The Performance during the Exercise: Legitimizing the Psychophysiological Approach

Abstract

Over the years, there has been a growing interest in the study of issues related to the psychophysiological processes underlying sports performance. A relatively recent perspective is supported by the concept that the brain acts as a central regulator of performance during exercise. This phenomenon is called pacing and is based on the premise that prior knowledge about the activity plays a fundamental role for individuals to self-regulate their efforts throughout the exercise. However, knowledge regarding this topic remains scarce, and further clarification is needed. This chapter reports new perspectives in relation to the existing evidence regarding the role of the brain as a central regulator of performance, questioning the complex interdependencies and interrelations between fatigue and physical exercise in the light of a psychophysiological perspective. A broader understanding of the cognitive basis of the psychophysiological phenomenon during the exercise is needed, bringing together concepts such as pacing behavior, decision-making, self-regulation of effort, prior knowledge of the duration of the task, and perception of effort.

1. Introduction

Exercise is characterized as a complex activity, in which the phenomenon of fatigue is enigmatic and stimulating, therefore requiring further investigation [1, 2, 3, 4]. Over time, efforts have been made to study this phenomenon in the field of sports sciences [5, 6, 7]. However, knowledge about fatigue remains ambiguous, unpredictable, and difficult to fully explain. There is a wide range of variables (training load, anxiety, etc.) that can affect the fatigue process during exercise and its synergies with the human body responses [8, 9]. For these reasons, there is no consensus in the scientific community regarding fatigue during exercise [4, 10, 11, 12], and therefore, there is no unique definition for the concept of exercise fatigue. Thus, reaching a single definition remains a scientific challenge.

Until today, classical fatigue theories continue to be the main focus of discussion on the subject. However, recent studies have emerged, identifying flaws and limitations in these theories, essentially because they do not consider significant factors in their analysis [1, 2, 4]. Following this line of investigation, the concept of fatigue has been evidenced in new investigations as a result of other aspects [13]. So far, the fatigue concept was specially based on physiological variables [14]. The new data do not fully support innovative approaches in relation to the phenomena considered until then. In fact, the studies suggested expanding the scope and focus of the fatigue research [2]. This is because the physiological perspective justifies part of the problem [15]. However, the remaining problem seems to be explained by multifactorial variables, raising the possibility of new perspectives and psychophysiological approaches [16, 17, 18].

Based on this perspective, effort regulation has emerged as a choice that athletes should take during exercise and that strongly influences performance [17]. This control of effort on the part of athletes has been called “pacing,” and it is assumed as a valuable concept in sport, supporting the existence of a psychophysiological perspective [16]. Although coaches and athletes are aware of the importance of pacing, it has been the object of study by researchers only in recent years, and so, it continues to have little expression in literature [19]. However, investigations have shown the existence of a psychophysiological system capable of controlling physical capacities and with apparent applicability in all types of exercises, including team sports [16]. The pacing phenomenon is directly linked to exercise, and until now, there was a propensity to look at pacing as a purely psychological phenomenon. It was not considered an object of analysis in the field of sports sciences, which mainly explores physiological phenomena associated with exercise [20]. Recent findings in brain research have shown that pacing is a phenomenon with strong interconnection between the psychological and physiological dimensions [16]. In fact, stress resulting from high-intensity exercise (which leads to exhaustion) can cause an unconscious or conscious inhibition of the athlete’s tolerance to pain [21]. This may cause the central nervous system (CNS) to regulate the exercise pace as much as necessary for the athlete’s pain to become bearable, allowing the task to be completed [4, 10]. Generally, researchers agree that the perceived discomfort of fatigue occurs just before the occurrence of physiological limitations in the muscles. However, the precise role of the CNS in detecting, causing, or even canceling the perception of fatigue remains unclear, and there is a gap in the literature regarding this phenomenon [12, 18]. This perspective does not come from the physiological system, it only emphasizes that effort regulation is consciously or unconsciously commanded by the brain. Thus, greater knowledge regarding the exercise operating mechanisms may bring about new approaches to explore the phenomenon of pacing in exercise [4, 10, 12, 16, 18]. Furthermore, if the pacing phenomenon acts as a regulatory system that allows the effort to be completed in the context of training and competition, factors such as previous experience and perfect knowledge of the task appear to be fundamental to success [4, 10, 16]. In fact, prior knowledge about the activity to be performed (i.e., duration, importance, demands of the game) is assumed to be a fundamental point to support a psychophysiological approach to the phenomenon of pacing [9, 18, 22]. That is how athletes are able to self-regulate their own performance during exercise [19].

Athletes’ previous knowledge about the duration of the task can induce changes in the performance, as demonstrated in ultra-marathon running [23]. In addition, the circumstance in which fatigue is expressed is another interesting factor for investigation, although little explored. The sports modalities’ specificities are important and help to interpret the results and sensations of perceived fatigue [16, 18]. This is because the type of required effort differs between modalities (i.e., collective or individual) [19]. Upon that, different reflections about the mechanisms of fatigue are necessary. It is also important to note that there is a research gap considering this subject in team sports, since the existing studies on the phenomenon of pacing are mostly related to endurance sports [16, 18, 19]. This may be related to the intrinsic and specific characteristics of each team sport. It can create issues in the analysis, possibly provoking a propensity for the study of specific variables, which could be considered as a reductionist approach to the problem. That said, based on the lack of identified research, it becomes important to enhance knowledge about the mechanisms of fatigue in sport.

2. The fatigue in sport and exercise

As presented above, there are several concepts about fatigue; nevertheless, it can be defined as “extreme tiredness after effort; reduction in the efficiency of a muscle or organ after prolonged activity” [24]. Previous research has shown that the fatigue phenomenon is more complex and has a broader spectrum of parameters, and more comprehensive concepts are necessary. The wide range of investigations carried out had a beneficial impact in expanding the available definitions of fatigue. It is difficult to accurately determine the development of fatigue as a concept in the sports sciences [1]. At the beginning of the twentieth century, two phenomena were identified that helped to characterize fatigue; (i) the reduction of muscle strength; (ii) tiredness as a sensation [25]. In fact, over time, there have been variations in the definitions of fatigue in the field of sports sciences such as; (i) “the difficulty in maintaining the necessary or expected strength” [26]; (ii) “the decreased ability to generate maximum strength” [27]; (iii) “a reversible state of strength depression, including a lower rate of strength increase and slower relaxation” [28]. That said, it is difficult to gather consensus on a single definition of fatigue. This is problematic when trying to consistently compare and interpret the different concepts. However, these inconsistencies allow the debate on the topic to be constantly open, mainly on its usefulness and applicability in the different modalities. Moreover, reaching an early definition of fatigue, which is only accepted for lack of alternatives, may help to reiterate the complexity of the fatigue phenomenon [1, 2].

2.1 Peripheral fatigue and central fatigue

Fatigue in sport and exercise can theoretically be categorized into two types (i.e., peripheral fatigue and central fatigue) [29, 30]. Peripheral fatigue is related to decreased muscle strength production caused by processes distal to the myoneural junction [31]. The concept of peripheral fatigue originated from studies carried out in the 1920s [32, 33, 34]. These studies led to the conclusion that, just before the end of the exercise, the muscles’ requirement for oxygen exceeded the heart’s ability to supply that oxygen. This process develops an anaerobiosis in the muscles in activity, causing the accumulation of carboxylic acid. Due to this change in the intramuscular environment, the continuation of the contraction becomes impossible, and therefore, the muscles reach a state of failure. These pioneering studies claim that fatigue is the result of increased intramuscular carboxylic acid, which is produced within the body only under anaerobic conditions [1]. These conclusions were supported by the fact that exercise performance improves with oxygen inhalation [35]. Furthermore, the authors also concluded that the main limiting factor in exercise tolerance was the heart’s ability to pump blood to active muscles. Thus, fatigue is possibly a consequence of the heart’s inability to supply oxygen and the cardiovascular system not being ready to remove waste through the oxidation of active muscles [1]. Based on this perspective, the cardiovascular system appears to restrict performance due to the difficulties induced by the breakdown of the supply of blood, nutrients, and oxygen to the active muscles [36, 37]. Insufficiencies in the heart’s pumping capacity, as well as the reduced density of capillaries, can limit the amount of ventilated blood that reaches the muscles, consequently limiting performance. This theory, known as the anaerobic/cardiovascular/catastrophic model of human performance exercise, predicts the failure of cardiac homeostatic balance [32, 33, 34]. Although this model has been criticized, this theory has prevailed and is probably still the most cited theory of exercise-induced fatigue, but some limitations are known and have been the subject of previous analysis.

An investigation [38] concluded that the maximum cardiac output limit of the heart was reached via the evolution of myocardial ischemia, when the heart loses the ability to pump more blood because it has reached the rate of oxygen consumption limit. Additionally, this investigation showed that obtaining a higher flow rate limits the blood flow to the muscles in activity, inflicting the anaerobiosis and limiting the capacity to remove lactic acid. The increase in the concentration of lactic acid directly influences the contractile capacity of muscle fibers, presenting an association with mechanisms that induce muscle fatigue. In addition, the same authors also considered the prospect that myocardial ischemia, as a result of reaching maximum cardiac output, which is a limiting factor in exercise and a threat to the integrity of cardiac tissue, can be avoided due to the existence of a governor, located in the brain or the heart, which protects from possible damages.

Even so, there is still a lack of scientific evidence showing that muscles’ energetic profile actually becomes anaerobic during exercise and close to fatigue; or even that oxygen consumption or cardiac output consistently reaches a peak. This peak would be a requirement for its implication in fatigue during maximal intensity exercises [1]. In addition, a healthy heart, even during maximal intensity exercises, does not assume the existence of myocardial ischemia. Upon that, the hypothesis about an existing regulator in the brain or in the heart lacks scientific support. The model also suggests that peripheral fatigue events would lead the brain to recruit additional muscle fibers in an effort to help these fatigued fibers. Thus, to maintain the intensity of the exercise, it will be necessary to engage more and more available muscle fibers at their maximum capacity. However, this prediction is contradictory to other aspects of the model, as the continuous muscular recruitment should aggravate the metabolic crisis that the model foresees to be the reason for the end of the exercise [29, 30]. The main issue of this theory is that fatigue is a catastrophic event, sustained by an assertive response, leading to the total failure of the active muscles to continue to produce strength. However, catastrophic muscle or organ failure clearly does not occur in exhaustion for healthy individuals during any type of exercise [1]. Additionally, for this model, fatigue is shown from the perspective of an exhaust failure system. However, skeletal muscle fibers are never fully recruited during exercise; muscle adenosine triphosphate (ATP) never falls below 60% of resting levels, and glycogen concentration decreases but is not depleted during exercise [39]. Even more, in many circumstances, fatigue occurs before high concentrations of metabolites, such as lactate, H⁺, extracellular K⁺, without disturbances in muscle Ca²⁺ kinetics and without high core temperatures or significant hypohydration [40].

All these observations contradict the prediction of the peripheral linear catastrophic model, which states that some type of homeostatic failure should occur to cause fatigue. However, the importance of the model peripheral component remained. In the Hill’s model (presented in the beginning of the twentieth century) [32, 34], the physiological aspect is accepted; although it proves unable to respond to the complexity of the fatigue phenomenon in a broader scope. The model considers a refutable role of the brain to disrupt myocardial ischemia; however, it ignores the role of neural control over all physiological systems [1]. While peripheral fatigue occurs through processes outside the CNS, it is believed that the origin of central fatigue lies in the CNS, with the loss of muscle strength occurring through processes proximal to the myoneural junction. Specifically, this refers to sites within the brain, spinal nerves, and motor neurons, and it is related to instances in which the CNS presents a decrease in neural impulse to the muscle [41]. Central fatigue is perceived as the failure of the central nervous system to drive the muscle to its maximum, resulting in some loss of strength [42]. The decrease in strength/performance production [43] is largely justified within the central nervous system (brain and spine—central) and anywhere outside the central nervous system (e.g., peripheral muscle). Comparatively, little research has been done on the role of the CNS in fatigue until the last decades [44], which is curious considering that it has long been suspected of being a central component of fatigue [1]. The impact of the research findings on peripheral fatigue and the limitations to measure central fatigue due to the lack of objective and direct tools explain the current research gap in this field. In fact, central fatigue is usually only accepted when experimental findings do not support any peripheral cause of fatigue [44].

The central nervous system plays an important role in maintaining homeostasis [45]. Therefore, the motor component of the brain is responsible for the production of motor drive and the recruitment of motor units during exercise [45]. Thus, the brain takes control of cognition and recognition of physical sensations that are perceived as fatigue. Perceived fatigue results from exercise, and it is felt as a “sensation” (common/frequent) during exercise. The workload can create a sensation so intense that it is perceived as a need to reduce the strength to successfully complete the activity (i.e., pacing). In some cases, it may be necessary to stop exercising altogether if the sensations felt are too intense [46]. For this reason, the various stages that athletes go through during exercise are indicative that physiological mechanisms are not the only ones responsible for regulating exercise intensity. Also, humans exhibit an anticipated aspect of exercise regulation, possibly with regard to factors such as perception of the effort required for the task, and motivation [1]. Physical and biochemical changes during exercise are physiological aspects that naturally must be considered. However, perceived fatigue should also be carefully considered due to the influence in behavior/performance. Therefore, it is important to study perceived fatigue with similar importance [1, 45, 46]. Considering that the catastrophic failure of the system does not occur, there is a possibility for the appearance of a psychophysiological model [16].

2.2 Psychophysiological evidence

The inability of the peripheral and central fatigue processes to convincingly explain sport and exercise fatigue allowed the researcher to predict explanations for the fatigue phenomenon [29, 30]. An interesting perspective that has recently emerged is the concept of the brain acting as a central regulator of the exercise performance [4].

As mentioned above, the peripheral catastrophic model remains the dominant model, and this is essentially due to the modifications made to the Hill’s model [32, 33, 34] with the incorporation of factors such as energy supply and depletion [47]. In fact, the introduction of energy supply to the model suggested that high-intensity exercise was due to the inability to provide ATP at rates fast enough to maintain the exercise high intensities [47]. Based on this model, the training process and the diet generate an increase in the storage capacity (for example, glycogen), and the increased use of metabolic substrates during exercise may result in a higher production capacity of ATP. Controversially, in the variation of energy depletion of the model, it was suggested that the amount of carbohydrates was the limiting factor [47]. This is probably due to the finding that fatigue during prolonged exercise is strongly associated with significant reductions in the liver and muscle glycogen [37, 48]. In addition, improvements in tolerance to hypoglycemia as a result of exercise allow exercise to continue [48]. Nevertheless, none of the models are fully accepted.

The concept of the existence of a central regulator, responsible for regulating muscle metabolic activity and performance via peripheral afferent feedback, was reintroduced by Ulmer in 1994 [49]. The author suggested that this central regulator anticipates the end point of an exercise. Anticipation was based on previous experience of the same exercise or knowledge of the duration of the task, which regulates the metabolic demand from the beginning of the exercise. This allows the task to be completed without catastrophic physiological failure. The control of the metabolic demand regulated by the brain is called teleanticipation. This central governor evidenced by Ulmer [49] has been analyzed in several research studies [40, 41, 45, 50, 51]. These studies were the starting point for the appearance of the so-called anticipatory feedback model of exercise regulation [4], which constitutes a psychophysiological approach of the fatigue phenomenon. This model assumes that the exercise is self-regulated from the beginning by the athletes based on previous experiences, knowledge of the expected distance, duration of the current exercise, and afferent physiological feedback regarding some variables (i.e., muscle glycogen levels, skin and body temperature) [40, 41]. Processing this information allows the brain to predict and regulate the most appropriate exercise intensity allowing an optimal performance without serious homeostatic disruptions [51]. These predictions are similar to the model that classified the perceived effort (RPE). Moreover, the physical, mechanical, and biomechanical variables required during exercise are constantly monitored by the brain, and it is through this afferent feedback that the athlete’s conscious RPE arises. During exercise, conscious RPE is continuously compared with standard RPE and will progressively increase and reach its desired maximum at the expected end of the exercise. The intensity of the exercise is modulated according to an acceptable level that the brain interprets as tolerated, taking into account the continuous comparison between the standard RPE and the real, conscious RPE [50, 52].

The anticipatory feedback model defends that fatigue, instead of a physical state, is a conscious sensation generated from the interpretation of subconscious regulatory processes [45, 51]. It is also suggested that RPE is not simply a direct manifestation of afferent physiological feedback, but that it also plays an important role in preventing excessive intensity of exercise duration. It acts as the motivating element behind the athlete’s decision to completely stop the exercise or adjust the intensity to guarantee its completion without significant or harmful physical damage [4]. Despite the lack of experimental research on the subject, some phenomena support this fatigue model [53].

2.3 The concept of pacing

Recently, numerous studies investigated the interaction between cognition and sports performance [54, 55]. The pacing behavior has been widely identified as an essential component of success in many sports and is directly related to a high spectrum of cognitive skills [56, 57, 58, 59, 60, 61]. Pacing has been described as a multifaceted process that requires a set of decision-making in which athletes need to decide when and how they will distribute their available energy throughout an exercise [60, 62, 63].

The ideal pacing behavior in time trial competitions is characterized by: the balance between the quick start to make optimal use of energy resources; preventing negative changes in performance resulting from early fatigue; and inefficient energy losses associated with speed fluctuations during the race [64]. To determine the most appropriate pacing behavior, a set of variables (i.e., biomechanical, physiological, psychological, and environmental) [46] are crucial to maintain internal homeostasis [65] and to avoid premature burnout [66, 67].

The concept of pacing supports the anticipatory feedback model and cannot be investigated from a purely physiological perspective [62]. The effort distribution is part of the exercise, which suggests that voluntary behavior (effort) may limit performance rather than the absolute capacity of a single physiological system [46]. The role of a central process and how it will be executed must be considered when developing a pacing behavior.

A heuristic model of decision-making was developed to integrate the theories of decision-making and pacing, in which heuristics were considered intuitions that require low cognitive demands [20]. However, the heuristic decision-making model did not consider the connection between perception and action that takes place in tactical pacing environments, in which some actions depend on the opponents’ behavior [20, 63, 68]. Later, a detailed explanation of the pacing phenomenon emerged and was presented as a behavioral result of the decision-making process and included human-environment interactions. Pacing leads athletes to make decisions in complex and demanding environments, where they are successively encouraged to modify, choose, and evaluate their behavior [69]. The brain and cognitive processes interact and act as an information processing system [70].

The competitive environments such as the stage of competition [71], the importance of competition, and the probability of qualifying time [72] can modify the athlete’s pacing behavior. During the competition, opponents are the most common affordances. However, there are other environmental factors that can influence the pacing behavior of athletes. Factors such as music [73], performance feedback [74], and weather conditions [75] can lead to voluntary reductions in exercise intensity. These reductions in intensity occur before any real physical need to do so and before the performance compromise occurs as a result of any failure of the physical system [76, 77].

The presence of pacing behavior in sports is important regarding the view of the anticipatory regulation proposed for the performance of the exercise. It seems that athletes perform the exercise less effectively when performing an exercise that is unfamiliar and whose demands are not entirely clear [78, 79].

Changes in exercise intensity during resistance exercises were reported in the initial phase of exercise before any peripheral physiological cause of fatigue [80]. These data suggest that the modification of exercise intensity (pacing) during exercise occurs in anticipation and not as a result of stress or failure of the physiological system [62]. Thus, the pacing strategies may be used to guarantee the completion of the exercise without any physical damage. The previous experience and knowledge of the demands of the exercise will play an important role [62].

The use of pacing strategies during exercise can provide support for components of the anticipatory feedback model [81, 82] as well as refute aspects of the peripheral linear catastrophic model. During the self-regulated exercise, it is observed that the pacing behavior depends on the environment, the demands, and objectives of the exercise and the afferent physiological feedback [83]. This is in agreement with the anticipatory feedback model. If an athlete’s pacing behavior is determined by the accumulation of metabolic products or depletion of energy reserves, as predicted by the peripheral linear catastrophic model, athletes would always begin exercising at an unsustainable pace [49, 74, 84, 85, 86]. Gradually, they would slow down due to the negative effect of peripheral variables, which is not actually put into practice. The peripheral linear catastrophic model states that the only possible stimulation behavior in exercise is linear [27, 29, 30, 45, 67]. The model simply does not allow the existence of other strategies. However, the evidence for these other strategies is abundant [16, 19].

Previous exercise knowledge/experience can be important information that the brain uses to select a more appropriate exercise intensity. Research on the use of pacing strategies in exercises has confirmed that the precision ability of pacing is improved with training and experience [84, 87].

2.4 The end spurt phenomenon

The end spurt phenomenon supports the anticipatory feedback model [23] and is characterized by a substantial increase in the intensity of the exercise when it approaches the end. This model disregards the effort during the entire exercise. Throughout the exercise, there is often a level of uncertainty about the precise end point of the exercise period and the type of effort that will need to be spent until the end. These aspects are responsible for influencing the athlete’s pace and can, at any time, force the athlete to make changes in the pace, which cannot be predicted before exercise. This type of uncertainty can result in the maintenance of a motor unit and metabolic reserve throughout the exercise [4, 82, 88]. The athlete cannot be sure of what can happen in the rest of the exercise period and (unconsciously) retains some type of reserve to remain prepared to respond to any potential physical challenges. This may allow the accomplishment of the exercise without significant interruption of homeostasis. When the end of the exercise approaches, the uncertainty decreases and the accumulated reserve is no longer accurate, so that the athlete can significantly increase the metabolic demand increasing speed/power. Actually, this is a possible evidence that fatigue is not caused by the inability of muscles to produce strength [23].

2.5 The knowledge about the exercise duration

Knowledge of exercise duration as a regulator of exercise performance plays an important role and is supported by investigations about the knowledge or no knowledge of the task duration [1, 79, 89, 90, 91]. This type of research is usually called “deception,” in which participants believe that the exercise will last for a certain length of time; however, at the end of that period, they are asked to continue exercising. In one of the first researches on the topic [90], participants were asked to run on a treadmill at 75% of their maximum speed. However, in the first phase, they were asked to run for 20 min and were interrupted after 20 min. In a second phase, they were asked to run for 10 min, and at the end of this period, they were asked to run for 10 min more. In a third phase, they were asked to run, but were not told for how long (they were stopped after 20 min). All phases were performed at the same running speed and lasted 20 min. The results indicated that the participants’ RPE had increased significantly between 10 and 11 min in the 10 min deception. The deception occurred immediately after revealing the information that the participants were required to continue the exercise for a longer time. These changes in the perception of effort and pleasure occurred despite the fact that there were no changes in running speed or in physiological responses to the exercise period. The significant increase in RPE after the participants were asked to prolong the exercise was also found in another research with similar protocols [91], reporting that the effect also increased in the last minutes of exercise, probably because the participants were aware that the exercise was close to ending. These findings are related to the end spurt phenomenon. An increase in feelings of pleasure at the end of the exercise may explain the end spurt happening. Furthermore, there was no increase in the effect on the trial when the participants did not know the duration of the exercise, and the effect continued to decrease throughout the trial [91].

A recent study [79] assessed how the manipulation of knowledge about the duration of a training task restricts the pace and tactical behavior of soccer players during the performance of small-sided games (SSG). Players were instructed to play the SSG for 10 min, but after completing the 10-min game, they were asked to play for another 10 min, and in another situation, they were previously informed that they would play for 20 min. The results indicate that the first 10 min of each scenario had a greater physical impact regardless of the initial information that had been revealed. During that time, tactical behavior has also showed greater variability. In addition, there was an increase in distance from teammates during the second 10-min period in which the duration was fully known. That may be due to a smaller pacing behavior. This study showed that prior knowledge of the duration of the task led to different physical and tactical behaviors of the players, and these data have been corroborated by other investigations [92, 93, 94]. These findings confirm the possibility of changes in the pacing patterns of the players, as a consequence of the knowledge of the duration of the task that leads to consider the possibility of the nonlinearity of the fatigue effect previously reported in other studies [23, 95]. These data suggested that the knowledge of the exercise duration assumes a fundamental role for the adequate regulation of the exercise performance, as in the anticipatory feedback model. The increases seen in athletes’ RPE when deception is revealed may reflect an interruption of the feed-forward/feedback mechanism, which is fundamental to RPE as suggested by other studies [4, 96]. Moreover, it is also verified that both the RPE and the physiological responses (oxygen consumption, heart rate) present lower values when the duration of the exercise is not known compared with the moments when the duration is known. However, no significant differences in exercise intensity were found [90, 91]. Thus, these responses may reflect a subconscious improvement in the effort economy in order to retain energy due to the unknown duration of the exercise period. That said, knowledge of the end point of the exercise plays a great role in the perceptual and physiological responses to that same period of exercise [97]. This fact is further evidenced by the observation that the responses of the RPE to the exercise are robust when the duration of the exercise is known, even when no information is provided to the athlete about the exercise [85].

Research results related to exercise duration prior knowledge provide additional evidence on some processes by which athletes can retain physiological reserves during exercises of uncertain duration [91]. These findings provide support for a central role of the CNS in regulating exercise performance [98], probably to ensure the maintenance of homeostasis and the guarantee of an emergency “reserve” of energy/physical capacity [12, 91].

2.6 The relationship between RPE and performance

The perceived effort during the exercise is reflected in changes in the sensation to regulate the athlete’s physical integrity [99]. The output (perceived effort) is based on a combination of sensory inputs and cognitive processes [100]. One of the most accepted parameters during the exercise is the RPE response, which represents a sum of afferent feedback signals [100] and supra-spinal mechanisms [86]. Based on the principles of self-regulation, it is suggested that the use of RPE methods to monitor training presents itself as an effective tool for all types of exercise.

RPE has been recognized as a valid and reliable indicator of the level of physical effort by the American College of Sports Medicine. The RPE characterizes the conscious perception of the effort experienced during the exercise, which gives it a considerable practical value for the athlete. Thus, exercises that require higher levels of energy expenditure and physiological effort usually present higher RPE. Furthermore, previous studies [4, 20, 63] reported the existence of a relationship between variations in the RPE during the exercise and the duration of the exercise, which highlights the assumptions of the anticipatory feedback model. They also suggest that the RPE is effectively a crucial regulator of the exercise performance. Additionally, the suggestion that RPE may vary from the beginning of the exercise through changes in the ambient temperature and intensity of the exercise [4, 101] before effective physiological changes supports the role of the RPE in anticipatory regulation of the exercise. This evidence suggests that RPE may not be a direct reflection of the athlete’s physiological state during exercise, but rather an anticipatory sensory regulator of exercise performance. Upon that, the RPE may undergo variations during the exercise in anticipation of the occurrence of physiological changes and not because of these changes. This supports the fact that the nature of fatigue should be considered as previously reported [90].

2.7 Other approaches

An interesting alternative to the brain regulation model has been proposed [11]. In this approach, there is an acceptance that the brain regulates muscle recruitment and limits performance; however, there is also reticence about the need for a central regulatory governor. This perspective suggests that the search for a central governor in the brain’s subconscious may be similar to current reductionist approaches that search for a single cause of fatigue. The anticipatory feedback model states that a central regulator in the brain maintains subconscious control over skeletal muscle fibers recruitment during exercise. However, the presence of a single region of the brain, exclusively dedicated to regulating exercise performance, is highly unlikely. It is antagonistic regarding everything known about the functioning of the brain as an integrated organ of maximum complexity where each region contributes to the general functioning of the brain [62]. This may also explain why the specific region of the brain considered the central governor was not found. This model also states that the perception of effort is fundamental to demote the individual to continue in dangerous levels of conscious exercise that can be theoretically redundant. This is because the subconscious regulator will prevent athletes from exercising at a dangerous level regardless of the motivation that may exist to continue. However, another author [4] states that the anticipatory feedback model could exist without the perception of effort being considered. An alternative suggestion was given with a simplified model that helps to explain some of the evidence attributed to the anticipatory feedback model. This model determines that the end of the exercise occurs when the effort required to continue exercising is similar to the maximum effort that the individual is willing to provide or when the individual believes that he/she has provided a true maximum. Therefore, the subject realizes that it is not viable to continue exercising [11].

The increase in the effort that the individual is willing to put into the exercise will improve his tolerance, as long as it does not exceed what the individual understands as his maximum effort [11]. The importance of the perception of effort remains clear, but the existence of a central regulator in the brain is not necessary. Additionally, it has been suggested that the gradual increase in RPE over time and at different rates in response to changes in exercise intensity and ambient temperature can be explained by other factors; a central regulator that uses perceived exertion as a mechanism of security would be an insufficient explanation. The RPE is generated through signals originating in the CNS, specifically referring to prolonged submaximal exercises with a constant workload [31]. These data are highlighted and refuted in other investigations [10, 11]. It was demonstrated that the RPE suffered changes almost since the beginning of the exercise as a result of the verified differences in the exercise intensity and in the ambient temperature. In addition, it is important to note that the increase in the CNS motor commands could not happen without afferent sensory feedback, which is similar to the anticipatory feedback model [10].

3. Conclusion

This chapter provided a broader understanding of the cognitive basis of the psychophysiological phenomenon during the exercise, bringing together concepts such as pacing behavior, decision-making, self-regulation of effort, prior knowledge of the duration of the task, and perception of effort. This reinforced the role and contribution of the cognitive component in the pacing behavior. Furthermore, the development of fatigue during exercise seems to result from a complex interaction between the physical and psychophysiological components responsible for changes in the exercise intensity, and it can be pointed out that central and peripheral fatigue can help to exercise the intensity regulation at the beginning of a rhythmic self-paced. Also, the perceived responses may be of higher importance to control the intensity of the exercise, especially in the final phase, this results from the attempt to retain a reserve of energy that allows maximum effort in the end. About the prior knowledge about the task duration, it can create a greater capacity to regulate the effort, leading athlete to better manage energy reserves throughout the exercise. It would be also interesting to continue to analyze the impact of psychophysiological factors on the perception and regulation of fatigue by team sports players according to recent studies of psychophysiological fatigue.

Acknowledgments

This work is supported by national funding through the Portuguese Foundation for Science and Technology, I.P., under the project UID04045/2020.

Conflict of interest

The authors declare no conflict of interest.