Sit on a stability ball, knees bent, feet on floor. Walk your feet out and lie back; put left hand on ball, right hand behind head. Use glutes to tilt pelvis up. Maintain this position throughout [A].

 

Contract abs (pull navel toward spine) as you roll your torso up, bringing ribs toward hips; then rotate right elbow toward left knee [B]. Keeping hips square and lifted, slowly lower. Do reps on right side, then left. Begin with 8–12 reps per side; gradually build to 16. Targets rectus abdominis and obliques.