I was taught that rectus abdominous is one muscle, and as such, it's fibers contract all or none. Some of my colleagues say there is an upper rectus ab and a lower rectus ab (below belly button). These people say that if you do a reverse crunch, you tend to work the lower part of the rectus ab more. Also if we use hip flexors in a supine position while stabilizing the pelvic and the lumbar spine, they say that the lower part of the rectus ab is working more. I do feel more tension in the lower part than in the upper part of the rectus when I do those stabilizing exercises. My question is, is it the lower part of the rectus ab that is working more or is the internal layer of the abdominal wall (transverse abdominal, etc, and all the other more internal pelvic muscles) that are working more vigorously to stabilize the pelvic girdle?
There seems to be many things to address with your question. The muscles of the trunk that are responsible for flexion, rotation with and without angles and lateral flexion are referred to as abdominal muscles. These and the other trunk muscles create contractions for stability and/or movements depending on what position the body is in at the time.
You are correct about feeling more tension in the "lower" part of the rectus abdominis when doing a reverse abdominal curl. I believe one of the reasons for this increased tension has to do with the amount of resistance the legs provide. The muscles of the hips and legs are essentially being lifted by the rec abs whose fibers run in a vertical fashion. I have heard people refer to the different portion of the rec abs as "lower" and "upper" but was unable to locate it any references to it.
The rec abs are responsible for spinal flexion, and like any muscle, when it contracts for movement, it get shorter, increases in tension and girth. The transversus abdominis (trans abs) compresses the abdomen and allows the rec abs to become more forceful by supporting the linea alba (the line down the center of the abs.). The quadratus lumborum and the hip flexors help in spine stability during the exercise.
The reverse abdominal curl is a great exercise for conditioning the entire abdominal musculature. There are several excellent articles on abdominal training by Paul Chek and Carlos Santana available on this site. I suggest you research those for more information.
- Rasche, P. and R.Burke. Kinesiology and Applied Anatomy: the Science of Human Movement. 6th ed.(1978) Lea & Febiger. Philadelphia
- Tortora, G. Principles of Human Anatomy. 4th ed. (1986) Harper & Row. New York
- Kendall, F. and E.McCreary. Muscles: Testing and Function. 3rd.ed.(1983)Williams and Wilkins. London