Question

Why sit-ups are more difficult to do if yours hands are placed behind your head instead of at your sides?

Answer 1

U sure this is a physics question

Answer 2

because when your hands are at your side, you can use your hands to "pull" yourself. It happens when you extend your arms, and then use it to balance yourself when you get up. So therefore, the force needed to rise is divided between the abdominal and the muscles of the hands. While if the hands are at the back of the head, you cannot use your arms to pull yourself nor to balance your body, thus the pressure is all on the abdominal

does that help?