Are you tired of doing the same exercises for abs? The jackknife crunch may be the answer to your problems. This exercise effectively works the core muscles. It should be noted that whenever the knees move toward the chest (the jackknife motion), the hip flexors are involved. The hip flexor muscles are actually part of the upper leg. They allow you to kick a ball or lift the knee from a standing position (as in running). Thus, if you are trying to isolate the abs, there are better alternatives. Instructions for the jackknife crunch are as follows:

Sit on the edge of an aerobic step. Lean back and place your hands on the step for stability. Start with your legs extended, then bring your knees in toward your chest. As you do so, crunch your abdominal muscles. The knee and crunch motions should take place simultaneously. Be sure to exhale as you contract your abdomen.