This is another exercise that really burns the lower abs, Babidiye says. [See: 7 Best Total Body Fat Burning Exercises] You can extend the legs fully or use a Pilates machine, she adds. Placement is crucial while performing this exercise, if you want to avoid injuries. “Activate the muscle from the bottom up.” It’s important to not swing back and forth. Stabilize your spine as you pull your knees towards the chest. Press your back against the pad and use your abs to raise your legs and knees.