5. Twisting Sit-ups: Lie on your back with your arms by your sides, knees bent & heels on the floor, hip-width apart. Brace your core & sit up, twisting to the left & reaching your right hand past your left knee. Lower back down to the starting position & repeat, this time twisting to the right. That’s one rep. Do this 30 times. Repeat the entire sequence 3 times.