Traditional crunches & sit-ups place large amounts of stress on the lower spine and neck. Repeated crunches and sit-ups encourage you to flatten your spine rather than allow for the curves that are naturally there. Core training will support the spine without compromising the natural integrity of its form. Without this support, the strain on your lower back can eventually lead to bulging or herniated discs, compressed vertebrae and nerve damage. Additionally, sit-ups and crunches do not strengthen the deep abdominal muscle, transversus abdominus. Here are a couple of videos featuring ways to focus on core at a variety of skill levels.
Wash Cloth Sliders
Stability Ball Rollouts:
Two Medicine Ball Push-up Planks:
Bosu Ball Abs & Cardio: