Get Fit With a Tennis Ball
Flatten your belly
Lie on your back with the ball in your left hand. Lift legs and torso so your body forms a "V," and begin to scissor legs up and down, alternating sides. As your right leg lifts, pass the ball underneath it to your right hand; repeat as left leg lifts, passing ball back to your left hand.
That’s one rep; do 3 sets of 10–15 reps four times a week to see a difference in two weeks. Just don’t forget to keep your core muscles contracted.