Adding a Swiss ball to basic crunches already ups the intensity by increasing your range of motion. Toss in this backstroke arm move and you’ll really feel the burn!
To do it: Lie with your hips and lower back in contact with a Swiss ball and your knees bent, feet hip-width apart and flat on the floor. Extend your arms straight out from your shoulders. Raise your head and shoulders and crunch your rib cage toward your pelvis, extending your arms in front of your chest. As you lower out of your crunch, circle your arms back and down, completing your circle by reaching them back up to the ceiling as you crunch up again. That's one rep. Work up to 3 sets of 15 reps.