SYNERGISM: The Critical Element
For a given series of exercises, there is always one specific sequence that affords maximum benefit to all the muscles involved.
When performed in that sequence, each exercise becomes more powerful than when performed in other sequences or alone. This is synergism: combining elements to create a whole greater than the sum of its parts.
The main factor determining ideal exercise sequence is the principle of Interdependency of Muscle Groups
Often two or more muscle groups cooperate in an exercise. When this happens, it's possible for a tired muscle to limit the effort aimed at a fresh one. Properly taking into account their interdependence helps prevent this.
In the case of the abdominals, it works this way:
Imagine the stomach muscles divided into upper and lower abs. This isn't an anatomical distinction, but for the sake of discussion a line is often drawn between the top two and bottom two abdominal lumps.
Fig.4 The division between upper and lower abs
The upper abs can be further divided into center and outer sections.
From now on, the term upper abs will refer to the center section; the outer sections we'll call by their anatomical name: the external obliques.
First consider just the upper (center) abs and the lower abs. They are interdependent in the following way:
When you do a lower ab exercise, you use both LOWER and UPPER abs.
When you do an upper ab exercise, you use almost exclusively UPPER abs.
Notice the upper abs play a role in working both areas. If you train them first, their fatigue will limit your lower ab work, preventing the lower abs from getting a good workout. The solution is to train the lower abs first, then finish the upper abs with exercises that concentrate on them.
There's a side benefit to proper sequencing:
Since the uppers become partially fatigued from the lower ab work, they don't have to be pushed very hard to get a good workout.
We can apply the same logic to oblique work. The obliques twist the torso-with support from the upper abs. Therefore oblique (twisting) exercises should precede upper ab (straight forward) exercises, so that upper ab fatigue doesn't become a limiting factor in training the obliques.