Medicine-Ball Reverse Crunch
Lie on the floor with your hands at your sides and angled out about 45 degrees from your body for balance. Elevate your legs off the floor, bend them to 90 degrees and place a medicine ball between your knees, squeezing your thighs together to hold it in place. Flex your abs to bring your knees toward your chest, allowing your glutes and lower spine to curl off the floor. Squeeze for a one-count in the fully contracted position, then lower your glutes back to the floor. Repeat for reps.