Differences in fiber number and fiber type proportion within fascicles. A quantitative morphological study of whole vastus lateralis muscle from childhood to old age.

Data are presented on the number of fibers and the proportion of different fiber types within fascicles of whole vastus lateralis muscles from 5 male children, 5 to 15 years of age, and compared with results from 25 male adults, middle aged and old individuals. The results verify a difference in the proportion of fibers with different properties within a fascicle. The proportion of type 2 (fast twitch) fibers on the border of fascicles is larger than the proportion internally: the children have (P less than 0.01) greater difference than the adults and the middle aged, whereas the old have (P less than 0.001) less difference than the other age groups. The mean number of fibers per fascicle increases (P less than 0.05) from childhood to adult age, and thereafter reduces (P less than 0.01). The results imply that fascicles continuously rearrange during the normal growth and development and the normal aging process. It is argued that the causes of the difference in fiber type proportions within fascicles are local factors in the muscle, secondary to the overall age related functional demands put on the fiber population.