Core training isn't doing sit ups, crunches and other spine bending exercises because these movements are done at the expense of your spine. Overtime, as these exercises damage the spine your core strength and stability will reduce. Core training means following two fundamentals;

1. The torso is designed to prevent movement
2. The limbs are designed to produce movement

Why Sit Ups & Crunches are not core training

These two exercises violate the first fundamental because they bend the spine. Side bends, back extensions and twists are the same, they produce movement of the spine rather than prevent it. These exercise slowly diminish the structures of the spine that lead to injury. This doesn't mean your spine cannot or should not bend, twist or extend but under the load of an exercise it increases the risk of injury. 

The 2 fundamentals are based on how your body is built

I have pulled these fundamentals from thin air, if you look to your anatomy it begins to paint a picture of how y...