Rock hard abs don't come from sit-ups alone. Incorporate more intense exercises, such as the hanging leg raise to challenge your entire core.
The hanging leg raise works the entire midsection, with particular emphasis on your hip flexors and the lower portion of your abdominals.
How To: Hanging Leg Raise
To perform the hanging leg raise, hold onto a fixed pull-up bar with your arms and legs extended. With your legs together, contract your abdominal muscles and lift your knees as high as possible by rolling up your spine and bringing your pubis toward your sternum. Slowly lower your legs to return to the starting position. Hold a dumbbell between your feet to increase the difficulty of the exercise.
Core Muscles Worked
The hanging leg raise requires flexion and extension of your torso. This activates the fronts of your hips, but also the six-pack muscles of your middle.
The muscles of your forearms, shoulders and back are used to grasp the bar, support your weight and stabilize your back and shoulder joints when you perform hanging leg raises.