During high-intensity aerobic exercise, glycogen is the main source of fuel for oxidative phosphorylation. Muscle glycogen stores are depleted after 90 minutes of exercise, however. If exercise is prolonged, utilization of free fatty acids and blood glucose increases. Because the availability of FFAs from adipose tissue is almost unlimited, a normal, healthy person can perform moderate exercise for many hours. In patients with metabolic myopathies, symptoms become evident during activities that require increased metabolic demands, such as exercise.
Metabolic myopathies result from defects in the breakdown of glycogen or utilization of FFAs.
The glycogenoses result from defects in the metabolism and utilization of glycogen or glucose by the muscle. Only the more common of these disorders are discussed. This autosomal recessive disease presents as exercise intolerance and myoglobinuria in patients older than 15 years. Fixed muscle weakness may develop later in life. The resting serum CK level is often elevated.