Introduction Endurance is a broad term. Different types of endurance (short -, medium - and long term) are fueled by different energy systems. The first one or two maximum muscle contractions are powered by the phosphates in the muscle cell. After that, short bursts of up to 12 seconds draw their energy from the creatine- phosphate reserves. These two modes of energy production are known as anaerobic (lacking oxygen) alactic (without significant production of lactic acid). Longer efforts, up to roughly 3 minutes, primarily make use of the anaerobic lactic system, also called anaerobic glykolysis, i.e., the utilization of sugar in the absence of sufficient oxygen. Finally, even longer work is primarily fueled by the aerobic system. Here, oxygen is available in sufficient amounts such that sugars and fats can be oxidized in the Krebs cycle. It is this system that will be in the scope of this article. The aerobic system is, amongst other things, relevant for recovery after training se