To help draw your abdominal muscles back together, you want to have a tight wrap around your midsection, starting the wrap around your hip bones and extending up to just under your ribs - this will help approximate the two sides of your rectus abdominus muscle as you work your strengthening. (hab-it.com)
Your transversus abdominus muscle as well, is rendered ineffective because it lacks a solid anchor to pull against, having lost the lordotic curve that perfectly interlocks each vertebrae of your lumbar spine. (hab-it.com)
Crunches mainly target the superficial muscles and don't hit the deeper transverse abdominus muscle, the «girdle» muscle that holds everything in. (cathe.com)