We all know the image of a well-muscled boxer or weightlifter doing the “self-clasping handshake” as a gesture of victory; and sometimes politicians have used this gesture to recognize a spirited response from the crowd.  But this position also presents an opportunity for isometric exercise, in which muscles contract but are prevented from shortening by an opposing force — in this case, the opposite arm.  So a bodybuilder might use the occasion of one triumph, as a chance to work out for the next competition.

The pectoralis major might be considered the “bodybuilder’s muscle” because it is the largest of the chest muscles, enhancing the male physique.  It performs several actions, by pulling (at what is known as the insertion point) on the front side of the humerus (arm bone).  The action shown here is horizontal adduction — bringing raised arms toward the midline.  An alternative example would be a musclebound villain strangling his puny foe.

But despite its caricature status, the pectoralis major is one of few muscles with a special ability:  It can undo its own action!  Since all muscles work by shortening (contracting), this is a rare feature indeed.  The key is that the muscle has two divisions (heads) — the origin (attachment point on the body) of one is above the insertion, at the clavicle (collarbone), the other is below the insertion, at the sternum and ribs.  Depending on which head is used, you can move the arm in opposite directions.

When the clavicular head contracts, the arm is brought forward and upward (flexion).  You can test this by facing a wall, with your arms at your sides.  Push against the wall with one arm, and use the other hand to feel the muscle contract, just below your collarbone.  The front wall of your armpit, formed by your entire pectoralis major, hardens as well.

Now try raising one arm up high and in front of you, as if eagerly answering a question in class.  Face the wall again, and push with your raised arm.  Now, you are attempting to extend the arm, bringing it back down to your side.  Using your other hand again, feel that the lower division (sternal head) is contracting, as is the front of the armpit again.  While the two heads can work together in some actions (“two heads are better than one”), they are antagonists in performing flexion vs. extension.

Repeat these exercises a few thousand times, and maybe you too can look like a cartoon!