Situps provide a convenient way to tone and develop your abdominal muscles, especially the transverse and rectus abdominus muscles. Defining the latter helps you achieve the much-coveted "six-pack abs." Situps also target the hip flexors and a muscle called the rectus femoris of the quadriceps. While doing situps alone will not give you a "six pack," they will improve your core strength without the necessity for any special equipment.
Working the Abs
Lie on your back. Bend your knees and place your feet flat on the floor or mat. Position your heels about 1 to 1 1/2 feet in front of your tailbone. Place your fingertips behind your head or cross your arms on your chest.
Squeeze your shoulder blades together. Exhale then tighten your abs and curl up toward your bent knees. Do not pull on your neck. Keep your head in line with your spine, tilting your chin forward slightly as you come up.
Keep the bottom of your feet, your lower back and your tailbone flat against the floor throughout the exercise. Curl toward your thighs until you are in a seated position. Pause for a count of two.
Inhale then slowly lower yourself until your back is in contact with the floor. Keep your abdominal muscles contracted throughout the downward phase of the movement. Repeat the upward and downward phases of the situp until you have reached your goal, or until you feel a burning sensation in your abdominal muscles.