Muscles involved: External and internal rotators, triceps
You can perform this exercise either standing, or seated on a back-less bench. Raising one arm over and behind the head while the other goes behind the back from below, try to grab the fingers of both hands behind your upper back.
With this exercise, the mobility of the shoulder joint is easily determined. Advanced stretchers will have no difficulty joining their hands, and some may even reach the forearms. Novices should use some help in order to improve. A training partner can be very handy, standing behind the person doing the stretching and pushing gently upon the elbows in an effort to bring the hands closer together. With each repetition, try changing the position of the arms in order to balance the stress upon the structures being stretched.