The core muscles are called that because the are responsible for all motions through out our thoracic cavity:
- bending forward (flexion), controlled mostly by our abs
- bending backward (extension) controlled by the lower back
- pivoting (spinal rotation) controlled by lower back, abs and obliques
- bending to the side (lateral bend) controlled by the obliques
When any of these muscle groups are trained more than another it causes muscular imbalances.
For instance, too many crunches and too little lower back work can cause the torso to tilt forward. This is obviously not a prime body position for a healthy back — and terrible from a sports standpoint. These imbalances can lead to serious injury.
When you train arms, you work biceps and triceps. When you train on core day, remember all core muscle groups.
Your body functions as a unit. Truly, it must be trained as a unit.