MUSCLE biopsy has revealed an abnormal predominance of type II fibers in some children who have suffered a prenatal cerebral stress.1 Type II predominance is expected during the first 25 weeks of gestation2 but not at term. The persistence of large type II fibers was considered the consequence of either a disordered cerebral influence on maturing muscle or a nonspecific effect of fetal hypomobility. This investigation was undertaken to help differentiate the relative importance of brain damage and disuse as an influence on the maturation of muscle fiber typing. Infants who had been immobilized in utero, as revealed by fixed joint deformities at birth (arthrogryposis), were selected as a study model. Disorders of the lower motor unit were excluded by muscle biopsy. Thus, in the study patients, fetal hypomobility had been secondary either to brain abnormalities or factors outside the fetal nervous system. Patient Material Seven