"Myoglobin is like an emergency oxygen source. Ordinarily oxygen would be carried in the haemoglobin of red blood cells. When the muscles are working, this oxygen source can be defective (hence anaerobic respiration). Muscle fibres contain the protein myoglobin, which is kind of like hemoglobin. Without going into any sort of detail, it give up its oxygen it is carrying easily and quickly to muscles allowing them to work longer and more efficiently during exercise."