Cross-sectional areas of fat and muscle in limbs during growth and middle age.

Age changes in the cross-sectional areas (CSAs) of fat and muscle tissues of limbs were investigated by using a B-mode ultrasonic technique for 245 males and 275 females aged 6 to 60 years. Four measurement sites were selected in the right limbs: the forearm, upper arm, lower leg and thigh. In an age span from 13 to 15 years, fat CSA for males decreased, while that for females increased at every site. Fat CSAs for both sexes increased in their thirties, but from then there was no more increase with aging. During early childhood, females had similar growth curves in muscle CSAs to those of males. An apparent sex separation in muscle CSAs took place at 13 years of age and after, the relative gender differences in muscle CSAs became larger with growth in the upper extremity than in the lower one. For both sexes, muscle CSA in every site remained almost constant during middle age. This age change in fat and muscle CSAs of limbs during the period of growth generally resembles that reported previously for total fat and fat free mass, respectively. However, age trend in fat CSAs during middle age, a lack of increase on reaching and after the forties differs from that described by prior studies for fat mass.