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Probably the best known abdominal exercise is the sit-up. This move targets your rectus abdominis, but a number of other muscles are also involved, from your abdomen all the way down to your ankles. However, any time you think of sit-ups and six-pack abs, you are picturing your rectus abdominis. When performing a sit-up, you bring you entire back off the floor as you raise your upper body toward your knees. A similar move, the crunch, involves lifting only your upper back off the floor.
Your rectus abdominis is one of the four main muscles making up your abdomen, and it is responsible for creating the so-called washboard abs. Your rectus abdominis starts at your pubic bone and stretches in a straight line upward to your ribs. It attaches to your fifth, sixth and seventh ribs. Any time you bring your ribs toward your thighs, you are engaging your rectus abdominis. It is also responsible for controlling the tilt of your pelvis and the curvature of your lower spine.
The assisting muscles - iliopsoas, tensor fasciae latae, rectus femoris, sartorius, obliques - are essential to get you through a sit-up motion. Your iliopsoas runs from your lower ribs and across your hip and attaches to your femur. It assists with flexing your hip and rotating your spine. Your tensor fascia latae is a thin muscle running from your pelvis to your tibia. It assists with flexing your hip and moving your legs away from the center of your body. Your rectus femoris makes up part of your quadriceps, and it assists with both bending your hip and straightening your knee. Your sartorius is a long, narrow muscle running diagonally across the front of your thigh. It assists with both bending your hip and your knee. Your obliques are other abdominal muscles that lie on the sides of your torso. They assist with flexing, rotating and bending your spine.
You might not expect to find that muscles in your lower leg are used during a sit-up, but that's exactly what your tibialis anterior does. This muscle, which runs from the top of your shinbone down to your first metatarsal, or big toe, helps you flex your ankle. As you go through the upward motion of a sit-up, you may feel slight tension through the front of your legs. This is your tibialis anterior engaging. While the sit-up does not strengthen this muscle, it does use it to keep your ankle stabilized.
The Proper Sit-Up
To engage all of these muscles correctly, do your sit-up with proper form. According to the American Council on Exercise, lie on your back with your knees bent and your feet flat on the floor. Your heels should be about 12 to 18 inches from your buttocks. Place your hands behind your head. Engage your core muscles and pull your rib cage together and toward your pelvis. Keep your neck relaxed and your head aligned with your spine. Your feet and buttocks should always touch the floor. Raise your upper body until your back is off the mat. Hold that position, then lower your back to your starting position.
Be aware that the sit-up may be problematic if you have lower back issues, according to a study published in "Physical Therapy." A crunch puts less stress on your lower back than a sit-up. When performing a crunch, keep your lower back on the floor throughout the exercise and raise your upper back toward your knees. While both exercises use the same muscles, the sit-up requires more hip stabilizing muscles than the crunch.