Lie on the floor facing down and hold the medicine ball with your feet. Make a curl a simple curl with the knees and then return the legs in to initial position.
You can make a throwing variation of the exercise with a partner who stands in above your waist. So when you make the curl, throw the ball from your feet, your partner catches it and then places it back for the lowering curl movement. Make 2 sets of 8 repetitions.
Sit on the floor with upper body bend backwards and hands on the ground for support. Bend the knees slightly and hold the medicine ball with them. Now, with no upper body movement (arms are helping the position), lift the legs and draw them towards your chest. Return to initial position and make another repetition. Make 1 set of 12 repetitions.