The Supine Pull-ups are a great workout for your back, They are many a times being referred to a s the reverse pull up or the reverse push up. They are not that hard to perform unlike the classic pull up. They give great results and can get you body ready to do more pull-ups than before.
This exercise is normally performed on Smith Machine. Instead of positioning the bar on the Smith machine to do shoulder workout, you position it approximately 4 feet from the floor, or as you find it suitable.
Position your body below the bar and grab it a bit wider from shoulder length (just like the classic pull-up or classic bench press workout). Stretch out your legs up front so that your body will form a straight line. Now, from that position move your body toward the bar and keep a straight body line all the way. Return to initial position and repeat the movement.
Remember that you must maintain a straight body line all the way; the abs will assist you in that, so it seems that they are working in the background as well. Great!
To increase the challenge of this exercise, feel free to elevate your legs on a bench or a chair; stability ball is also great. This way the body in initial position will form a parallel lining with the floor and the abs will work full time in preserving that straight line.
When the stability ball is in the game, the abs will work even more to maintain the body balance; to avoid movements from side to side.