Executive functions, self-paced exercise and cycling performance

Abstract

The main aim of the present thesis was to understand the role of executive (cognitive) functions in self-paced aerobic exercise (cycling). A self-paced exercise is a physical activity in which the effort has to be distributed in the best possible way to achieve the objective of the event (e.g., to cover a given distance as quickly as possible or to cover the largest possible distance in a given time) (1,2). Self-paced exercise requires the monitoring and control of feedback from the muscles and cardiorespiratory systems to the brain (3).. From an applied point of view, we could consider that the self-paced aerobic exercise is a goal-directed behaviour towards an objective that involves several cognitive processes, and in particular of executive functions (e.g., inhibitory control or working memory) (18). Consequently, any change at cognitive level (and brain related to the cognitive processes under study) will affect physical performance. To understand this relationship, in an introductory chapter we summarized the role of executive functions on the self-paced exercise, and the empirical evidence of the neural basis. We also summarized the different manipulations that have been designed to investigate the role of the executive functions on self-paced exercise. In the following chapters, we describe the three studies we have conducted to investigate the role of executive functioning on the self-paced exercise. First, we investigated the ergogenic effect of tramadol on physical and cognitive performance. Next, we attempt to understand the effects of transcranial direct current stimulation (tDCS) (applied to the left dorsolateral prefrontal cortex) on objective and subjective indices of exercise performance. Finally, we investigated the role of cognitive (executive) load during self-paced exercise. Analgesics drugs are widely used in sports to treat pain and anti-inflammatory processes associated with injuries (4). However, it has been detected that there is a tendency among athletes of all levels to use these drugs, not only to treat minor injuries, but also to train and compete (5). Therefore, in addition to its peripheral effects, there is a possibility that athletes are using these drugs for their effects in the central nervous system to increase physical performance during training and competitions. Some of these drugs could have an effect on the activation of higher brain structures (e.g., prefrontal cortex or anterior cingulate cortex) involved in pain and cognitive processing. One of these drugs commonly used by cyclists is tramadol (6). Tramadol could improve physical performance by reducing perceived exertion, pain perception and/or mood. However, tramadol is known for its frequent side effects, such as drowsiness or nausea, which can have a negative effect on cognitive functions and exercise performance (7). Therefore, in a series of two experiments, our objective was to study the effect of tramadol on physical performance and cognitive processing in cycling. Experiment 1 revealed that tramadol improved physical performance by approximately 5% during a self-paced (time-trial) cycling test of 20 minutes (8). Tramadol appeared to allow participants to achieve greater mean power output without modifying brain electrical activity, rate of perceived exertion or mood. The results of this experiment seemed to support the hypothesis that tramadol could improve physical performance, however, this was not corroborated in Experiment 2. Experiment 2 was designed to replicate Experiment 1 and to test the hypothesis that tramadol might have an effect on sustained attention during exercise (8). To do this, participants completed an oddball task while performing the 20 minutes cycling test. The cognitive task consisted of a random presentation of a sequence of visual stimuli of a frequent blue circle (non-target), a small rare blue circle (Target 1) and a red square (Target 2) on a screen. Contrary to the results observed in Experiment 1, tramadol did not improve physical performance or affect sustained attention at the behavioural level compared to the placebo condition, that is, neither the accuracy of the response nor the reaction time differed significantly between the experimental conditions. However, during the sustained attention task, we found that tramadol caused a lower brain activity (i.e., greater suppression with respect to the baseline) in the alpha frequency band linked to stimulus processing (relevant to the task) in the condition of tramadol compared to placebo. Higher alpha activity has been considered as an indicator of greater alertness (9). On the contrary, another study that used a similar task, interpreted the reduction of the alpha band when unusual objectives are presented, as a higher mental effort to detect infrequent objectives (10). Therefore, our results could point out to the need for greater allocation of cognitive resources to detect infrequent targets when participants received tramadol versus placebo. In the next study, following a different approach, we hypothesized that the stimulation of a brain area related to executive functions could improve self-paced exercise performance, if self-paced exercise relies on executive functions. Even if several studies had investigated the effect of tDCS on physical performance, none of them had addressed the question of whether anodal stimulation over the left dorsolateral prefrontal cortex might affect self-paced aerobic exercise performance (11). Interestingly, the results of our study did not support the idea that anodal tDCS affects neither self-paced performance nor oscillatory brain activity, both at rest and during exercise (12). This finding added more inconsistency to the ambiguous results published to date. Therefore, in view of our null results and the inconsistency of the literature, we decided to perform a meta-analysis to check whether the tDCS had a real effect on indexes of exercise physical performance. The meta-analysis showed that tDCS has a small, albeit positive, effect on exercise performance (Hedges' g = 0.34) (13). None of the included moderators (e.g., location of stimulation, intensity or duration) in the analysis explained the variance in the data. In addition to the small effect size, we detected that the positive results could be overestimated by methodological artefacts and publication bias. To date, with our (and others) meta-analysis we cannot establish tDCS is an effective tool to improve exercise performance (13–15). In a final study, we try to fill an existing gap in the literature on the effect of the cognitive (executive) load during self-paced exercise, since in most of the previous studies the cognitive task was performed prior to a physical exercise (16). The cognitive load was manipulated by using a working memory task (n-back) with two levels of difficulty to induce two levels of cognitive load, low (1-back) and high load (2-back). We anticipated that if the self-paced exercise is determined by executive processing and the n-back also requires executive processing, self-paced exercise performance would impair due to the inability to self-regulate efficiently. However, although the task with more cognitive demands was more demanding (lower precision and slower response times) than the one with low cognitive load, and therefore greater difficulty in terms of executive demands, the results did not provide enough evidence to affirm that high load worsens physical performance or modifies perceived perception of effort (17). These results are in line with the previous findings of this thesis, suggesting that self-paced exercise might not rely on executive functions. In conclusion, the results of the series of experiments carried out during this thesis do not support the idea that self-paced exercise depends to a large extent on executive processing. Although tramadol could affect physical performance in Experiment 1, oscillatory brain activity was not affected during exercise and the mechanisms of possible improvement are unknown (we are currently conducting another study to clarify the previous results). That was also true in the tDCS’ experiment, since brain stimulation did not affect physical performance or brain activity. Finally, a greater cognitive load during self-paced physical exercise did not seem to affect physical performance. Crucially, the results of this thesis may indicate that executive functions may not play a decisive role in the self-paced exercise, contrary to what is assumed. However, we acknowledge that the role of executive functions in the exercise could be mediated by several factors, such as sports expertise. References 1. Edwards AM, Polman RCJ. Pacing and awareness: Brain regulation of physical activity. Sports Med. 2013;43(11):1057–64. 2. Pageaux B. The psychobiological model of endurance performance: an effort-based decision-making theory to explain self-paced endurance performance. Sports Med Auckl NZ. 2014 Sep;44(9):1319–20. 3. St Clair Gibson A, Lambert EV, Rauch LHG, Tucker R, Baden DA, Foster C, et al. 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