This exercise will mobilize your hips and strengthen your core. You can also do this exercise on the floor without a ball.

Sit on an exercise ball and roll down until your shoulders are on the ball and knees are bent.
Extend your hips upwards by pushing with your heels trying to make your thighs and torso parallel to the ground.
Lower your butt to the floor, then push hips your hips up again.
Return to the start position and alternate pushing your hips up and down.
Do 8 to 10 repetitions.