Metabolic myopathies and physical activity: when fatigue is more than simple exertion.

Fatigue can have many causes in active people. A metabolic myopathy-though uncommonly identified as a cause of fatigue during sporting events-must be considered in certain circumstances, and the diagnosis may be helpful for preventive and genetic counseling. In general, symptoms of disorders of glycogen breakdown and glucose utilization (glycogen storage diseases) occur during high-intensity exercise. Symptoms of disorders of fatty acid transport or oxidation and mitochondrial disorders occur after endurance exercise. Important investigations include forearm ischemic testing, electromyographic and nerve conduction studies, muscle biopsy (histology, enzyme, and DNA testing), and exercise testing. Most patients with metabolic myopathies can participate in sports with appropriate exercise adaptations and dietary manipulation.