Kinesiology of Exercise


Many athletes, bodybuilders, and fitness buffs ignore the sit-up and do the crunch instead. The crunch is like the sit-up except that you only raise the head and shoulders through a limited ROM. There is no questioning the fact that the crunch is an effective exercise, but the sit-up can be as, or even more, effective. 

Thus, it should remain in your arsenal of abdominal exercises. If you cannot do the sit-up because you have back problems or are predisposed to back problems, you should have the problem diagnosed so that you do not have to avoid doing the sit-up. It does not cause problems when done correctly.

Major Muscles and Actions Involved

The upper rectus abdominis and internal oblique and external oblique are involved in spinal flexion. The upper rectus abdominis is not a separate group of muscles; rather, only the upper portion of the entire muscle is in action, as substantiated by EMG (electromyographic) studies. The rest of the muscle remains under tension. In this exercise your head and shoulders are lifted and move toward your hips.

Sports Uses

The sit-up exercise is important for all athletes who throw implements with maximum force (baseball, football, javelin, shot) and for those who perform acrobatic type movements such as diving, trampolining, and gymnastics. Upper abdominal development is very important to bodybuilders because these muscles show the ripped effect. Athletes who must handle heavy loads or must stabilize their spines, need strong abdominal (and lower back) muscles to maintain firm midsections.

Exercise Analysis

  • There is considerable controversy over whether your legs should be straight or bent at the knees when you do the sit-up. In general, your knees should always be bent when your legs are held down or secured in some other way. When you attempt to do a sit-up with your legs straight, the psoas muscle (which is a hip joint flexor and is attached to the lower vertebrae) comes into play. This muscle can cause the lower spine to hyperextend (arch) if the abdominals are too weak, which in turn can cause lower back problems if the stress is sufficiently great.
  • It is important to understand that the pull of the psoas on the spine is critical only when your body is in a straight-line position on the floor. Once you have slight flexion of the spine, as you do in the beginning position him of the sit-up and crunch exercises, and you can hold that position, it is impossible for the psoas to hyperextend the spine. Flattening the lumbar area of the spine by contracting your abdominals and tilting your pelvis backwards as you begin to do the sit-up will also remove the possibility of hyperextension.
  • Performing sit-ups in this manner is not dangerous. If it were, many other exercises would be dangerous as well, including the hanging leg raise; the knee-up and other exercises which involve hip flexion before spinal flexion. This also includes athletic activities such as running, especially sprinting; and even walking fast when the hip flexors are strongly involved.
  • The sit-up has also been negatively criticized because the hip flexors are involved when your legs are secured. However, there is nothing wrong with hip flexor involvement. In fact, it is beneficial to develop these muscles in order to keep your pelvis properly aligned. Thus, by doing a sit-up with your knees bent and your feet secured, you get development not only of the upper abdominals but of the hip flexors as well. In essence, you work the mutual relationship between these muscles.
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