The U.S. Army has started phasing out sit-ups for some soldiers, and the Navy and Marine Corps are considering new fitness requirements. Personal trainers and military advisers now believe that sit-ups are dangerous, because they put too much pressure on the spine. They say men and women should replace them with the plank pose, which is when you stay in the upward part of a push-up.
From our very good friends over at blog.bonati. com