The Twist: Balancing a ball on your legs forces you to keep your lower body still and your pelvis in a neutral position, maximizing the workload on your abs.
Lie faceup with knees over hips, shins parallel to the ground, and toes pointed. Place ball on your lower legs and extend your arms at your sides, palms on the ground. Raise your head and shoulders oﬀ the ground and lift arms just above hip height beside you. Pulse arms up and down 100 times, inhaling for 5 pumps and exhaling for 5.