A test that measures the amount of myoglobin in the blood. Myoglobin is an oxygen-binding protein in cardiac and skeletal muscle. Exercising muscle uses up available oxygen as it metabolizes available fuel (that is, glucose and fatty acids) for energy. The presence of myoglobin in muscle provides an extra reserve of oxygen so that exercising muscle can maintain a high level of activity for a longer period of time. When skeletal muscle is damaged, myoglobin is released into the bloodstream. Ultimately, it is excreted in the urine (see Urine myoglobin ).