It is my opinion, with a little education behind it, that crunches, sit ups, V-ups, the trunk rotation machine in the gym, etc. are doing more harm than good for 99% our readers! Really! Why? I'm glad you asked!
First, these exercises (flexing the core with the resistance of gravity or added weight) are risky for your low back, especially the discs of your lumbar spine. In labs and in clinical practice, scientists and physicians study what causes spinal disc bulges and herniations (at least we do at Cumberland Chiropractic and Sports Medicine, right here in Lebanon, TN). The results are conclusive. Repeated forward bending and twisting causes maximal pressure on the nucleus (the jelly at the center of each disc) and stress on the annular fibers (the "tupperware container" of that jelly). Wanna do the damage faster? Add weight on your shoulders or heavily contract your abs while doing so. Therefore, we believe that the risk of performing these exercises outweighs the benefit.
Core stability is a word that is so popular yet misunderstood, that we should touch on some myths and facts about it. Our “core” is the center of our body; everything from ribs to hips. It houses the lumbar spine and is surrounded by vertical muscles in the front and back, hoop muscles around the sides and a lid and floor formed by the diaphragm and pelvis. “Stability” is the ability to resist movement and to support when demand is applied. A bridge is STABLE when it supports cars from the top and resists wind from the sides.