This variation of the abdominal crunch targets not only the rectus abdominis (the "six pack"), but also the obliques; the muscles that reside on either side of the rectus abdominis. Virtually all ab exercises are executed via a "crunching" motion, whereby the upper torso rolls forward to flex the abdomen. However, this particular exercise is different in that a twisting motion is added to the movement. As you reach with the ball, the torso rotates and pivots which naturally engages the oblique muscles to a greater degree than the traditional ab crunch.

The following are instructions for how to perform the ab crunch with unilateral reach (e.g. one side of the body):

Lie down on your back (supine) with your knees bent. Place feet flat on the floor. Grasp the ball with both hands. Once you are in position, crunch your abdomen by rolling your upper torso forward. At the same time, reach with the ball with arms fully extended toward the side of the RIGHT knee. Exhale as you reach with the ball. Return to the starting position

Repeat the reaching motion for several repetitions on the RIGHT side of the body. Once all repetitions are completed, perform the same movement on the LEFT side of the body.

In order to really get the most out of this exercise, try to hold the ball near the knee for a second or two, then return to the starting position. This exercise is not recommended for anyone with back pathologies.