To make leg lifts more challenging, you can try V-ups, which add an upper-body challenge to the leg raise movement. As you lift the legs, you also perform a situp, extending your arms up straight toward your feet. If you have access to a workout bar, you can try hanging leg raises, as well. You can also add ankle weights, to test your core strength even more. For a different challenge, you could place a block or a mini exercise ball between your thighs or ankles—this will help target and engage your thigh muscles, too, particularly your abductors.