Muscle fiber size in healthy children and adults in relation to sex and fiber types

BACKGROUND: In adult males, cross‐sectional area (CSA) for type II muscle fibers is generally larger than for type I fibers. In this cross‐sectional study the aim was to compare sex‐related CSAs of various muscle fiber types during childhood‐to‐adulthood transition. METHODS: Percutaneous biopsy samples were obtained from vastus lateralis in 10‐y‐old children (10 males and 5 females) and in young adults (9 males and 7 females). Fiber types were classified by myofibrillar ATPase and CSAs from NADH‐dehydrogenase staining. RESULTS: Type IIA were larger than type I fibers in adult males, but not in adult females or children (age x sex x fiber type, P < .002). When including all participants, body weight and sex explained 78% of the variation in type IIA CSA but only body weight contributed for type I. CONCLUSIONS: Sex‐specific patterns in CSA of the muscle fiber types appears to develop during the transition from childhood to adulthood.