If you want to better strengthen your abs when doing sit-ups, do them with your feet unrestrained, according to a study published in the Journal of Strength and Conditioning Research.
British researchers had members of the British Army do sit-ups and curl-ups in a variety of ways, including with feet restrained (i.e., someone holding them or hooking them under a stationary object) and feet unrestrained. The subjects did each exercise for two minutes while the researchers measured muscle activity in abdominal muscles as well as the rectus femoris, a quadriceps muscle that acts as a hip flexor.
The researchers found that muscle activity in the rectus femoris was greatest when the Army members did sit-ups with their feet restrained; that is, the hip flexor muscle did more of the work in this type of exercise than when the subjects did sit-ups with their feet unrestrained. As the researchers point out, the feet-restrained method has also been shown to increase forces in the lower back, potentially leading to injury there. Sit-ups with feet unrestrained, the researchers concluded, are therefore better for strengthening abdominal muscles.
The video below shows a challenging version of feet-unrestrained sit-ups performed on an exercise ball.