1. Computed tomography has been used to establish the proportions of fat, muscle and bone present in the normal human forearm.
2. Subjects were healthy young volunteers, eight males and eight females. A series of six cross-sectional scans at right angles to the long axis of the forearm was obtained, the scans being made at equidistant intervals between the olecranon process and the ulnar styloid.
3. The volumes occupied by fat, muscle and bone were calculated for the complete forearm. Total forearm volume calculated by this method agreed closely with that measured by water displacement.
4. For the male subjects, forearm composition was 72.1 ± 4.4% muscle (mean ± SD), 15.0 ± 5.3% fat and 12.9 ± 1.2% bone. Female forearms had less muscle (P<0.001) and more fat (P<0.001) than those of the male subjects: 58.5 ± 4.0% muscle, 29.3 ± 5.0% fat and 12.3 ± 1.4% bone.
5. Forearm muscle volume was significantly correlated (r= 0.75; P<0.05) with lean body mass in the male subjects; the range of values for lean body mass in the female subjects was too small to permit calculation of the equivalent relationship.
6. Forearm fat content, as a percentage of total volume, was proportional to whole body fat content as estimated from skinfold thickness (males, r = 0.84; P< 0.01; females, r = 0.77; P< 0.05).