Kawakami Y, Abe T, Fukunaga T

J Appl Physiol 1993 Jun;74(6):2740-2744

Muscle-fiber pennation angles were measured in vivo with the use of ultrasonography to investigate the relationship between fiberpennation and muscle size for 32 male subjects (from untrained subjects to highly trained bodybuilders). From the image of aB-mode ultrasonogram, fiber pennation angles and thickness of triceps brachii were determined, the former as angles betweenechoes from the interspaces of fascicles and from the aponeurosis of long and medial heads of triceps and the latter as the distancebetween the fat-muscle and muscle-bone interfaces. The pennation angles were in the range of 15 and 53 degrees for the longhead and 9 to 26 degrees for the medial head, which were similar to or greater than the published and the present data on humancadavers. Significant differences were observed between normal subjects and bodybuilders in muscle thickness and pennationangles (P < 0.01), and there were significant correlations between muscle thickness and pennation angles for both long (r =0.884) and medial (r = 0.833) heads of triceps, suggesting that muscle hypertrophy involves an increase in fiber pennationangles.