The hanging leg raise is one of the best exercises for emphasizing the lower abs because your upper body remains stationary as you lift your lower body using your abs to flex the lower part of your spine. This is a difficult exercise to perform because gravity is maximal when your body is vertical.
If you can’t complete at least six reps of hanging leg or knee raises, start by doing reverse crunches to build up your lower-ab strength. This exercise is the same as the hanging leg raise except your body is horizontal, which lessens the resistance. Once you can do three sets of 20 reverse crunches on the floor, move to a decline ab board and do reverse crunches on a 45-degree angle to further increase your lower-ab strength. When you can do three sets of 15–20 reps, you should be able to complete at least 10 reps of the hanging leg raise.