Pec minor's insertions onto the ribs make it a secondary respiratory muscle. When the lungs need extra capacity it pulls the ribs up to increase the space in the chest cavity. If you breathe high up in your chest all of the time, rather than allowing your diaphragm to pump the lungs up and down, you'll be overloading pec minor (see the breathing section in the relaxation pages). This area of the chest is also where the Lung meridian hits the surface of the body before travelling down the arm, often indicated in elbow and wrist problems.
The general relaxation exercises and those specific for the shoulder all aim to open out the chest and take the strain off pec minor. If you have a history of lung problems will need to pay particular attention to how your chest moves when you are breathing.
Because most musicians hold their instruments in front of them there will always be a tendency to close off the chest, straining the anterior shoulder muscles. Guitar players particularly tend to bring their right shoulder over the top of the guitar when sitting. The next section on the coracobrachialis and bicep brachii will complete the treatment of the anterior shoulder muscles.