The determinants of task failure during high-intensity exercise.

Some of the most physically-demanding sports are those that require prolonged high-intensity exercise. Such tasks place high demands on both the aerobic and anaerobic energy systems, and can only be sustained for several minutes. A better understanding of the underlying mechanisms of fatigue and its limitations would provide greater insight into how to delay the onset of fatigue. This will be relevant for athletes trying to improve their performance, but also for the elderly and patients with chronic diseases whose daily-life activities are limited. Although various theories about the development of fatigue and task failure have been described in the literature, no consensus exists of the precise mechanisms underlying fatigue and task failure. Therefore, the main objective of this thesis was to study high-intensity exercise, using a multi-disciplinary approach to gain a better understanding of the differences processes determining task failure. Firstly, the energy-depletion model was evaluated, which states that task failure coincides with the depletion of a finite store of anaerobic energy. To this end, participants exercised on a cycle ergometer at a supra-maximal intensity until task failure, after which the power output was dropped to a lower, but still supra-maximal level. According to the energy depletion model, participants should not be able to continue cycling. However, the results showed that the participants continued cycling for a substantial time at this high-intensity workload. It was therefore concluded that anaerobic energy depletion could not have been the direct cause of task failure. The results subsequent calculations also indicated that the common methods to quantify anaerobic energy production, the critical power model and the maximal accumulated oxygen deficit method, might not be accurate. The purpose of the second study was to evaluate the potential involvement of subconscious mental processes during high-intensity cycling exercise, and to challenge the assumption that peripheral factors exclusively cause task failure. This was achieved by manipulation of perceived time by a displayed clock during high-intensity cycling exercise to task failure. Interestingly, participants cycled for a significantly longer time while watching a fast-running clock (x 1.1) compared to a slow (x 0.9) clock. From these results, it was concluded that central subconscious processes are involved in the regulation of task failure. Additional measurements of the activation patterns of four muscles in the lower leg (using electromyography) revealed a small difference in the recruitment strategy between the fast and slow clock condition, which might explain how participants prolonged their exercise duration. In the final study, participants performed repeated bouts of dynamic one-legged knee-extension exercise. A twitch-interpolation technique was used to monitor the contribution of central and peripheral factors to muscle fatigue during high-intensity exercise. The results revealed that central fatigue developed during the repeated bouts of high-intensity exercise. However, as no significant reduction in voluntary activation was observed after the first exercise bouts, central was not a prerequisite for task failure. Furthermore, task failure is thought to occur as a result of a critical reduction of maximal muscle force. However, inter-individual variations in maximal voluntary force after task failure suggested that not solely peripheral fatigue, but that a combination of different factors determines the moment of task failure during high-intensity exercise. Based on the results of the studies presented in this thesis, it can be concluded that muscle fatigue with central and peripheral components, but also (subconscious) mental processes contribute to task failure during high-intensity exercise. It is proposed that during exercise the integration of these fatiguing processes lead to an increase in the perception of effort, ultimately resulting in volitional task failure at the moment the maximal tolerable level has been reached. Ultimately, a model is proposed aiming to unify existing theories and provide an overview of the potentially limiting factors and mechanisms leading to task failure during high-intensity exercise.