Lie on your back, facing up. Make fists and place them under your rear end to elevate your hips. Lift your straight legs about 5 inches above the ground. Quickly lift one leg about 6 inches and lower it as you lift the other leg. Quickly alternate this up and down movement as if kicking your legs while swimming. Count four kicks per rep, 1-2-3-1, 1-2-3-2, 1-2-3-3 and so on, doing 10 reps. Keep your neck in line with your spine during the exercise.