Figure 60-2 The composition of the diaphragm and intercostal muscles changes markedly during the first 2 years of life. The number of type I muscle fibers is inversely related to age and may account in part for the ease of inducing respiratory fatigue as the work of breathing increases. (Data from Keens TG, Bryan AC, Levison H, et al: Developmental pattern of muscle fiber types in human ventilatory muscles. J Appl Physiol 44:909–913, 1978.)