A systematic literature review was conducted to identify psychological determinants of endurance performance. First, this review identified 25 studies that examined the effects of practical psychological interventions on endurance performance. These psychological manipulations were judged to be ethical, feasible and accessible to a sport practitioner, coach or athlete. Twenty-one additional studies were identified that drew attention to other psychological factors that affect endurance performance.
Practical Psychological Interventions
This review found substantial support for using practical psychological interventions to improve endurance performance. Association, dissociation, goal setting, hypnosis, imagery, pre-performance statements, PST packages and self-talk were found to improve performance in endurance tasks. Of the 24 studies that aimed to improve endurance performance, 22 found that at least one intervention improved performance. With the exception of research conducted on association and dissociation, however, none of the studies compared the effects of these different interventions on endurance performance. For example, PST packages were not compared with their individual components (i.e. goal setting, imagery, relaxation or self-talk), and only one study  compared a PST intervention (positive self-talk) with alternative interventions (association and dissociation). Therefore, and because only one study  was classed as strong in quality, it is difficult to draw a strong conclusion about whether one practical psychological intervention should be chosen over others. There was, however, consistent support for PST packages, with five studies finding that PST packages improved endurance performance across three sports, with athletes, in real-life and simulated competition, and in multiple posttests [39–41, 45, 48]. The relative contribution of each intervention component is not known , however, and support was also found for goal setting, imagery and self-talk interventions in isolation. Therefore, a PST package might be more time consuming for an athlete without further improving their endurance performance. A cautious comparison of effect sizes and PND values does not suggest that there are substantial additive effects, although teaching multiple psychological skills might be advantageous if it allows athletes to choose one or more psychological skills that complement their needs and preferences.
Psychological Mediating Variables
Although many practical psychological interventions improved endurance performance, little is known about the psychological mechanisms underlying these improvements. Surprisingly, only three practical psychological intervention studies [44, 49, 53] appeared to target and measure psychological mediating variables. Understanding mediating variables could help sport practitioners and athletes to choose an intervention that might be particularly valuable for that athlete. For example, an intervention that increases self-efficacy could be useful for an athlete who doubts the attainability of their goals during the most demanding periods of a race. Additionally, understanding mediating variables could help a coach or practitioner to adapt the intervention to meet the needs of the athlete, while maintaining the ‘essence’ or intention of the chosen intervention. Measuring mediating variables could also help researchers understand why an intervention was not efficacious for a proportion of participants—that is, the intervention might have had an inconsistent effect on the mediating variable. The findings of this review suggest that practical psychological interventions aimed at increasing motivation, increasing efficacy strength or reducing perception of effort could improve endurance performance. Researchers could therefore design an intervention that targets these psychological factors or examine the effect of an intervention on measures of these psychological factors. Psychological theories could also determine which psychological factors are targeted and measured. As explained in Sect. 4.3, the lack of theoretically informed interventions could account for the paucity of studies investigating psychological mediating variables.
Placebo Control Conditions
Increased expectations of performance improvement might account for the effects of some psychological interventions. The placebo effect refers to a favourable outcome that arises purely from a person’s belief that they have received a beneficial treatment . A recent literature review  reported placebo effects of varying magnitudes from studies that measured the performance of sub-elite athletes in strength, pain tolerance and endurance tasks ranging from 1 km running to 40 km cycling. Similarly, a recent meta-analysis of 14 studies reported a moderate effect size (0.40) for the placebo effect across exercise modes and performance variables, and a small effect size (0.22) for endurance exercise . In the present review, nine of the 20 effect sizes [45, 46, 61, 62] calculated for practical psychological interventions in group-design studies without a placebo control group were less than 0.22. As well as expecting to improve, participants might have believed that the researchers hoped or expected that they would perform better post-intervention, and they might therefore have offered different amounts of effort in these performance tests (i.e. demand characteristics) . It is difficult to judge the contribution of expectation effects in this review, because only two studies [47, 53] included a placebo control group. Additionally, some of the included studies [39, 50, 54] appeared to heighten participants’ expectations of performance improvement through the wording of the instructions they gave . Furthermore, relatively few studies used research assistants who were blinded to the participants’ allocation, deceived participants about the research question, played down the likely benefits of the intervention (if necessary) or looked at the endurance performance of high-level and motivated athletes in competition; each of these factors might have increased the likelihood of expectation effects in the included studies . It is acknowledged that enhanced expectations can be an important component of a performance enhancement intervention. Nevertheless, it is important for the credibility of sport psychology as a profession that recommended psychological interventions are shown to have greater effects than the expectations they instil in athletes. Unlike other sport science disciplines (e.g. nutrition), psychologists are unable to create a placebo treatment by removing the key ingredients from an intervention. We therefore encourage sport psychologists to compare psychological interventions with alternative control treatments  or inert solutions, pills or capsules that are described as beneficial for endurance performance. Alternative control treatments are similar in duration, perceived value and procedure to the experimental treatment, but they target unrelated dependent variables . We also suggest that researchers measure each participant’s expectation of performance improvement .
Limitations of Practical Psychological Intervention Studies
Additional limitations were consistently identified across the included studies investigating the effects of practical psychological interventions on endurance performance. Only six of the 18 studies that chose group designs reported using random assignment to experimental and control groups, which is an indicator of strong experimental research . None of those six studies, however, measured the endurance performance of athletes in competition. Andersen  argued that few randomized, controlled trials have shown that psychological interventions improve the performance of athletes in competition. Further, there are few sport psychology intervention studies that have measured the performance of athletes in competition . Illustrating this point, only two of the included studies [44, 45] examined the effects of an intervention for athletes in real-life competition. These interventions were inconsistent in improving endurance performance, perhaps because of confounding variables (e.g. the specific competition) or because the margins for improvement are small for trained athletes in competition. Alternatively, the benefits of practical psychological interventions for competitive athletes might not be observable in their short-term competitive performances. Instead, psychological strategies that help athletes to improve their performances in training—where performance incentives are likely lower—could lead to meaningful long-term improvements in competitive performances through a physiological mechanism (e.g. adaptation) or a psychological mechanism (e.g. increased self-efficacy). Nevertheless, research that measures endurance performance in competition could complement well-controlled studies by demonstrating whether research findings generalize to ‘real-life’ performance. It is acknowledged that gatekeepers to athletes, such as a coach, might be hesitant to accept that only a proportion of the athletes will receive a potentially beneficial intervention. Researchers and gatekeepers might therefore agree that control participants will be offered the intervention after the study is completed.
The long-term benefits of practical psychological interventions are unclear. For example, none of the studies that delivered instructional or motivational statements before performance included multiple posttests, and the novelty of these and other interventions might wear off. Alternatively, continued practice of a psychological skill could lead to additional improvements in endurance performance. Identified group-design studies did not include more than two posttests, and the second posttest was conducted up to 1 month after the first [45, 60]. Single-subject designs included up to nine post-intervention performances , and these studies typically demonstrated that improvements in endurance performance were maintained. However, the effects of practical psychological interventions on endurance performance after three or more months are unknown. It would be valuable to know if participants maintained their improvements in endurance performance, but it would also be difficult to attribute long-term changes in endurance performance to the intervention. Therefore, it would also be valuable to know whether participants continued to use the taught intervention after they finished their commitment to the study [39, 40]. None of the studies reported this information.
When participants were required to make a commitment to a practical psychological intervention, 12 of the studies (67 %) did not report the numbers of withdrawals and dropouts (and the reasons for them). This information is important because participants might drop out when they do not believe that an intervention will be beneficial, when an intervention is not enjoyable or when an intervention is perceived to be inconvenient or too much work. Therefore, dropouts could lead to a greater reported mean improvement in the experimental condition. Furthermore, only three studies referred to the experience or qualifications of the person performing [44, 55] or overseeing  the intervention. This information would be valuable so that readers can judge whether the expertise of this person influenced the effects of the intervention. Similarly, practitioners could judge whether they have sufficient expertise to perform the intervention.
Additional Psychological Determinants
External motivators, mental fatigue, priming, emotion suppression, efficacy strength and the experimenter’s sex affected endurance performance. In particular, experimental research consistently demonstrates that mental fatigue undermines endurance performance, whereas external motivators can have a beneficial effect on endurance performance. Mental fatigue, induced by prolonged and demanding cognitive tasks, consistently increased perception of effort and had a detrimental effect on endurance performance [75–77]. As external motivators, head-to-head competition [31, 66, 67], verbal encouragement [69, 70] and a combined intervention of financial incentives and verbal encouragement  improved performance in various endurance tasks, although the introduction of a financial incentive did not affect endurance performance . It is difficult to establish how these interventions improved endurance performance and to explain the inconsistencies in the results, because the effects of the interventions on psychological variables were not measured in these studies. Although head-to-head competition and verbal encouragement might increase participants’ motivation to perform, these interventions might also act as sources of self-efficacy (vicarious experience and verbal persuasion, respectively, e.g. Bandura ) or they could reduce perception of effort. Measuring these mediating variables could clarify the psychological mechanisms underlying the observed change in endurance performance. Finally, endurance performance can be affected by priming interventions [52, 78]; additional research is required, however, to determine whether these interventions offer a feasible means of performance enhancement.
Only three practical psychological interventions [49, 53, 55] tested (or were clearly informed by) a specified psychological theory. Psychological theories can help researchers to identify key factors that determine behaviour (e.g. endurance performance) and that can be targeted by novel or refined interventions —that is, theoretically informed studies could identify the psychological mechanisms through which interventions affect endurance performance, and researchers and practitioners could target these mechanisms with an intervention. Theoretically informed interventions might therefore produce greater or more consistent effects .
Although psychological theories have illuminated the effects of psychological factors on perception of effort [90, 91], the psychobiological model of endurance performance is the only model based on psychological theory that specifically explains how psychological factors affect endurance performance. The psychobiological model is based on motivational intensity theory , and it argues that perception of effort and potential motivation are the ultimate determinants of endurance performance (for an overview of the psychobiological model, see Smirmaul et al. ). Perception of effort is the conscious sensation of how hard, heavy and strenuous the exercise is , and potential motivation refers to the greatest amount of effort that a person would be willing to offer to satisfy a motive . The psychobiological model predicts that any psychological or physiological factor that increases potential motivation or reduces perception of effort will improve endurance performance, and that any psychological or physiological factor that reduces potential motivation or increases perception of effort will undermine endurance performance . Similarly, the psychobiological model suggests that all psychological and physiological manipulations that affect endurance performance do so because they affect either potential motivation or perception of effort . In support of the psychobiological model, motivational self-talk [49, 53], PST  and dissociation  reduced perception of effort and improved endurance performance; mentally fatiguing tasks [75–77] and emotion suppression  increased perception of effort and undermined endurance performance; and subliminally presented visual cues  influenced both perception of effort and endurance performance. Future research that includes psychological mediating variables (e.g. perception of effort) could clarify the psychological mechanisms through which psychological manipulations affect endurance performance. Although the psychobiological model emphasizes perception of effort and potential motivation, researchers are encouraged to include additional psychological mediating variables, such as self-efficacy, that could shed light on the psychological mechanisms underlying changes in endurance performance.
In the present review, five studies explicitly examined the effect of traditional associative and dissociative attentional strategies on endurance performance [47, 54, 56–58]. Traditional classifications of attentional focus have proposed that athletes who use an associative strategy monitor their bodily sensations and use this feedback to adjust their pace; athletes who dissociate direct their attention away from these uncomfortable sensations . More recent theoretical perspectives, however, argue that attentional strategies can be categorized more precisely [9, 99]. Brick and colleagues  recently proposed a five-category model of attentional activity. According to this model, athletes ‘actively self-regulate’ when they attempt to control or monitor their thoughts, feelings or actions. This associative strategy allows an athlete to optimize their pace or efficiency of movement without elevating perception of effort. Examples include self-talk and relaxation strategies that are used during endurance performance  and pre-performance; mental preparation strategies, such as setting process goals ; and visualizing successful execution of skills . The findings of the present review suggest that active self-regulation strategies could be valuable for athletes who aim to optimize their endurance performance.
Implications and Recommendations for Practice
PST interventions involving imagery, self-talk and goal setting offer a promising tool for improving the performance of endurance athletes. It is unclear whether teaching multiple psychological skills is more beneficial than teaching one psychological skill, particularly when the intervention is tailored to meet the needs of an athlete. As mental fatigue increases perception of effort and undermines endurance performance, endurance athletes should avoid mentally draining activities before they compete. For example, endurance athletes might avoid thought suppression strategies (e.g. thought stopping) and situations that require them to suppress their emotions or behaviour (e.g. interviews with the press) before they compete, because they could be mentally fatiguing [81, 100] and therefore detrimental to endurance performance. As a further suggestion, coaches could use head-to-head competition and verbal encouragement during training to facilitate maximum effort when required (e.g. sprint interval training).
Music could be valuable during training, as well as events and competitions that permit its use. Although excluded from this review, there is substantial evidence that music elicits positive affect and feeling states during exercise at all intensities, reduces perception of effort during exercise below the lactate threshold and facilitates endurance performance [27, 28]. Self-selecting music for its motivational qualities is encouraged [27, 28]. The benefits of music, however, should be weighed against potential risks, such as not hearing safety-related cues (e.g. road traffic), distraction from technique or distraction from pacing-related cues (e.g. bodily sensations and other competitors). Placebos and various forms of deceptive feedback can also be used to improve endurance performance; the practical application of these manipulations during training and competitions, however, raises significant ethical issues [26, 101].
Implications and Recommendations for Research
Theoretically driven studies could systematically examine the mechanisms through which psychological interventions affect endurance performance, and they could therefore encourage development and refinement of performance enhancement interventions that have consistent and strong effects in endurance events. Research examining the effects of interventions in real-life competition could particularly add to the endurance literature. Researchers are also encouraged to compare different performance enhancement interventions, using randomized, controlled experimental designs. Inclusion of placebo control conditions could help readers to judge the effects of interventions beyond expectation effects. Furthermore, these studies should include more than one posttest, report whether participants continue to use the intervention following their commitment to the study, report the number of participants who drop out from the study and their reasons for doing so, and provide expertise-related information on the person delivering the intervention.
As an alternative to measuring performance in real-life endurance competition, researchers could use head-to-head competition and verbal encouragement to ensure that participants offer maximum effort during an endurance task. This could help researchers to test the effects of interventions when participants are in motivated performance situations. To reduce the risk of confounding variables, care should be taken to apply head-to-head competition and verbal encouragement consistently across experiment trials. For example, a research assistant who is blinded to the study aims or hypotheses could provide verbal encouragement using a consistent verbal encouragement protocol , a blinded and independent researcher could analyse audio recordings of the delivered verbal encouragement and attempt to predict the experimental conditions, and head-to-head competition procedures could be standardized .
Few practical psychological interventions appeared to be designed specifically for the demands of endurance sports. More often, interventions were informed by research on mental preparation or interventions across a range of sports. This is surprising because endurance activities have physical, technical, logistical and psychological demands that should be taken into account when an intervention is being designed . Qualitative research has drawn some attention to the demands faced by endurance athletes and the cognitive strategies used by high-level endurance athletes [104–108]. Future research could shed greater light on the demands facing endurance athletes or test interventions that are designed to help athletes to cope with these demands.
It is surprising that only four studies [75–77, 81] examined interventions that undermine endurance performance. Ethically approved future research could address the effects of other psychological states (e.g. debilitative anxiety), psychological strategies (e.g. thought suppression) and situations that athletes encounter (e.g. insufficient time for mental preparation) that could be debilitative to performance, in experimental endurance tasks.
Little is known about whether participant characteristics influence the effects of psychological manipulations. The results shed little light on whether sex [46, 60, 79, 80] or athletic ability [69, 82] are moderating variables. Nevertheless, personality type appears to affect participants’ responses to verbal encouragement , participants with high task and ego orientations respond more favourably to goal-setting interventions , and hypnotic susceptibility influences whether hypnosis-based interventions improve endurance performance . Further research on moderating variables (e.g. competitive level, competitive distance, achievement-goal orientation) could shed light on whether certain interventions are particularly beneficial for specific groups of athletes, and this evidence base could increase the effects of sport psychology interventions.
Lack of blinding procedures was often a source of bias. Researchers who are aware of the intervention status of participants might unintentionally affect participants’ performance expectations. Where resources are available, researchers are encouraged to collect data using research assistants who are blinded to treatment allocation, particularly when verbal encouragement is given during endurance performance. It is acknowledged that researchers may be unable to disguise the research question when they are testing the effects of an intervention. Researchers could therefore inform participants that they do not know what impact (if any) the intervention will have on their endurance performance , or they could include an alternative control treatment .
Finally, researchers could consider a more diverse range of sports and distances. None of the located studies examined rowing or triathlon performance in field settings, and none of the research was focused on endurance-distance race walking, speed skating or cross-country skiing. There is also a lack of studies examining the effects of interventions in long-distance events (e.g. half marathons, open-water swims, ultra-distance events); only two studies [44, 48] measured performance in endurance tasks that took longer than 1 h to complete.
Limitations of the Systematic Review
This literature review synthesized studies on the psychological determinants of endurance performance. A heterogeneous selection of studies was included, and there are insufficient studies to provide sport- or distance-specific guidance. Outcome measures that range from 100 m breaststroke swimming  to ultra-endurance events (e.g. long-distance triathlons) could satisfy our definition of endurance performance. The technical, physical, logistical and mental demands  of the included sports and distances will undoubtedly vary, and the comparability of these performance measures could therefore be questioned. Individual differences also need to be taken into account; interventions seemed influential for only a proportion of group-design participants. While the findings of this systematic review should inform evidence-based practice, practitioners interested in performance enhancement should also consider the demands of the specific sport and competitive distance, as well as the needs of the individual athlete .
This systematic review synthesized the peer-reviewed studies that have been published to date, because these studies comprise the evidence base that is available to practitioners, theorists and researchers. Publication bias might partially account for the abundance of interventions that significantly affected endurance performance, because studies might not have been put forward or accepted for publication if the examined intervention did not have an effect . Indeed, a recent study reported statistical evidence that publication bias is a pervasive problem across all areas of psychological research .
Each included study was evaluated using a modified version of the EPHPP Quality Assessment Tool for Quantitative Studies . This evaluation tool is not specific to the sport context, and it was therefore adapted. The tool evaluates information that is reported in the manuscript, and reporting practices could vary between healthcare and sport science. Nevertheless, researchers are encouraged to report randomization and blinding procedures (when performed) and the numbers of withdrawals and dropouts (and the reasons for them), because this information is important for judging bias. An evaluation tool that is specific to sport science research and is sensitive to its research practices would be valuable. Similarly, an evaluation tool that recognizes the strengths of single-subject research in sport psychology (see Barker et al. ), as well as the different quality criteria applied to these designs, would be welcomed. The tool was useful, however, for identifying common sources of bias across all of the studies, such as blinding and withdrawals and dropouts, and comparing the quality of the included studies.