This exercise is great for abdominal muscles, hips and upper body. To perform it you will only need an exercise ball.
Put knees on an exercise ball and hands on the floor so your body forms a straight line. Arms are perpendicular to the floor and are carrying the weight.
Bring the ball in using abdominals and hip flexors. Return to basic position. If you want to make this exercise even more difficult put your feet on the ball.