The dominant muscle in the upper chest is the pectoralis major. This large fan-shaped muscle stretches from the armpit up to the collarbone and down across the lower chest region on both sides of the chest. The two sides connect at the sternum, or breastbone.
The pectoralis major moves each shoulder joint in four distinct directions, and also keeps the arms attached to the body. Injuries to this muscle are rare, but symptoms include pain in the chest, bruising, and decreased strength of the muscle.
The pectoralis minor is located underneath the pectoralis major. This thin triangle-shaped muscle runs up and down along the upper ribs.
The major muscles in the upper torso include:
- Trapezius: This muscle extends across the neck, shoulder, and back. It allows for movement of the shoulders and shoulder blades.
- Rhomboid major: Attached to the shoulder blade, this muscle is one of many that aids shoulder movement.
- Infraspinatus: This rotator cuff muscle helps raise and lower the arm.
- Teres major: This muscle helps rotate the upper arm.
- Serratus anterior: Located in the rib cage, this muscle keeps the shoulder blade against the chest wall and helps rotate the shoulder blade upward.
- Deltoid: This muscle gives the shoulder its rounded shape as well as raises and rotates the arm.
- Latissimus dorsi: This flat rectangular muscle of the back helps the arms rotate as well as move away from and toward the body.
Men typically have more developed muscle than women. This occurs because of the hormone testosterone, which the body begins to produce at higher levels during puberty. Besides aiding in muscle development, testosterone helps fuel the reproductive system, increases bone mass, and helps grow body hair.