The situp has largely been supplanted by the crunch in recent years. Most people performed the situp to work the muscles of the abdomen — and the crunch more effectively isolates those muscles. So for bodybuilders, eight-time Mr. Olympia Arnold Schwarzenegger recommends crunches over situps in "The New Encyclopedia of Modern Bodybuilding." However, many athletes who require not only abdominal strength but hip strength still perform situps. Find out exactly which muscles situps use to decide if it's the best exercise for you.
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 situp should be performed with the back at least slightly rounded at all times to protect the spine. This contraction works the rectus abdominis.
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 situp.
Tensor Fasciae Latae
This is where the situp begins to differentiate itself from the crunch. The tensor fasciae latae attaches 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 situps, 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 situps.
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 tensor fasciae latae in flexing the hips.