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 5,9,% slow twitch oxidative (SO) fibers.
Maximum oxygen consumption averaged 45.2 ml/kg min-1 when 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 intracellular 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 ultrastructure of skeletal muscle in six-year-old children was not different from
normal adult tissue.