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.
Muscles in Action
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.
Important Facts to Know
- 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.