The oxygen cost of exercise refers to the rate of oxygen uptake measured during aerobic exercise performed at an intensity below the lactate threshold. Because oxygen is required to generate energy (adenosine triphosphate, ATP), the oxygen cost of exercise indicates the energy demands placed on the body during exercise. Endurance training causes adaptations that decrease the oxygen cost of exercise, which means that less energy is required for exercise. This adaptation indicates an improvement in efficiency (sometimes also referred to as economy).