The pectorals (pecs) are the chest muscles. In shape they are roughly large squares with rounded corners. They cover the upper part of the rib box. A large tendon attaches the pectoral muscle on each side to the humerus, (upper-arm bone). A pectoral contraction pulls via that tendon on the upper arm bone and draws it forward.
In order to demonstrate pectoral function to yourself, raise your arms out to the sides and stiffen them. Now without dropping them, move your arms to the rear as far as possible. This is the starting position of the pectoral contractile function. Then slowly contract your pecs to pull your arms directly forward and toward each other, actually contracting so completely that your arms cross each other in front of your chest I have just demonstrated the full range of motion your arms can achieve as a result of pectoral contraction. (http://www.psychwatch.com/sport_psychology.htm )
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