Having your feet anchored when performing a situp activates your hip flexors, which assist you as you curl your body up from the floor. While this is an effective way to practice the proper situp without the same level of intensity, it increases the strength of your hip flexors and not your abs. It can also strain your lower back. Instead of anchoring your feet, perform modified situps that will strengthen your abdominals and allow you to perform a proper situp.
Perform a situp with proper form. Start by lying on the floor with your knees bent and feet flat. Interlace your fingers behind your head and draw your navel in toward your spine. Flatten your back onto the floor by contracting your abdominal muscles. Maintain stability in your lower body and begin curling up, first by lifting your head, then your neck, followed by your shoulder blades, and finally your lower back. Pause for one count and then slowly lower your body back to the starting position while resisting gravity on the way back to the floor.
Perform butterfly situps, a variation of the basic situp. Sit on the floor, bend and flare your knees out to the sides and put the soles of your feet together. Start by extending your arms, putting your palms together and resting your fingertips on your shoes. Slowly lower your back to the floor while keeping your arms forward. When you feel your shoulder blades touch the floor, contract your abdominals and sit back up to the starting position.
Hold a medicine ball or hand weight to increase the intensity of the situp. Sit tall on the floor while holding a light medicine ball or hand weight with both hands. Start by bending your knees, putting your feet flat on the floor, bending your elbows and holding the weight a few inches over your head. While keeping the weight in position, slowly lower your back to the floor. Pause, contract your abdominals, return to the starting position and repeat. Avoid swinging your arms to gain momentum.
Practice core exercises to strengthen your abdominals. The rectus abdominis is the primary muscle used in a situp, but the transverse abdominis, internal and external obliques, and muscles of the lower back assist the movement. Add exercises, such as the plank, cable rotation, back extension and reverse crunch, to your workout routine to strengthen all the muscles of your core.
Perform the situp over a stability ball. Even though there is now the added challenge of balancing on the ball, situps on a stability ball can be easier than performing them on the floor. Lie face up over a stability ball with the natural curve of your spine positioned slightly below the apex of the ball. Interlace your fingers behind your head and engage your abdominals as you would for the basic situp. Curl your body up into a seated position and then slowly lower back to the starting position while keeping the ball from rolling.