Crunches are an isolated movement, but your abdomen doesn't work in isolation. It works in organic movements. When you hit a baseball your abdomen works with your arms, legs and the rest of you to hit the ball. Your abdomen's main job is to transfer force and stabilize the spine. Crunches mainly work the muscles that make up your hip flexors. Your body automatically uses the strongest muscle in any given movement. During crunches, the muscles of your hip flexors are the strongest. One layer of your abdomen is minutely worked, if at all, during crunches. This is your rectus abdominus. Your rectus abdominus isn't useful to develop abdominal strength, let alone aesthetically pleasing abs.
Function
There are four layers of the abdomen. The outermost layer is the rectus abdominus. The next layer is the external obliques. Your external obliques are located on your sides. Under your external obliques are your internal obliques, also located on your sides. Either working alone or with one another, these muscles create spinal movements--spine flexion, lateral bends and twisting. The innermost layer of the abdomen is the transverses abdominis. Its job is to rotate force, resist force and stabilize the spine.
Considerations
Crunches, sit-ups and most abdominal machines are bodybuilding type exercises. There are many myths and misconceptions surrounding them. These exercises weren't chosen for the exercise science behind them. Sits-ups work your hip flexors; your abdomen is hardly used. "Ironman" magazine says, "any movement in which you bring your knees toward your chest is hip flexion. This includes leg raises, hanging leg raises, kneeups and Roman-chair situps (reverse leg raises)."
Spinal Stability Identification
Make stabilization exercises part of your abdomen work out routine. Your abdomen needs to be able to stabilize itself so it does not compensate. This can lead to injuries and muscle imbalances. The abdomen supports and stabilizes posture. This happens when you're still or static and when you move. Plank, side plank and bridge are examples of static exercises that work the abdomen. Squats and dead lifts are exercises that will work your abdomen when you move.
Force Resistance Identification
Along with spinal stability, the abdomen's job is to resist and slow down force. If you do a forward jump and land with an excessive forward lean, this is most likely the result of a weak abdomen. When you jump, force is coming from the ground. An excessive forward lean happens because your weak abdomen couldn't hold your spine straight against the force. Examples of exercises to work force resistance are broad jumps and overhead medicine ball throws.
Force Rotation Identification
Part of the abdomen's job is to redirect force usually coming from the ground. You do this while playing basketball, when you catch a pass from a teammate, turn, and throw it to another teammate. An example exercise to work this function of your abdomen is woodchops.
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