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Serratus Anterior
A couple of inches below our armpits lay a bumpy or "serrated" muscle that
helps keep the shoulder blades (scapulae) stable and flush against the ribs
where it's supposed to be. A possible sign of a weak serratus anterior is a
"winged" shoulder blade (a poking out).
Find an elevated surface such as a bench or a chair. Place hands slightly
more than shoulder width apart.
Keeping body in line (relatively straight from back of the head to the
heels) let your shoulder blades collapse inward and than push them out -
WITHOUT moving your elbows.
Repeat 12-20 times.
http://www.diet-blog.com/archives/2008/02/05/training_your_weak_links_upper_
body.php