toward and across your chest when your elbows point out to the sides, such as chest flies, activate the sternal head. Moving your upper arm down and back, such as in pullover exercises, activates the sternal pecs, particularly when you shoulders are internally rotated.

Clavicular

The clavicular head of the pectoralis major extends from your collarbone to the front of your humerus bone. Movements that involve lifting your upper arms forward, such as barbell front raises, activate the clavicular pecs. The clavicular head of the pectoralis major also moves your upper arms out to the sides of your body during activities such as dumbbell lateral raise exercises. According to an article published in "The Sport Journal" by Miguel Jagessar and Michael Gray at The University of Trinidad and Tobago, close-grip incline barbell bench press exercises maximally activate the clavicular pecs.

Minor

The pectoralis minor extends from the outer surface of your third, fourth and fifth ribs to the carticoid process, which is a small hook-like portion of the top edge of your shoulder blades. The minor pecs are particularly active during protraction and inferior rotation of the shoulder blades. Protraction movements, including pushup and bench-press exercises, spread the shoulder blades away from your spine. Inferior rotation movements, including pullups and chinups, rotate your shoulder blades downward.

Subclavius

The subclavius pec muscle is a small, long, triangular muscle that extends from your first rib and the cartilage around the first rib to the middle of your collarbones. The subclavius is particularly active during activities that elevate your first rib and draw the collarbones down, such as lifting heavy objects. Activities that roll your shoulders forward, such as working with your arms in front of you at the computer or while driving, also activate the subclavius. The subclavius pec muscle also protects nerve bundle and circulatory bundles that go to your arms during collarbone fracture injuries, which are the most common long-bone fractures in humans.