This knee tuck, also known as the jackknife, is an effective abs exercise that works your arms and legs at the same time.
To do it: Assume a pushup position with your arms straight. Rest your shins on a Swiss ball. Your body should form a straight line from your head to your ankles. Keeping your shoulders over your hands, brace your core and roll the ball toward your chest by pulling it forward with your feet. Return to the starting position by lowering your hips and extend your legs back out. That's one rep. Work up to 3 sets of 15 reps.