Hands behind head causes pulling with arms instead of focus on abs:
Many ways to do sit ups - quite popular is the US Marine Corps style:
Please see my comments below Ma Ann and I truly hope they help clarify why the attempts were done the way they were, how they are most popular today and will draw people to your site.
Hi MaAnn: If you consider what most people actually do to keep their abs in shape, their target for completion of situps is what they can do in 30 to 60 seconds and the US Marine Physical Fitness Test Rules help to get them focused on a common format. That is why I set this as a goal and I do believe it will draw more viewers to your site. I have certainly informed everyone I know on faceBook and linkedIn of this and send them to your site:
The starting position is lying on your back with your knees bent and feet flat on the floor. The arms are folded across and the chest, and must maintain no gap between the forearms and the chest at all times. A second person is permitted to hold the lower legs or ankles. On the command ‘go,’ start the crunch by raising your upper body forward until the elbows or forearms touch the thighs, and then lower the torso until the shoulder blades touch the ground. This is one complete sit up.
Hi Ma Ann - please see below: How to Do Marine Corps Situps
Lie flat on your back with your knees bent and your feet flat on the floor. Your buttocks should be flat on the floor close to your heels.
Cross your arms flat across your chest so your hands grip the inside of your elbows.
Slowly lift up your upper body toward your knees. Do not move your arms from your chest; they must remain flat against your chest. When your forearms touch your thighs pause briefly.
Lower your upper body slowly down until your shoulder blades touch the floor to complete one situp.