Eligibility criteria

The PICO method was used in the selection of literature (Table 1). The selected literature must also be available in English and fully published in a peer-reviewed journal. Specifically, studies were eligible when the study outcomes included any form of skilled performance (see Methodology: Data Analysis) in sports, which is the main outcome of the current review. Interventions were used to induce mental fatigue prior to the test of skilled performance with all types of cognitive tasks, and the population only included athletes who are very good at sports, particularly on the court. They can be classified into various levels, such as amateur, well-trained, elite and professional etc. In addition, these studies had to include a control group that induced no or less mental fatigue than the mentally fatiguing tasks.

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Table 1. PICO (Participation, Intervention, Comparison, Outcome).

https://doi.org/10.1371/journal.pone.0258307.t001

Data analysis

To investigate a more in-depth investigation, the current qualitative review will examine the main outcomes of three aspects: offensive, defensive and decision-making skills (see details in the Introduction part). The action of engaging an opposing individual/team with the objective of scoring points or goals is referred to as offensive skill, whereas, defensive skill refers to the action of preventing an opponent from scoring [2]. In addition, athletes are usually required to pay sustained attention for making accurate, quick decisions based on the retrieval and processing of information from a dynamic environment [5, 6]. Athletes must excel in three aspects to achieve peak performance.

Search strategy and selection of literature

The following keywords, truncation and Boolean operators, were used individually and in combination in the search in five databases (S1 Table). A search was also conducted on Google Scholar and based on the sources of reference in the selected reviews for additional literature that may not have appeared in the search results using the main databases.

The PICO method was used to determine whether the articles should be included or excluded (Table 1). First, the titles and abstracts were screened. Then, potential full-text articles were identified and selected. Two reviewers worked independently in this process (S.H. and S.K.G.). Disagreements were resolved by a discussion between the two reviewers. If necessary, a third reviewer (S.K.L.) was consulted to reach a consensus. Fig 1 summarises the selection process.

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Fig 1. PRISMA summary of the study selection process.

https://doi.org/10.1371/journal.pone.0258307.g001

Protocol and registration

The protocol of methods and planned analyses applied in this systematic review was registered in PROSPERO (ref. CRD42020201715). Although there are protocols in PROSPERO that examine the effects of mental fatigue, none of them focus on skill performance in sports. Hence, the novelty of the proposed protocol was ensured.

Quality assessment

A systematic review is essential for evaluating relevant studies. Similarly, an overview should brief the assessment on the quality of the selected systematic reviews [37]. In addition, the data should be handled with care to prevent a “garbage in, garbage out” scenario that is the basis of conducting a systematic review [38].

The quality of the methodology used in this study was assessed using the quantitative assessment tool “QuaIlSyst” [39], which includes 14 items (Table 2). The scoring is based on how well a specific criterion was met (no = 0, partial = 1, yes = 2). The letter “NA” represents items that do not apply to the study design and are thus excluded from the calculation of the summary score. Each study’s summary score was calculated by adding the total score obtained and dividing it by the total possible score. The scores, ≤ 55%, 55–75%, and ≥ 75, indicate low, medium, and high quality, respectively. Any low-quality study should be excluded from the systematic review [39].

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Table 2. “Qualsyst” of quality assessment.

https://doi.org/10.1371/journal.pone.0258307.t002

Selection of literature

There were initially 1242 hits from PubMed, Web of Science, EBSCOhost (CENTRAL, SPORTDicus), and Scopus, as well as two hits from a reference search and Google Scholar. All duplicates were removed, and after screening the title and abstract, the entire text was read; ten literatures were chosen for this study (Fig 1). However, 2 out of the 13 literatures were deemed to be of low quality and were thus excluded from this review. Thus, this systematic review was based on 11 studies that focused on skilled performance among athletes in various sports. The details are presented in Table 3.

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Table 3. Overview of publications details.

https://doi.org/10.1371/journal.pone.0258307.t003

Overview of sports

The comprehensive search yielded studies involving ball sports, of which only Le mansec et al. [44] reported on an individual sport (table tennis), while the others reported on team sports (soccer and basketball). Soccer was the subject of 9 out of 10 studies on team sports [40–43, 47, 48, 50–52].

Interestingly, the majority of studies have focused on high-level athletes, including professionals [44, 47, 48, 51] and elites [40, 46]. In addition, most studies have focused more on offensive skills rather than defensive skills.

Intervention inducing and the manifestation of mental fatigue

Badin et al. [40], Moreira et al. [46] and Gantois et al. [48] and Filipas et al. [50] used a 30-minute computerised Stroop task to induce mental fatigue. Gantois et al. [48] also investigated a 15-minute exposure to examine the dose-response of the Stroop task. Smith et al. [41], Smith et al. [42], and Smith et al [43]. have investigated the 30-minute exposure of the printed version Stroop task. In addition, Le Mansec et al. [44] required that participants complete a 90-minute AX-continuous performance test (AX-CPT). Fortes et al. [47] and Fortes et al. [51] have studied the effect of exposure to social network apps on smartphones for 15, 30, and 45 minutes, in addition to a 30-minute video gaming session, which is more realistic in today’s world. In general, continuous exposure to cognitive tasks for ≥ 30 minutes successfully induces mental fatigue when compared to two 15-minute interventions, as observed in studies involving smartphone exposure [47] and a computerised Stroop task [48].

To prove the condition of mental fatigue, seven studies have chosen subjective manifestation, all of which used the Visual Analogue Scale (VAS) [40, 41, 45–49] rated from “0” to “100”, signifying “not exhausted at all” to “completely exhausted”. However, the other five studies used VAS to report motivation and no significant difference in performance was reported between the mentally fatigued and control group in the given tasks in five of the studies [40, 41, 43, 44, 52]. Interestingly, greater motivation was observed in the mentally fatigued group than in the control group. [46]; the findings demonstrated that motivation is not diminished by mental fatigue. It is consistent with the psychobiological model of endurance performance, and effects of mental fatigue on skilled performance are not caused by motivation. However, only one study reported that mental fatigue likely increased RPE [40]. Most physiological variables that were assessed were general parameters, such as heart rate and blood lactate, which did not change in the presence of mental fatigue. Interestingly, Moreira et al. [46] found a higher concentration of salivary testosterone in mental fatigue athletes. The general physiological indicators, such as heart rate and blood lactate, did not show significant differences.

Cognitive performance was measured in four of the selected studies, which included 11 experiments, to evaluate the effects of mental fatigue [46–48, 51]. The results demonstrate that the 30-minute cognitive tasks result in a significant reduction in reaction time and performance accuracy among the athletes.

Skilled performance outcome

As a prediction, the extracted literature has mainly focused on the skill demands based on the role or position of the athletes, and technical skills have been divided into two categories (offensive and defensive skills). It is worth noting that previous studies focused more on offensive skills than defensive skills. The details are presented in the following sections.

Technical skill

Overview of sports

Unsurprisingly, all of the selected 11 studies have focused primarily on ball sports (soccer, basketball and table tennis), as the cognitive demands in ball sports are extremely challenging [54, 55]. In sports, the trajectory paths of athletes’ movements and the ball are usually unpredictable; interruptions may occur, causing changes in direction, as well as various occlusions and segmentation, such as objects blocking or disappearing from view. Therefore, athletes playing ball sports must remain alert throughout the game [34, 54] that increases the chances of mental fatigue. In line with previous studies, the predominant focus on soccer athletes demonstrates that these athletes may be more susceptible to mental fatigue than other athletes due to the longer duration of exposure to cognitive tasks in ball sports than other sports [34].

Most of the selected studies have focused on high-level athletes, which suggests that the chances of mental fatigue are exceptionally high in athletes who are under extreme pressure due to high-intensity training and competition demands [13, 34]. However, studies on athletes in other categories are also important, because many studies have shown that factors unrelated to sports activities are also responsible for mental fatigue in athletes during games [47, 51, 56, 57]. Future studies should consider this particular scope.

Intervention inducing and the manifestation of mental fatigue

The Stroop task was used in most of the selected studies (see Table 3). Generally, the Stroop task is a classical method for measuring inhibitory control and sustained attention, which are executive functions [18]. According to a previous study, these functions are highly related to skilled performance in sports [22, 23, 34]; a decrease in such abilities may impair athletes’ performance. Also, in line with recent studies, a 30-minute Stroop task can be used to induce effective mental fatigue in athletes of all levels [29]. Contrastingly, McMorris [35] indicated that shorter cognitive tasks have a larger negative effect than longer ones due to learning effects. The longer duration influence was found to be more negative in the current review [47, 48]. Because the types of cognitive tasks and subsequent tests are different. Brown et al. [31] found that shorter and longer cognitive tasks did not affect submaximal exercise. McMorris [35] also raised this issue, stating that the short versus long duration of cognitive tasks should be addressed in the future.

Consistent with Smith et al. [58], the current review showed that subjective manifestation is reliable for measuring mental fatigue. Previous studies have shown that subjective ratings can detect fatigue when objective assessment is not possible [59, 60], Moreover, it has been suggested that subjective mental fatigue precedes any decrements in performance [60]. Particularly, VAS appeared to be an effective instrument, and it has been widely implemented [40, 41, 43, 44, 50, 52], as well as for minimum time spent and easy interpretation [59].

The effect of mental fatigue on sports skilled performance

Potential mechanism

Understanding the potential underlying mechanisms may assist in the development of interventions to counteract the negative effect of mental fatigue on skilled performance. To consider the mechanisms, it is inevitable to consider the perception of effort in the literature of physical performance, which has demonstrated that it is the only parameter to mediate the effect of mental fatigue on endurance performance [15, 29]. However, the current review did not find consistent evidence, as the level of RPE was not significantly different in the majority of the studies. Importantly, Moreira et al. [46] found that mental fatigue elevates salivary testosterone that may be associated with the increased errors of technical skill. Indeed, testosterone is thought to be involved in regulating activity of the mesolimbic reward system involving dopamine [66]. Moreira et al. [46] proposed that lower testosterone levels may influence dopamine transmission in mentally fatigued athletes. This finding shows that mental fatigue decreases dopamine in the ACC, impairing executive functions.

Technical skill impairment has been strongly related to attentional direction [40, 42, 43]. Indeed, mental fatigue has been shown to shift attention from goal-directed to stimuli-directed that are irrelevent to subsequent performance [16, 67]. Furthermore, it decreased the ability to anticipate ball movement and prepare to control it [43]. Smith et al. [42] even suggested that this negative effect may be more likely in the match setting, where the additional irrelevant stimuli are increased compared to a control experimental environment. Moreover, athletes must constantly monitor and adjust their skills to improve their performance. However, a previous study indicated that mental fatigue leads to an impaired capacity for performance monitoring and adjustment [68]. The ability to monitor error performance is thought to rely predominantly on intact ACC functioning [69]. Lorist et al. [68] found that people typically slow down to perform when an error occurs. Thus, when athletes are mentally fatigued, they may slow down to exert technical skills to avoid errors, allowing opponents to counterattack.

Athletes’ decision-making ability is also impaired due to the shift in attention focus. In soccer, decision making highly relies on the human brain’s ability to perceive relevant information from a complex environment while blocking out irrelevant distractions [48, 70]. This process is controlled by selective attention [71, 72]. Selectivity prevents athletes from reacting reflexively to environmental stimuli and allows them to make flexible decisions. However, it was impaired by mental fatigue.

Another possible explanation is that athletes are unable to respond to cues to prepare for upcoming decisions because mental fatigue negatively impairs preparation and planning [73]. To make a good decision, athletes should interpret and/or anticipate the movement of teammates, opponents and/or the ball etc. However, the execution of preparation and planning processes is susceptible to mental fatigue and frontal negativity [73].

Furthermore, the impairment has been attributed to the reduced ability of the visual search strategy. In team sports, mentally fatigued athletes tend to spend more time fixating on their opponents while spending less time fixating on their teammates [41, 47]. Therefore, athletes have little focus on the movement of their teammates, even though they may understand the movements of their opponents, impairing the passing decision-making skill, as evidenced by three extracted studies [47, 48, 51].

The implications and future directions

The mechanisms of mental fatigue in sports in terms of skilled performance are complicated. Understanding these mechanisms may assist in the development of future interventions. Prior cognitive tasks have been shown to activate the ACC, inducing the concentration of adenosine to rise. Indeed, the ACC plays a key role in many executive functions that are in charge of performing skills among athletes. Future studies may look into strategies, such as implementation intentions to aid in action automation; mindfulness music, and nature exposure to enhance directed attention; biofeedback to monitor/adjust performance and improve executive functions while countering mental fatigue.

The impact of duration after cognitive tasks should be considered by future studies, for the condition of metal fatigue is acute. Although the precise duration of mental fatigue is unknown, Smith et al. [58] reported the prolonged mental fatigue condition in an individual after 60 minutes in an assessment using the Psychomotor Vigilance task and VAS; the findings also demonstrate that the individual may recover from acute mental fatigue in 60–135 minutes.

The fact that most of the investigations in the selected literatures have converged on soccer suggests that soccer athletes may be more prone to mental fatigue than athletes in other ball sports, or athletes in other sports in general. Previous studies have consistently reported that soccer athletes face extremely challenging cognitive demands when competing [13, 34]. However, attention should also be given to athletes from other sports, as recent studies have shown that mental fatigue may be caused by factors unrelated to sports activities (i.e., smartphones, video playing, fixture congestion, etc.).

Most importantly, athletes who play an offensive role may be more susceptible to mental fatigue than defensive athletes in some team sports, such as soccer. This suggestion should be considered by coaches. Offensive athletes may need to do more mental training.

However, during high demand cognitive tasks, athletes are prompted to perform an executive function to maintain vigilance. If this process is prolonged, mental fatigue may set in. This knowledge provides opportunities for future studies to improve skill performance in sports by increasing athletes’ ability to perform an executive function.

Limitations

This systematic review poses a few noteworthy limitations. First, this systematic review, conducted rigorously, is not a meta-analysis. Moreover, the systematic review focuses only on skilled performance, defined as technical and decision-making skills, excluding physical performance in sports. Because three of these skilled performances are closely related and contribute to athletes’ performance, future studies should investigate them together to obtain more comprehensive results. Finally, selecting articles only written in English may limit the representation of the results even further.