Body composition of the area at the level of the 4th lumbar vertebral section was studied in 62 humans who had no abdominal problems (32 males, 30 females) by X-ray CT (computed tomography) images. The total cross-sectional area, subcutaneous fat, abdominal cavity, vertebra, vertebral foramen, and muscles (rectus abdominis, abdominis lateralis, quadratus lumborum, psoas major, erector spinae, transversospinalis) were measured, and compared for sex, age and body type differences. The subjects were classified, by age, into six 10 year groups from 20 to 70, and by Rohrer Index, into three types. The following results were obtained. 1) The total cross-sectional area was smallest in the twenties and increased in the thirties, in both sexes. There were no significant changes after the thirties. 2) The mean ratios of cross section area of each tissue and organ to the total body cross section area for all ages were : In males, the abdominal cavity and muscles were each about 34%, and these were followed by subcutaneous fat. In females, the subcutaneous fat and abdominal cavity were each about 34%, and were followed by muscle. 3) Comparison of the components by age yielded: In males the relative order was muscle, abdominal cavity, and subcutaneous fat in the twenties and thirties, and above the forties the relative order was abdominal cavity, muscle, and subcutaneous fat. The muscle was equal to the subcutaneous fat in males in their seventies. In females, the subcutaneous fat ratio was largest in the twenties to fifties and this was followed in order by muscles and abdominal cavity in the twenties, and by abdominal cavity and muscles above the thirties. In the sixties and seventies, the abdominal cavity ratio was largest in females. 4) In both sexes, the real amount of subcutaneous fat and abdominal cavity tended to increase with age. The real area of the vertebra increased slightly, but the vertebral foramen tended to decrease with age. The amount of muscle decreased remarkably in the males, and gradually with age in the females. 5) In males, the order of the mean areas of the muscles were: abdominis lateralis>erector spinae>psoas major>quadratus lumborum ≅ transversospinalis>rectus abdominis muscle. Decrease of area of each muscle with age was greatest in the forties and seventies for psoas major and quadratus lumborum, in the fifties for erector spinae, and in the seventies for transversospinalso true alis, but there was no change in the rectus abdominis or abdominis lateralis. All of this was in females, except that the quadratus lumborum and rectus abdominis muscle were equal and were smallest. 6) Among the three body types, the real areas of all components except the vertebral foramen tended to increase in the order of A type, C type, and D type in males. In comparing ratios, only the subcutaneous fat ratio increased in the same order (A, C, D) . In females, the real area of subcutaneous fat increased in the order of A type, C type, and D type. There was a discrepancy between A type and C type in the real area of the abdominal and cavity and the muscles. The subcutaneous fat ratio increased in order of A type, C type, D type, and the muscle ratio decreased in that same order.
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