Muscle can use ATP exclusively as the direct source of energy for contraction. The muscle ATP stores cannot provide more than 1 or 2 kcal of muscular work. tthe energy for resynthetizing ATP is supplied by three processes : the breakdown of creatine phosphate, anaerobic glycolysis and aerobic processes. These three mechanisms are characterized by different inertia, maximal output and capacity. Taking into account the part of aerobic processes in energy production, the physical fitness of an individual is usually expressed by its maximal oxygen uptake (VO2 max.). When the energy requirements cannot be met by aerobic reactions, the subject contracts an oxygen deficit which is compensated for during the recovery period by an oxygen uptake exceeding the rest requirements. During exercise tidal volume and ventilatory frequency are increased. The increase in ventilatory output is directly related to the workload until 75% of the maximal aerobic power is reached. For higher relative workloads the increase in ventilatory output is steeper. This increased ventilation allows the organism to limit the decreases in PaO2 and pH during exercises of high intensity.