In the photo of this article the feet of the participant are held in a fixed position. This is not an ideal form of trunk flexion. There is a better way to do this technique which can change the mechanics and the outcome of the abdominal exercise. At the Legacy Center in Dallas we have taught trunk flexion to 100’s of clients, with and without back pain, including military personnel without issue.
http://www.wsj.com/articles/why-you-can-stop-doing-sit-ups-1450722637