The seated ball twist is a very good exercise for core development and strength. Sit on the floor with a slight bend in your legs. Hold a therapeutic ball in front of you with your arms fully extended. Lean back slightly, pick your chest up high and pull your naval in. Now turn your shoulders to one side and touch the ball to the floor and then turn to the other side and do the same. A complete revolution counts as one repetition. Be sure you arms stay extended throughout this exercise. Breathing pattern is to exhale as you touch the ball to the floor and inhale as you bring the ball across your body.
For Beginners I recommend you do 2 sets of 20 repetitions twice per week. For those of you who are more advanced, I recommend 3 sets of 30 repetitions three times each week.