Growth and development of human muscle: A quantitative morphological study of whole vastus lateralis from childhood to adult age

TLDR

Muscle volume increases from childhood to adulthood through changes in fiber size and type composition. Cross‑sections of whole vastus lateralis from 22 healthy males aged 5–37 were stained for myofibrillar ATPase and morphometrically analyzed, with fiber area, size, number, and type proportions examined via linear regression. Muscle cross‑sectional area rises from childhood to adulthood because mean fiber size increases, and the proportion of type 2 fibers rises from ~35 % at age 5 to ~50 % at age 20, likely reflecting type 1 to type 2 transformation without changing total fiber number.

Abstract

Abstract The mechanisms underlying the increase in volume of muscle tissue, and the functional development of muscle fibers from childhood through adolescence to adult age, have been studied. Cross sections of autopsied whole vastus lateralis muscle from 22 previously physically healthy males, 5 to 37 years of age, were prepared enzyme histochemically (myofibrillar ATPase) and examined morphometrically. The data obtained on muscle crosssectional area, size, total number, and proportion of type 1 (slow‐twitch) and type 2 (fast‐twitch) fibers were analyzed using linear regression techniques. The results show that the increase in muscle cross‐sectional area from childhood to adult age is caused by an increase in mean fiber size. This is accompanied by a functional development of the fiber population: the proportion of type 2 fibers increases significantly from the age of 5 (approx. 35%) to the age of 20 (approx. 50%), which, in the absence of any discernible effect on the total number of fibers, is most likely caused by a transformation of type 1 to type 2 fibers.