Medicine Ball Sit Ups.php
A medicine ball sit up is like a regular sit up, with a medicine ball. Surprise!
All kidding aside though, that is what it is. Grab a medicine ball (you decide the weight) and start rocking back and forth. On your way down, extend your arms above your head, almost(!) touching the floor with the ball. On your way up, extend your arms like you are about to pass the ball to a partner.
Be sure not to overextend (so don't lock your elbows). And on your way up, don't lean too far forward (this will disengage your abs, opening yourself up to back injuries).
Other ab exercices
Flutter Kicks
Hanging Leg Raises
Leg Raises
Russian Twists
Scissor Kicks
Sit Ups