In my experience most people like to think that crunches are the only way to get the “firm” look in their abs for that six pack…however, I have found that isometric exercises such as the plank have helped not only improve the look of the rectus abdominus but the function by working the core as a whole. It’s all about risk versus reward in this situation. I would rather have my clients goal be more about the core with mountain climbers, planks, russian twists, etc. then crunches. I use them sparingly as there are many more exercises that can get the same benefit with less risk.