Pectoralis Major Muscle
The pectoralis major is a thick, fan-shaped muscle, situated at the chest of the human body. It makes up the bulk of the chest muscles in the male and lies under the breast in the female. It is primarily responsible for movement of the shoulder joint. The pectoralis major muscle plays an important role creating actions such as; throwing a ball side-arm, lifting a child, flapping the arms and arm-wrestling. This is a great stretch to do before and after playing baseball. If you’ve done the Strengthener of the Month, you’ll also want to stretch those muscles out.
Retrieved from: http://en.wikipedia.org/wiki/Pec_major#Function
Exercise
Clasp your hands together behind your back with your thumbs down
Extend your arms behind you and slowly and gently pull your arms upward
Retrieved from: http://www.wikihow.com/Perform-Shoulder-Stretches