The aim of the present study was to quantify changes in human skeletal muscle pennation angle (F theta) values during growth and adult life. The human gastrocnemius medialis muscle of 162 subjects (96 males and 66 females) in the age range 0-70 years was scanned with ultrasonography. The subjects were laying prone, at rest, with the ankle maintained at 90 degrees with all muscles relaxed. F theta increased monotonically starting from birth (0 years) and reached a stable value after the adolescent growth spurt. There was a significant (p < 0.05) linear relationship between F theta and muscle thickness (TK). F theta = 0.84 (+/- 0.09) * TK + 3.15 (+/- 1.13). Human gastrocnemius medialis F theta and TK data found in the literature seem to fit the F theta-TK plot in a coherent manner, independent of the physiological or anatomical characteristics of the subject. The present findings indicate that F theta is not a constant parameter but evolves, as is the case for bone length and height, as a function of age.