The sit-up has largely been supplanted by the crunch in recent years. Most people performed the sit-up to work the muscles of the abdomen — and the crunch more effectively isolates those muscles.
However, many athletes who require not only abdominal strength but hip strength still perform sit-ups. Find out exactly which muscles sit-ups use to decide if it's the best exercise for you.
Read more: Straight-Leg Situps
Sit-ups target the abdominal muscles, including rectus abdominis, external and internal obliques, iliopsoas and rectus femoris.
1. Rectus Abdominis
The rectus abdominis is the wall of abdominal muscle that connects to the lower rib cage and to the hips. When built up so that it bulges against its crossing tendons, it creates the six-pack effect. Its purpose is to tilt the rib cage and the pelvis toward each other.
Like all abdominal exercises, the sit-up should be performed with the back at least slightly rounded at all times to protect the spine. This contraction works the rectus abdominis.
2. Internal and External Obliques
The external obliques also attach to the rib cage and the pelvis, but to either side of the rectus abdominis. They are the primary muscles for twisting the body back and forth and for tilting the rib cage from side to side. When contracted simultaneously, however, they aid the rectus abdominis in crunching the rib cage directly toward the pelvis, such as occurs during a sit-up.
Read more: Good Situps for Your Lower Abs
This is where the sit-up begins to differentiate itself from the crunch. The iliopsoas attaches to the lower spine and high up on the hips and to the upper front of the femur. Their function is to bend the body at the hips. Usually this is to lift the thigh toward the torso, but in the case of sit-ups, it's to lift the body toward the thighs.
Proportionately, they are very weak compared to their antagonist muscles, the gluteus maximus, which are some of the largest and strongest muscles in the body. This is one good reason to work them with sit-ups.
4. Rectus Femoris
The rectus femoris is one of the four heads of the quadriceps, the large muscles of the front of the thigh. All four heads of the quadriceps attach to the patella, or knee cap. Their primary function is to straighten the leg at the knee.
While the other three heads of the quadriceps attach to the upper femur, the rectus femoris crosses the hips, attaching to the pelvis. There, it aids the iliopsoas in flexing the hips — or lifting the torso toward the thighs — during a sit-up.
The Accessory Muscles
Depending on your technique, additional muscles help with sit-ups. Neck flexors, chest muscles and even shoulder extensor muscles can all activate to assist in lifting your torso off the ground. To reduce the use of your chest and shoulders, cross your arms over your chest rather than placing your hands behind your ears. Also, keep your neck relaxed throughout the movement.