Want a killer butt and core? Forget separate core workouts. Adding this ingredient to your current moves will amplify results and sculpt your middle in one efficient session.Why the bosu ball? “By adding an unstable surface, a BOSU ball increases activation of the rectus abdominus and facilitates extra activity per exercise than a stable surface would,” says Meaghan.Words/workout: Meaghan TerzisPhotography: James Patrick1. Sit-up3 sets / 12-15 reps1