Energy Systems: A New Look at Aerobic Metabolism in Stressful Exercise

For many years the notion of the aerobic and anaerobic systems has been used to define the type of work being carried out by athletes during high intensity sports. The traditional view is that the resynthesis of ATP is supplied by three integrated systems comprising the Creatine Phosphate (PCr), lactic and aerobic systems [1]. This now appears to have been incorrect [2]. The three-system model does not explain why certain timelines of performance seem to exist in the aerobic area of metabolism. The balance of how each of these systems contributes at different times has been reviewed by a number of authors. As the anaerobic contribution declines, the aerobic increases with continued activity duration [3,4]. Larsen (2010) referred to the first and second ventilatory thresholds, which in other papers have been defined as the aerobic threshold, lactate threshold, max-lass, anaerobic threshold and onset of Blood Lactate Accumulation (OBLA) [5-7]. These points have been seen as important in determining the transition from the predominance of the aerobic system to the “lactate system.” This definition is incorrect however, as the thresholds speak more about the relationship between clearance rates and lactate production than lactate production alone. The literature is littered with confusion as to where each occurs and what it means, mostly due to the mix of protocols [8] and subjects used (elite athletes, middle-aged sedentary, college students etc). The effect of this was to create confusion as to what went on, where and why. Although some activities may rely predominately on a single system (either very short or very long duration activities, where the percentage contribution from of the other systems is small), the timing and intensity of other activities require a significant contribution of more than one energy transfer system. All activities activate each energy system to some degree, depending on exercise intensity and duration.