The knee raise is performed by simply hanging from a pull-up bar and slowly raising the knees up towards the chest by contracting the abdominal muscles. Hold your knees at the top of the movement for a count of one, then slowly lower back to the starting position. That’s one repetition.
A more advanced form of this exercise can be done by keeping the legs straight (or slightly bent at the knees), and raising them up until they’re parallel to the floor – again, focus on contracting the abs, holding the contraction at the top of the movement, then slowly returning to the starting position.
The Reverse Crunch
Reverse crunches are most easily done while lying on a weight bench, with your back on the bench and your feet on the floor. You’ll want to hold on to the bench at either the end above your head, or on the sides your hips. Like the knee raise, focus on contracting your abdominals to bring your knees up towards your chest. At the peak of contraction, hold the position for a count of one, then slowly return to the starting position. That’s one reverse crunch. Again, quality over quantity! Take your time, and focus on each repetition in order to get the maximum benefit from this exercise.
To add resistance to the reverse crunch, hold a medicine ball between your feet as you perform the exercise.
Bicycle Crunches
This is a very effective abdominal exercise that's done by lying face up on an exercise mat with your feet in the air and your knees bent at a 90 degree angle. Place your hands behind your head (again, don’t interlace the fingers in order to avoid injury). From this position, contract your abdominals in an attempt to touch your left knee with your right elbow. Return to the starting position, then perform the movement again, this time contracting your abdominals while trying to touch your left elbow to your right knee. Return to the starting position again. This is one repetition.
It is very tempting to try to move quickly during this exercise since the abdominals are subject to constant resistance for the duration of the set. Once again, the more focused your efforts are, the better the results will be. Try not to rush yourself, and perform every part of the exercise slowly and deliberately.