30 subjects with mitochondrial disorders, 25 with metabolic myopathy, 25 with muscular dystrophy and 25 healthy controls will be recruited. A blood sample will be taken, and GDF-15 and other muscle markers will be measured. 10-15 subjects with mitochondrial disorders, 10-15 with metabolic myopathy and 10-15 healthy individuals will further be investigated with an exercise test, and blood samples will be taken afterwards.