When working the abdominal muscles most players do the crunch and/or variations of the crunch which develops mainly the upper portion of the abdominal musculature. To develop the lower abdominals which play a very important role in quick movement on the field or court, you should do the reverse sit-up, sometimes called the reverse crunch.
Keep in mind that the lower abdominals play a critical role in rotating the pelvis backward. This is needed to allow the necessary forward movement of the thigh which is the key to a quick first — and second — step. The lower back muscles play the major role in rotating the pelvis forward to allow for a long push off.
To execute, lie on your back on an exercise mat or carpeted floor. Keep your arms alongside your body with the palms down and raise your thighs with your knees bent until the upper legs are vertical. This is the starting position.
Then inhale slightly more than usual and then hold your breath as you raise your pelvis up and toward your shoulders until your hips are off the floor. Keep your knees bent tightly as you do this so the action is isolated to the lower abdominals.
Push down with your hands to help raise your hips (and legs) and to ensure adequate rotation of the pelvic girdle. In the ending position, your knees should be close to your chest. Keep your head and shoulders as relaxed as possible throughout the upward movement. Exhale as you return to the initial position, stop and then repeat.
In the beginning movement, you may find it helpful to press down with your hands against the floor to assist you to get the pelvic girdle rotated up and over. But you should not do this continuously as it may irritate the shoulders. More importantly, if you want maximum muscle involvement, you must use your lower abdominal muscles to start the pelvic girdle rotation.
Thus, once you can do the exercise easily, place your arms over your head resting on the floor. In this position, you will have to rely solely on pulling with your lower abdominal muscles. For some athletes the exercise then becomes very difficult.
Also, do not return your feet to the floor during execution because doing so creates momentum in your legs which makes it easier for you to rotate the lower pelvis upward. As a result, you will be using the hip flexors in the initial part of the exercise rather than your abdominals.
The reverse sit-up is an excellent stretching and strengthening exercise. When you bring your knees up close to your chest – when your pelvic girdle rotates off the floor – you stretch your lower back muscles. This is not a passive stretch but an active one because your abdominal muscles must contract strongly to get you into this position. Thus, you strengthen your abdominals as you stretch your lower back.
More information on the role and execution of this exercise see Explosive Running, Explosive Basketball Training or Women’s Soccer; Using Science to Improve Speed.