BELL, R.D., J.D. MacDOUGALL, R. BILLETER, and H. HOW-ALD. Muscle fiber types and morphometric analysis of skeletal muscle in six-year-old children.Med. Sci. Sports Exercise, Vol. 12, No. 1, pp. 28–31, 1980. Needle biopsies from the vastus lateralis of 13 six-year-old Swiss children were analyzed for muscle fiber type populations and morphometrical characteristics. No significant differences existed between the males and females for fiber type distribution, maximum oxygen consumption, or any of the ultra-structural parameters investigated. The vastus lateralis muscle consisted of 19.7% fast twitch glycolytic (FG) fibers, 21.5% fast twitch oxidative glycolytic (FOG) fibers, and 58.8% slow twitch oxidative (SO) fibers. Maximum oxygen consumption averaged 45.2 ml/kg min-1when the subjects were considered as a single group. Morphometrically, it was found that the mean volume density of the central mitochondria was 5.54%, the mitochondrial/myofibrillar volume ratio was 6.68%, and the intra-cellular lipid volume was 0.46%. There was a significant correlation (r=0.69) between the mitochondrial volume density and the distribution of SO fibers as determined histochemically. It was concluded that the fiber type distribution pattern and infrastructure of skeletal muscle in six-year-old children was not different from normal adult tissue.