Morphology of skeletal muscle in children. An assessment of normal growth and differentiation.

Muscle fiber size and relative proportion and size of type 1 and 2 fibers were studied in postmortem samples of diaphragm, rectus abdominis, quadriceps, and deltoid of 28 infants and children. The rate of fiber growth varied with age, muscle, and fiber type. Type 1 fibers were smaller than type 2 fibers in preterm and term neonates and histologic identification of pathologic type 1 fiber size disproportion may thus not be possible in the young infant. Isolated groups of type 1 fibers unaccompanied by other evidence of neuropathy may be seen in normal children and may reflect normal variation in motor unit innervation patterns. Definition of normal muscle growth provides a standard for evaluating diagnostic material and allows insight into normal neuromuscular maturation.