The relative proportions of type I and type II muscle fibers in the external sphincter ani muscle at different ages and stages of development--observations on the development of continence.

The external sphincter ani muscles were partially or completely removed from 11 fetuses, 3 infants, 2 children and 12 adults, none of whom had had anorectal disease and all of whom had died of diseases not affecting the anorectal organ of continence. Muscle fiber-typing was carried out. In the fetuses ages 26-40 weeks, we found a predominance of Type II (rapid) fibers. With increasing gestational age Type II fibers started to diminish along with a simultaneous increase in Type I (slow) fibers. At the same time, stromal restructuring took place. Three infants, ages 1, 2 and 10 months, showed further increases of Type I fibers, until finally, in the two older children studied (5 and 8 years), there was a predominance of Type I fibers. This persisted in adults between ages 56 to 70, but after age 70 Type I fibers diminished, so that the ratio of slow to rapid fibers in the 78 to 81 year-old group was 3:2. This study shows that in contrast to other skeletal muscles the fiber distribution in the external sphincter ani muscle is not established from the beginning, but is the result of a developmental process. The predominance of Type II (twitching) fibers explains the state of reflex continence of the young infant. With increasing maturation of tonic Type I fibers, an additional voluntary component to continence is made possible with the help of the supporting pelvic musculature. This maturation is determined by the increasing strain on the pelvic floor as the child learns to sit and walk.(ABSTRACT TRUNCATED AT 250 WORDS)