The "pressed-heels" sit-up is performed like a typical sit-up, except both feet are flexed so that toes point up and heels are pressed into the ground, says Michele Olson, Ph.D., professor of exercise physiology at Auburn University in Alabama. "It was thought that pressing the heels into the floor would increase the effort of the abdominal muscles while decreasing the load to the lower spine. But actually, EMG (electromyogram) data has shown that pressing the heels into the floor did the opposite." The bent-knee Pilates hundred, on the other hand, effectively works the abs with very low hip flexor activity, Olsen says. "And because only the shoulder blades are lifted off of the floor, there is decreased stress to the spine." To do the bent-knee Pilates hundred (pictured), lie faceup with knees and hips bent 90 degrees. Inhale and extend arms to ceiling, palms facing forward. Exhale and lift head, neck, and shoulders off floor, pressing arms down by hips. Inhale for 5 counts and exhale for 5, pumping arms up and down on each count (while keeping torso still). Do 10 reps total.