This is a much smaller muscle than the pectoralis major. It's located kind of in your armpit. It lifts your ribs, and works with the scapula.

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The pectoralis minor (TA: musculus pectoralis minor) is a broad, flat muscle which lies underneath (deep to) the large chest muscles (pectoralis major). It runs from the upper front of the shoulder (insertion: medial border upper surface of the coracoid process of the scapula) to make a diagonal downward and inward toward the breastbone (origin: outer surfaces of 3-5 ribs near the costal cartilage.

This muscle helps to bring the shoulder blade downward, forward, and toward the middle of the body (depression and rotation). It stabilizes the shoulder blade when the arms are pushing against a load. To a small extent, the pectoralis minor muscle also assists breathing.

The pectoralis minor muscle acts in concert or in opposition with a number of major muscles of the upper body, most notably the pectoralis major, intercostal, deltoid, upper trapezius, and levator scapula.

The pectoralis minor muscles are attached to the pectoral branch of the thoracoacromial trunk of the medial pectoral nerve (C8, T1). Blood is supplied by the pectoral branch of the thoracoacromial arterial trunk.

Injury and Problems
Strains and sprains of the pectoralis minor are uncommon, but by no means unheard-of. The muscle is seldom injured on its own, but more commonly in conjunction with one of its agonists or antagonists, such as the pectoralis major or deltoid.

A common problem with this muscle is extreme tightness (hypertonicity) often associated with overuse (a lot of swimmers have tight pec minors, to name only one example). Poor posture (as with individuals who slouch over a keyboard all day) or stress may also cause tightness of the pectoralis minor. In addition to being uncomfortable, tight pectoralis minor muscles may sometimes cause neck problems or, in extreme cases contribute to thoracic outlet syndrome (TOS). TOS is a very painful and potentially harmful situation in which the muscles of the upper torso create pressure on the outlet from the thorax into the arm, causing pain, numbness, and weakness in the upper extremities. When the pectoralis minor muscle contributes to this condition, it is termed Pectoralis Minor Syndrome.

Pec Minor Care and Maintenance
The pectoralis minor muscle is easy to stretch. One of the simplest stretches is to stand with your back against a doorjamb, edge of a door, or corner (one that sticks out, for reasons that will be obvious). With arms downward, bend elbows put hands, palm up atop one another, as though you are expecting someone to put some money in your palms. Then rotate the arms (at the shoulder) outward (lateral rotation) as far as they will go. Bring back to centre, reverse (so that the other hand is on top) and repeat. Do this stretch several times, slowly.

Another variation on that same theme requires a massage bolster or other cylindrical pillow thing (a foam roller could conceivably be used here, but they are pretty hard—please use caution when lying your spine on anything like this). Lying with that object along (parallel to) the spine, extend the arms straight out (perpendicular to the body) and bring them backward as far as possible. Hold for at least 20 seconds. Repeat.

An exercise and stretch for the pec minor is accomplished lying on the floor face up. Lift small weights in hands to a point where the arms are straight up above the chest, palms facing one another. Try to keep the arms straight and lower them slowly (the slower the better) backward, attempting to bring them behind the head. This works for both pec minor and pec major, strengthening, and stretching

An excellent massage/assisted stretch has the client lie on his/her back, then the arm is brought up over the head. The therapist/trainer pulls the arm slightly at the wrist or forearm, then completes a long, deep massage stroke starting around the collarbone and travelling slowly up the armpit and triceps. The move may be reversed, stroking down the arm to the pectoral muscles combining with kneading moves. Obviously care must be taken with female clients, as the pec minor lies beneath the breast.

Another caution point is illustrated as follows: recently I was working on a young lady, performing the above-described pec stretch. I worked down the arm to the elbow, then turned around and worked back toward the chest. As my hand got to the underarm area, I held the heel of my palm against the area for a moment, working that muscle with a light circular motion. Something lightly hit my side and I simultaneously heard my client squeak. Looking down, I saw she was waving a hand at me and making a face as though she were being badly tickled. It's really better to ask about these things before working that area under the arm—she could have hit a lot harder, after all.


References:
Special thanks to my colleagues David K. and Sue Z. for their help
Liberto, Thomas, "Hands-on Techniques for Pectoralis Minor" Massage Magazine, July 2008, pp. 62-7.
Taber's Cyclopedic Medical Dictionary, 19th Edition (FA Davis, Philadelphia, 1997).
Gray, Henry, Gray's Anatomy, revised 15th American edition (Bounty Books, New York, 1977).
Kapit, Wynn, and Elson, Lawrence M. "the Anatomy Coloring Book" second ed. (Addison Wesley, New York, 1993).
Sanders, Richard J, "Thoracic Outlet Syndrome", http://www.ecentral.com/members/rsanders/

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