When done properly, sit-ups are a very good and safe abdominal exercise. The characteristics of a very good ab exercise are that the motion bends the spine in it’s full variety of motion while getting the stomach muscles from full expansion to complete flexion.
Requirements when it comes to sit-up: neck blades touch ground at bottom of rep; started to a full sitting position at the top.
Most of the standard sit-up style uses a comfortable exercise mat. Using the exercise mat underneath the lumbar curve allows a contraction of the abdominal muscles from the full extension when the back is arched or extended 16-31 degrees to the full flexion when the lower back will come from the floor and spine is approximately 31 degrees from the floor.
Furthermore, switching the legs out, because of the feet is bent, places the hip flexors at a disadvantage, and that means you will truly be making use of your abs, and perhaps not your hip flexors to complete the motion. For a further more in depth description of how and why the floor pad works, you have to try it yourself, from a good gym that provides this kind of accessories within your location, or simply just by visiting your local neighborhood gym.