The muscle tissue is by far the most energy demanding organ in the body. Studies on energy liberation and muscle substrate levels in normal and sick children can give an important contribution to the knowledge and understanding of the problems in each disorder. Muscle metabolic data are presented from infants with symptomatic ventricular septal defect (VSD), children with coarctation of the aorta, children with coeliac disease and with late walking debut. The difference in muscle substrate levels and muscle enzyme activities in normal children 2 months to 15 years of age are also presented, as well as the effect of exercise and physical training.