Radiographically determined widths of bone muscle and fat in the upper arm and calf from age 3-18 years.

In the Harpenden Growth Study arm and calf radiographs were taken on 280 boys and 225 girls twice a year over varying periods. Widths of bone, muscle and fat halfway down the arm and at maximum calf diameter were measured, with widths of bone cortex and medulla where possible. Mean distance and velocity curves are given for chronological age 3-18 years together with curves based on time from peak-height velocity (PHV) and time from peak muscle velocity over the pubertal period. Muscle widths have their peak velocity more nearly coincident with the sitting height peak than with PHV; in the average child the whole muscle spurt lasts two years from start to finish. Calf muscle is much more pronounced in girls in comparison with boys than is arm muscle; this is true at all ages, with sex differences at maturity amounting to 10% for calf and 20% for arm. Humerus cortex has a marked spurt in both sexes, with the peak contemporaneous with the muscle peak. Both humerus and tibia medulla widths have a spurt in boys, but none in girls, where the means do not change from age 11 onwards. The average girl actually loses fat in the arm for a year at puberty, a result which contrasts with the velocity curve derived from mass cross-sectional data. Correlations between widths of bone in arm and calf average 0.5 during the pre-adolescent years and 0.4 at maturity; those between muscle widths in arm and calf 0.4 in pre-adolescence and 0.4 again at maturity. Between-tissue correlations are very low at all ages.