When any of my clients ask me to design them an abdominal program, I make sure to make it is the most comprehensive program it possibly could be. To insure this, the program must work all the different directions the muscle fibers run and use all the abdominal muscle actions.
The four parts of a complete ab workout are as follows:
Ribs to Hips
The most commonly performed movement, puts emphasis on the upper part of the rectusabdominis
Diagonal movements, rotations, and twists
These exercises put more emphasis on the obliques, the muscles that run on the sides of the front of your torso.
Hips to Ribs Lifting the hips towards the ribs with a bent knee or straight leg will put extra stress on the lower part of the rectusabdominis
Isometric ab exercises like the forearm plank (above), work on core stability and activation of the transverse abdominis