Effects of Psychological Interventions to Enhance Athletic Performance: A Systematic Review and Meta-Analysis

Background Psychological interventions are commonly applied in sports to help athletes enhance their performance, but the effect psychological interventions have on actual performance is unclear despite decades of research. Objective We conducted a systematic review with meta-analyses to investigate the effects of a wide range of psychological interventions on performance in competitive athletes. Methods A study protocol was preregistered in PROSPERO, and a literary search was performed in the MEDLINE, PsycINFO, Web of Science, and SPORTDiscus databases. Psychological intervention studies were eligible by using a group design and a quantitative performance outcome with athletes competing at a regional or university level or higher. Included studies were assessed regarding intervention characteristics, research methodology, and risk of bias. A multi-level meta-analysis framework with cluster robust variance estimation was used to quantitatively synthesize the results. Results A total of 111 studies met the inclusion criteria, and 25 of these studies (37 effects) could be synthesized into five meta-analyses in which there were similarities in the type of psychological intervention, comparator, and experimental design. Meta-analyses I (multimodal psychological skills training vs control), II (mindfulness- and acceptance-based approaches vs control), and III (imagery vs control) consisted of parallel-group studies, and random-effects models were used to calculate the standardized mean difference. Meta-analyses IV (attentional focus strategies, external vs internal) and V (regulatory focus performance instructions, prevention vs promotion) consisted of counterbalanced crossover design studies, and random-effects models were used to calculate the standardized mean change using change score standardization. Significant results were found in three of the meta-analyses (I, II, and III). Psychological skills training (g = 0.83, 95% confidence interval 0.21–1.45), mindfulness- and acceptance-based approaches (g = 0.67, 95% confidence interval 0.01–1.32), and imagery (g = 0.75, 95% confidence interval 0.14–1.36) outperformed controls with moderate effects. However, when non-randomized trials and subjective performance outcomes were removed in sensitivity analyses, the overall estimates of the effect size were no longer significant in any of the syntheses. Conclusions The significant moderate effects for psychological skills training, mindfulness- and acceptance-based approaches, and imagery are not stable, and further trials with robust research methodology, such as randomized controlled trials, are requested for all types of psychological interventions aiming to enhance performance in athletes. Moreover, improved reporting standards and the provision of datasets in open science repositories are important to consider in future trials in sport psychology. Clinical Trial Registration PROSPERO CRD42017056677. Supplementary Information The online version contains supplementary material available at 10.1007/s40279-023-01931-z.


Introduction
As sport psychology moved into applied field research during the 1970s [1,2], an interest in evaluating intervention effects naturally followed.Simultaneously, advancements of meta-analytic procedures in psychological sciences enabled ways to investigate aggregated effects [3].Feltz and Landers' [4] meta-analysis of the mental practice of motor skills to enhance skill learning and performance was among the first systematic effect evaluations in applied sport psychology, and several have followed since.Two overviews of reviews were recently conducted.Lochbaum et al. [5] identified 30 meta-analyses of both correlational and intervention studies related to the sport psychology literature.Lange-Smith et al. [6] also identified 30 reviews but included various types of review methodologies in addition to meta-analyses (e.g., narrative), only including reviews of intervention studies.Thus, scrutiny of the evidence relating to psychological Extended author information available on the last page of the article However, the research question about whether and which psychological interventions have an effect on actual sport performance in samples verified as athletes still needs to be addressed with further clarity.This systematic review therefore investigates a wide range of psychological interventions with potential effects on performance in athlete samples with a verified competitive level at a regional or university level and higher.Furthermore, we examine the characteristics and quality of this research.

Data Availability and Transparency Statement
A study protocol [21] was developed using the guidelines of the Preferred Reporting Items for Systematic review and Meta-Analysis Protocols (PRISMA-P) [22,23] and preregistered in the PROSPERO database.The search strategy (Online resource file 1) and a log of rejected studies (excluded full-text decisions according to PICO [Participants, Interventions, Comparator, and Outcome]; Online resource file 2) are available as supplementary information.Meta-analytic datasets and code are available online in the figshare repository [24].We followed the PRISMA reporting guidelines for systematic reviews and meta-analyses for the final report [25,26].

Literature Search Strategy
A literature search was conducted in February 2017 and replicated twice, in February 2019 and April 2022.The searches were performed in the following databases: MEDLINE (OVID), SPORTDiscus, PsycInfo, and Web of Science.The MeSH terms identified for searching Medline (OVID) were adopted in accordance with corresponding vocabularies in PsycINFO and SPORT-Discus.As SPORTDiscus is a database specialized in sports, it includes controlled terms for several rare sports.Those sports were included in the controlled vocabulary searches because of their exploding functionality.They were not specified in the free-text searches.Each search concept was complemented with relevant free-text terms.The free-text terms were, if appropriate, truncated and/ or combined with proximity operators.No language restriction was applied at the search stage.The searches were performed by librarians at the Karolinska Institutet University Library in collaboration with GR.The search strategies are available inOnline resource file 1.Additional hand-search procedures were applied.Reference lists of included studies and references citing included studies (in Web of Science) were screened for eligibility.
Furthermore, other reviews and meta-analyses related to sport psychology were also screened for additional studies.

Eligibility Assessment and Data Extraction Procedures
Through the screening process, two authors conducted all assessments independently.There were three pairs of assessors: GR-SA, GR-MJF, and GR-TL.Disagreements at the title/abstract level were decided by a third assessor in another pair of assessors.Eligibility coding, risk of bias ratings, and data extraction were conducted by two authors independently with two pairs of assessors: GR-SA and GR-MJF.Disagreements over the full-text eligibility coding, data extraction, or risk of bias ratings were solved through discussions with the third assessor in the other pair of assessors (SA or MJF).See Fig. 1 for the PRISMA flow diagram of the eligibility process.A log of rejected studies with decisions on excluded full-text assessments can be found in Online resource file 2.

Criteria for Inclusion in the Systematic Review
Only studies written in English and published in scientific journals were considered for inclusion.Gray literature, conference papers, and dissertations were not considered.Eligibility criteria were defined using the PICO format [27] and described below.

Participants
Athletes eligible for inclusion were currently competing at a regional or university level or higher.Sufficient information regarding the performance context had to be stated in the report to verify the current level of performance.All sports were considered.Recreational, novice, junior/youth level athletes, and athletes competing on a local level were excluded.If samples were recruited from a competition (e.g., regional level) in which ineligible participants could take part (e.g., recreational participants in a marathon), additional information regarding the sample had to be described to be considered.

Designs
All group designs, both within-group and between-group (non-randomized and randomized) studies, were eligible for inclusion in the systematic review.For a study to be included in a meta-analysis, additional criteria were used (see Sect. 2.6).Single-case research designs were not included.

Psychological Interventions
Several psychological intervention techniques were included.However, for an intervention to be considered psychological, it had to be a method in which the independent variable primarily consisted of a psychological or psychotherapeutic focus.Methods in which the independent variable primarily consists of or directly focuses on technical, instructional, tactical, and biological target factors were not considered.
The eligibility criterion was defined in this way to ensure that the investigated intervention effects were not due to, for example, the technical implication of a movement execution instruction.Therefore, interventions such as biofeedback/ neurofeedback, attentional strategies and instructional selftalk with a sole technical focus on movement execution, quiet eye training, and the restricted environmental stimulation technique were not considered in this systematic review.

Outcomes
Only studies using a quantitative measure of sport performance were eligible.Both objective performance and subjective ratings of performance were considered.Objective outcomes had to be a result or a direct measure of performance of the studied sport.Indirect performance was not considered.Examples of indirect performance are performancerelated measures (e.g., VO 2max for an endurance athlete) and other performance correlated measures (e.g., psychological constructs) that are not the performance itself (which for the endurance athlete could, e.g., be the completion time of an endurance activity).Additionally, for measures to be considered a direct measure of performance, they had to be a primary concern for the studied sport (e.g., endurance could be considered direct performance only in endurance sports).Subjective ratings of performance were either self-rated or by an external person (e.g., coach or expert rated).Subjective ratings had to originate from performances.Ratings of general athletic abilities (e.g., concentration skills, handling of emotions) were not considered.Other outcome measures in the included studies in addition to performance were not extracted to avoid a skewed representation of such measures as studies containing only non-performance or indirect performance outcomes were excluded.

Risk of Bias Assessment
A risk of bias instrument adapted to psychological intervention research was developed.The primary aim for the item selection was the assessment of internal validity aspects rather than a full quality of trial assessment.Furthermore, the instrument was composed to enable the assessment of a range of group designs.Initially, other risk of bias or quality assessment tools, both general or traditionally used in medical research [28][29][30][31] and adapted versions to psychology/ psychotherapy research [32][33][34][35], were screened and served as an item pool.GR compiled and selected items referring to internal validity issues.Additional items were considered and discussed among the study authors.A final selection of 12 items was chosen.The score range is zero to − 13, and a lower value indicates a higher risk of bias.All items of the instrument with the risk of bias assessments of the metaanalysed studies are presented in Table 1.The risk of bias assessments for the other studies in the systematic review can be found in Online resource file 3.

Inclusion Criteria in a Meta-analysis
All eligible studies in the systematic review were assessed for inclusion in a meta-analysis.For studies to be included in the same meta-analysis, the following characteristics of studies were considered: • Similarity of psychological interventions and techniques.
• Similarity of comparator across studies.
• Comparable experimental design across studies (between-group comparisons vs counterbalanced withingroup comparisons of two active experimental conditions).• Sufficient study data were available to enable effect size calculation.Attempts were made to contact study authors for additional data if necessary to enable inclusion in a meta-analysis.Estimates of missing standard deviations were calculated when sufficient information was provided [36].

Statistical Analysis
Analyses were conducted in the statistical software R (version 4.1.1)using the metafor (version 3.8-1) [37] and clubSandwich [38] packages.A multilevel meta-analysis (MLMA) framework [39] with cluster robust variance estimation (RVE) was used when studies included more than one performance outcome to handle dependency among effects, with an adjustment for small samples [40,41].Effect sizes were clustered within studies, and there was no information about sampling correlation; therefore, constant sampling correlation was assumed [40].r = 0.5 was used for all MLMA, and sensitivity analyses were conducted with varying levels of r.A meta-analysis either included only parallel-group designs (inter-individual comparisons at post-experiment) or counterbalanced crossover designs (intra-individual comparisons between active experimental conditions that assume no carry-over effects between conditions).In meta-analyses I (psychological skills training [PST] vs control), II (mindfulness-and acceptance-based [MA] approaches vs control), and III (imagery vs control) consisting of parallel-group studies only, random-effects models were used to calculate the standardized mean difference (SMD) [42].In meta-analyses IV (attentional strategies, external vs internal) and V (regulatory focus instructions, promotion vs prevention) consisting of counterbalanced crossover designs, random-effects models were used to calculate the standardized mean change (SMC) using change score standardization [43].When the repeated-measures correlation was not known in the SMC analyses, r = 0.5 was used with accompanying sensitivity analyses.All primary IV.
Statistical heterogeneity was examined with I 2 statistics.In the MLMA models, the variance of both within-study and between-study heterogeneity was calculated.Publication bias was investigated through a test of excess statistical significance (TESS) and the proportion of statistical significance test (PSST).Z scores larger than 1.654 in either the TESS or PSST tests were interpreted as evidence for excess significance bias in the model [44].An adapted version of Egger's regression was also conducted to test for funnel plot asymmetry [45,46].A modified measure of precision was used as a predictor to remove artifactual correlations between effect sizes and their related standard errors [46,47].A visual inspection of funnel plots was also conducted.
Cohen's [48] rule of thumb for effect-size magnitude categorization was used (small, 0.20-0.50;moderate, 0.50-0.80;large, > 0.80).Moderators were not investigated in any metaanalysis because of the risk of misleading results with fewer than ten studies [49].Each sensitivity analysis commenced from the main results of the meta-analysis; studies were never subsequently removed.Datasets and code can be found online in the figshare repository [24].

Results
First, the overall findings of the systematic review are described.Then, each meta-analysis is presented separately.Thirty-seven effects from 25 studies (k studies = 25, j effects = 37) distributed over five meta-analyses were synthesizable.For the meta-analysed studies, risk of bias ratings are found in Table 1, and study information is presented in Table 2.

Results
The estimated overall effect size for PST interventions compared with controls was significant, Hedges' g = 0.83 (95% confidence interval [CI] 0.21-1.45,standard error [SE] = 0.25, t = 3.36, p = 0.017), in favor of PST.A forest plot is presented in Fig. 2. Overall heterogeneity was substantial (I 2 = 70.39%), of which all 70.39% of the variance was estimated to be due to within-study heterogeneity and none due to between-study heterogeneity.No indication of publication bias was found.Both the TESS (− 0.37) and the PSST (0.17) were non-significant.Furthermore, the funnel plot was visually inspected for asymmetry (Fig. 3), and the relationship between the effect size estimate and precision was not significant (t = − 0.19, p = 0.853).
Sampling correlation: the average effect size and other model parameters were quite stable when sampling correlations were set to r = 0.2 and r = 0.8, respectively.

Included Primary Studies and Comparator
Mindfulness-and acceptance-based interventions that were compared to no-intervention controls were synthesized.Mindfulness-and acceptance-based interventions are inspired by developments that occurred in clinical psychology during the 1990s and early 2000s.These interventions (sometimes referred to as the "third wave" of cognitive and behavior therapy) target processes such as acceptance, mindfulness, values, metacognition, and attention [52].
A key theoretical implication of MA approaches was the accumulating evidence of the paradoxical effects in efforts to control, reduce, or suppress distressing thoughts and emotions [53][54][55][56], which was central to traditional PST to achieve optimal performance states through self-regulatory processes [50,51].Interventions represented in the current meta-analysis were acceptance and commitment therapy/ training [57], mindfulness-based stress reduction [58], mindfulness-acceptance-commitment approach [59], mindfulness sport performance enhancement [60], meditation, and mindfulness meditation training.All included studies were parallel-group designs (k = 6, j = 9), and a random-effects MLMA with RVE was conducted.

Results
The estimated overall effect size was significant, g = 0.67 (95% CI 0.01-1.32,SE = 0.25, t = 2.67, p = 0.047), in favor of MA over control.A forest plot is presented in Fig. 4. Overall heterogeneity was substantial (I 2 = 70.11%), of which 59.39% of the variance was attributed to betweenstudy heterogeneity and 10.72% was attributed to withinstudy heterogeneity.No indication of publication bias was found.The TESS (− 4.24) and the PSST (− 1.55) were non-significant.The funnel plot was visually inspected for asymmetry (Fig. 5), and there was no significant relationship between the effect size estimate and precision (t = − 0.84, p = 0.430).
Sampling correlation: the average effect size and other model parameters were quite stable when sampling correlations were set to r = 0.2 and r = 0.8, respectively.

Included Studies and Comparator
Studies that compared unimodal imagery interventions with a no-intervention control condition were synthesized.Imagery comprises mental constructions of athletic scenarios for preparatory and practice purposes.Imagery can be used for motor skill acquisition, cultivation, or to mentally practice/prepare for competitive circumstances (e.g., highpressure situations) [51].If there was more than one imagery condition per study, then the conditions were combined.All included studies were parallel-group designs (k = 5, j = 7), and a random-effects MLMA with RVE was conducted.

Results
The estimated overall effect size was significant, g = 0.75 (95% CI 0.14-1.36,SE = 0.22, t = 3.49, p = 0.027), in favor of imagery over control.A forest plot is presented in Fig. 6.Overall heterogeneity was moderate (I 2 = 39.77%), of which all 39.77% of the variance was attributed to between-study heterogeneity and none was due to within-study heterogeneity.No indication of publication bias was found.The TESS (− 0.86) and the PSST (PSST = − 0.11) were non-significant.The funnel plot was visually inspected for asymmetry (Fig. 7), and there was no significant relationship between the effect size estimate and precision (t = − 0.92, p = 0.398).
Sampling correlation: the average effect size and other model parameters were quite stable when sampling correlations were set to r = 0.2 and r = 0.8, respectively.

Meta-Analysis IV: Attentional Focus, External
Versus Internal

Included Primary Studies and Comparator
Attentional focus strategies are applied to enhance concentration and optimize performance by intentionally directing focus toward certain stimuli during performance.A common distinction is to either adopt an internal (e.g., focus on body or skill movements) or an external focus (e.g., skill-related external stimuli such as the ball leaving the bat or skill-unrelated stimuli such as sounds in the environment) of attention [11].Counterbalanced crossover designs comparing internal and external attentional strategies' effects on sport performance were synthesized (k = 4).If there was more than one internal (or external) condition in a study, conditions were combined [36].As no study had more than one performance outcome, a univariate random-effects model was conducted to calculate the SMC.The repeated measure correlation was set to r = 0.5 except for one study [67], for which the correlation was known (r = 0.651).
The TESS and the PSST were non-significant in both directions of the model, for external (TESS = 1.04;PSST = 0.69) and internal attentional strategies (TESS = − 2.21; PSST = − 0.97).The funnel plot was visually inspected for asymmetry (Fig. 9), and there was no significant relationship between the effect size estimate and precision (t = − 3.36, p = 0.078).

Sensitivity Analyses
Repeated-measure correlation: correlation was set to r = 0.25 and r = 0.75, respectively, except for Neumann and Thomas [67], for which the correlation was known.Neither r = 0.25 nor r = 0.75 altered the results in a significant way.

Included Primary Studies and Comparator
Regulatory focus theory was developed by Higgins and proposes two different modes in self-regulation, a focus on accomplishment and aspirations (a promotion focus) or on safety and responsibility (a prevention focus) [68,69].Regulatory focus moves beyond classic motivational theories that suggest pleasure seeking and pain avoidance as the fundamental guide for behavior.Regulatory focus theory also emphasizes the fit between individual-level orientation (promotion or prevention chronic orientation) and performance instructions (promotion or prevention focus instructions), which may moderate the behavioral effect.Investigations on athletic samples and the regulatory focus instructions' effect on performance began approximately a decade ago [70].
Counterbalanced crossover designs comparing the effects of prevention and promotion regulatory focus instructions on sport performance were synthesized (k = 3; j = 6).A randomeffects MLMA with RVE was conducted to calculate the SMC.The repeated-measure correlation was not known and set to r = 0.5 across studies.

Results
There was no significant difference in effect between prevention and promotion focus performance instructions, SMC = 0.07 (95% CI − 0.42 to 0.55, SE = 0.11, t = 0.65, p = 0.588).A forest plot is presented in Fig. 10.Heterogeneity was low, I 2 = 20.64%, of which 18.6% was due to between-study heterogeneity and 2.04% was due to withinstudy heterogeneity.The test of excess statistical significance and the proportion of statistical significance test were non-significant in both directions of the model, for prevention (TESS = − 1.42; PSST = − 0.70) and promotion performance instructions (TESS = − 0.79; PSST = − 0.35).A visual inspection for funnel plot asymmetry was conducted (Fig. 11), and there was no significant relationship between the effect size estimate and precision (t = 0.58, p = 0.594).

Sensitivity Analysis
Repeated-measure correlation: varying repeated-measure correlations were tested (r = 0.75; r = 0.25) but did not alter the main results in a significant way.Sampling correlation: the average effect size and other model parameters were quite stable when sampling correlations were set to r = 0.2 and r = 0.8, respectively.

Meta-Analysis I: PST (Multimodal) Versus Control
Multimodal PST interventions significantly outperformed the no-intervention control on sport performance outcomes in athlete samples with moderate effects.However, sensitivity analyses showed a decline in effects and a non-significant result when non-randomized studies were removed from the synthesis [71][72][73][74].Additionally, when removing subjective performance outcomes [72,73], the effect estimate declined and was non-significant.Taken together, the sensitivity analyses cast doubt on the robustness of the effect estimate, emphasizing the importance of interpreting the results cautiously.
The mean risk of bias rating score was − 5.71 (range − 4 to − 8); see Table 1.The consistent weaknesses in research design across studies were that no single study had a preregistered study protocol, and did not prespecify a primary outcome or adjust for multiple testing.Only Thelwell and Maynard [74] conducted an intention-to-treat analysis, and only Blakeslee and Goff [75] investigated deterioration or side effects.
In future research, several concerns should be addressed to further strengthen the scientific robustness of PST research.Seven studies met the eligibility criteria to be included in the synthesis, and only three of these were RCTs.This suggests that additional trials are needed in general and randomized studies in particular.Furthermore, the sports included were soccer [71], equestrian sports [75], biathlon [76], cycling [77], cricket [74], and two studies with athletes from various sports [72,73].This implies a variation of sports represented in the synthesis, although the total number of sports is still limited.Future research would also benefit from comparing PST with other active psychological interventions.Direct comparisons would provide better insight into what type of intervention may be most beneficial in certain sports or performance contexts.

Meta-Analysis II: MA Approaches Versus Control
Mindfulness-and acceptance-based approaches significantly outperformed no-intervention control on sport performance outcomes in athlete samples with moderate effects.However, the significant effect did not remain through the sensitivity analyses.The removal of a potential statistical outlier [61], the only non-randomized trial [62], and the use of subjective performance outcomes [63,64] altered the average effect The plot does not account for the fact that some effects sizes are dependent because of being clustered within the same study into a non-significant result in all sensitivity analyses.This calls for cautious interpretation of the robustness of the results.The limited number of studies (k = 6, j = 9) is another reason to make cautious interpretations.
The mean risk of bias rating was − 4.67 (range − 3 to − 7); see Table 1.Consistent weaknesses in research design were the lack of a prespecified study protocol and not investigating side effects.Except for Ruiz and Luciano [62], studies did not clearly predefine a primary outcome or adjust for multiple testing.Studies were randomized except for Ruiz and Luciano [62] and investigated intervention adherence except for John et al. [61] and Zadeh et al. [64].
The limited number of studies (k = 6, j = 9) in this metaanalysis precluded extensive conclusions; however, MA interventions showed significant moderate effects to enhance performance in athlete samples.Additional trials investigating the effect of MA interventions on athlete performance are needed, preferably with robust research methodology designs such as RCTs.

Meta-Analysis III: Imagery Versus Control
Imagery significantly outperformed control with an overall moderate effect.However, the effect was no longer significant when the non-randomized trials were removed in a sensitivity analysis, which also improved heterogeneity.This suggests that the results should be interpreted cautiously and that additional trials with robust methodologies, such as RCTs, are needed to further establish imagery intervention effects on the performance of athletes.Additional trials would also enable investigations of moderating effects.
The mean risk of bias rating was − 4.6 (range − 2 to − 8); see Table 1.A consistent weakness in research design was the lack of a preregistered study protocol.Only Lu et al. [66] investigated side effects.Another common weakness was the lack of investigating intervention adherence/manipulation checks, except for Lu et al. [66] and Seif-Barghi et al. [80].Additionally, only Lu et al. [66] and Robin et al. [81] conducted intention-to-treat analyses.Of the total five studies, three were randomized [80][81][82], and the other two were non-randomized [65,66].
Sports represented in the analysis were karate [65], soccer [80,82], and basketball [66,81], making team ball sports the main context of study.Additionally, only one study included female athletes [66].This is a limitation for the generalizability of the results.

Meta-Analysis IV: Attentional Focus, External Versus Internal
There was no significant difference associated with applying an external or internal attentional focus on sport performance in athlete samples.The number of studies in the analysis was limited (k = 4), which resulted in a poor basis for funnel plot inspection.Additionally, the statistical heterogeneity was considerable.It cannot be ruled out that the obtained non-significant results were due to low power, which is affected by the number of studies, sample size, and heterogeneity in the analysis [83,84].
The risk of bias score was − 5 across all studies with the exact same item ratings (Table 1).Consistent design weaknesses were the lack of a preregistered study protocol, no randomization of order of experimental conditions, dropouts were not reported, and no side effects were investigated.
Sports and outcomes included were baseball batting performance [85,86], table tennis forehand performance [87], and golf putting performance [67].Although there are many differences between these sports, there are some similarities regarding the motor skill of hitting a ball in all performance outcomes.The internal focus conditions were skill-focused across studies.However, the external focus conditions varied across studies with both skill-irrelevant/extraneous external focus [85][86][87] and skill-relevant external focus [67,85].Hence, stimuli foci can differ between two external focus conditions, although both are still considered external focus strategies.This can also be the case for different types of internal focus strategies.However, this meta-analysis can only draw conclusions based on the broad distinction between internal and external attentional focus strategies, which is another limitation of the current synthesis.
Prior reviews have concluded that an external focus is preferred over an internal focus for sport and motor performance [11,88].Such conclusions could not be drawn based on the current synthesis.However, the results of the current synthesis should be interpreted with caution.In future research, further studies with robust research designs comparing internal and external focus strategies in athlete performance are warranted for further evaluation of potential effects.If sufficient experiments are conducted across different sports and motor skills in athletes, more nuanced conclusions can be made regarding the most effective strategy for a given skill in a specific sport.Counterbalanced crossover designs such as the studies in the current meta-analysis are a feasible research design alternative, especially if carryover effects are assessed and controlled for with manipulation checks.This also has the potential to increase sample sizes.However, randomized parallel-group studies are another useful option if the prerequisites are sufficient.

Meta-Analysis V: Regulatory Focus Instructions, Prevention Versus Promotion
There was no significant difference in the effect of applying prevention-or promotion-oriented regulatory focus instructions in athletes on sport performance outcomes.The limited number of studies in the synthesis (k = 3; j = 6) suggests that the results should be interpreted cautiously.
The mean risk of bias rating score was − 6 (range − 4 to − 7); see Table 1.Consistent weaknesses in research design were the lack of a prespecified study protocol and not investigating intervention adherence with, for example, manipulation checks.No forms of side effects were investigated.Randomization of order for the experimental manipulations was only utilized in Wegner et al. [89].Sports represented in the synthesis were table tennis [90], soccer [91], and volleyball [89].
The non-significant results were expected and in line with the core hypothesis of the regulatory focus literature, namely, that the regulatory focus instruction effect is moderated by the fit of the chronic regulatory focus orientation to the individual [89][90][91].The current meta-analysis showed no significant differences in effect and was limited to regulatory focus instructions only and does not contain data on chronic regulatory focus orientation.In future research, an individual participant data meta-analysis that considers chronic regulatory focus orientation may further elucidate the effect of regulatory focus instructions on athlete performance.Fig. 6 Forest plot for meta-analysis III illustrating the effect of imagery on sport performance outcomes in athletes in comparison to controls.Effect sizes are the standardized mean difference (SMD [Hedges' g]) with a 95% confidence interval (CI).Black squares are individual effect sizes and the square size represent the relative weight.Black rhombuses are the summary effect size estimate for the random-effect (RE) model with model-based variance estimates and with robust variance estimates (RVE).a and b are separate performance outcomes in the same study Fig. 7 Funnel plot for meta-analysis III: imagery versus control.Black dots are individual effect sizes with the standardized mean difference (Hedges' g) on the x-axis and the standard error of the effect sizes on the y-axis.The dashed vertical line represents the summary effect size estimate in the meta-analysis model.Positive standardized mean difference values favor imagery and negative standardized mean differences favor controls for the performance outcome.The plot does not account for the fact that some effects sizes are dependent because of being clustered within the same study

General Discussion
Scrutiny of the evidence relating to psychological interventions' effect in sport has proceeded over four decades investigating different types of interventions, sport populations, and performance outcomes [6].This systematic review investigated a wide range of intervention types in studies focusing on only athlete samples and direct performance outcomes to bring further clarity to the understanding of psychological intervention effects in competitive athletes.Furthermore, the characteristics of such research were examined.In total, 111 studies met eligibility for the systematic review, and 25 (j = 37) of these were quantitatively synthesized in five separate meta-analyses (I, PST multimodal vs control; II, MA vs control; III, imagery vs control; IV, attentional, external vs internal; V, regulatory focus instructions, prevention vs promotion).Moderate significant effects were found in meta-analyses I, II, and III, in which multimodal PST, MA interventions, and imagery outperformed no-intervention controls for sport performance outcomes in athletes.However, when non-randomized trials were removed in sensitivity analyses (in meta-analyses I, II, and III), the effects were nonsignificant.Additionally, when subjective performance outcomes were removed in sensitivity analyses (in metaanalyses I and II), the effects were no longer significant.Therefore, the moderate effects in meta-analyses I, II, and III cannot be considered stable and should be interpreted with caution.Meta-analyses IV and V differ from I, II, and III as active experimental conditions (psychological techniques) were compared in counterbalanced crossover design studies.Such experimental designs (a.k.a.Latinsquare designs) are based on paired data and assume no carry-over effects between conditions.Furthermore, the purpose of the counterbalancing procedure is to remove bias due to order and periodic effects [92].In meta-analysis IV, no significant differences were found when comparing an external or internal focus of attention on athlete performance, although the results should be interpreted with caution owing to the limited number of studies in the synthesis.Finally, in meta-analysis V, there were no significant differences in applying prevention-or promotionoriented regulatory focus performance instructions.Taken The considered outcomes in this systematic review had to be a direct measure of sport performance, and similar criteria have been used in prior meta-analyses in sport psychology [93,94].For a meaningful understanding of whether the effect is relevant or not, its real-world meaning should be contextualized when possible [95], and effect size interpretation in sport psychology is more straightforward when studies use outcomes that incorporate performancerelevant behavior [96].As the meta-analyses in this systematic review included a variety of sports and performance outcomes, a nuanced interpretation of the real-world meaning of the performance change is difficult to make and not suggestible based on the overall effect estimates.Furthermore, the starting point of an effect size interpretation is preferably its CI [97].This should be considered for the significant results in meta-analyses I (Fig. 2), II (Fig. 4), and III (Fig. 6).All CIs are comparatively wide, which is another reason for interpreting the results carefully.
An advantage of including a wide range of psychological interventions is to enable comparisons across interventions.Prior reviews have commonly focused on one type of psychological intervention [4,[7][8][9][10][11][12][13][14][15], with some exceptions [17,19], including the current review.Unfortunately, only two studies in the systematic review directly compared PST and an MA intervention [98,99], which was not enough to conduct a meta-analysis.However, further direct comparisons of PST and MA may provide better insight into what type of psychological intervention may be beneficial in various sport contexts.
Many risk of bias assessment tools assess only one type of study design, for example, RCTs, such as the Cochrane Collaboration's tool for assessing risk of bias [30].As this systematic review contains a wide range of group designs, an instrument was developed to enable assessments across Fig. 10 Forest plot for meta-analysis V illustrating the effect of applying either a prevention-or promotion-oriented regulatory focus instruction on sport performance in athletes.Effect sizes are the standardized mean change (SMC) using change score standardization with a 95% confidence interval (CI).Positive SMC values favor a prevention instruction and negative SMCs favor a promotion instruction on the performance outcome.Black squares are individual effect sizes and the square size represent the relative weight.Black rhombuses are the summary effect size estimate for the random-effect (RE) model with model-based variance estimates and with robust variance estimates (RVE).a and b are separate performance outcomes in the same study Fig. 11 plot for meta-analysis V: regulatory focus instructions, prevention vs promotion.Black dots are individual effect sizes with the standardized mean change on the x-axis and the standard error of the effect sizes on the y-axis.The dashed vertical line represents the summary effect size estimate in the meta-analysis model.Positive standardized mean change values favor a prevention instruction and negative standardized mean changes favor a promotion instruction on the performance outcome.The plot does not account for the fact that some effect sizes are dependent because of being clustered within the same study varying designs (uncontrolled, non-randomized, randomized, and with either parallel-group or counterbalanced crossover designs) but with criteria relevant to the risk of bias in experimental trials of psychological interventions in general.This was the reason for the deviation from the study protocol for which the initial plan was to use CONSORT items in full [21].Because group designs varied, an item could formally assess different design features, although it pertained to an adequate rating of bias for that type of study design.For instance, Item 2 assesses whether randomization is conducted.For parallel-group designs, this refers to traditional randomization procedures.However, for counterbalanced crossover designs, the item assesses whether the order of experimental conditions was randomized to account for bias due to periodic effects [92].Therefore, comparisons of risk of bias ratings should only be performed between studies with comparable experimental designs, as in metaanalyses I (PST, mean risk of bias score = − 5.71, range − 4 to − 8), II (MA, mean = − 4.67, range − 3 to − 7), and III (imagery, mean = − 4.6, range − 2 to − 8) which contained only parallel-group studies (see Table 1).

Limitations
The results of a systematic review and meta-analysis may be influenced by study reporting (reporting bias), as some excluded studies may in fact be eligible, although underreporting makes this difficult to verify [100].The use of performance levels as an eligibility criterion was to enable answers to the research question regarding the performance context of competitive athletes and not recreational, novice, or junior participants.Nor were local level athletes considered in this systematic review.Similar approaches for defining and assessing sample eligibility criteria have been used in prior reviews, for example, [93].This can distinguish between competitive athletes and samples that, for example, may still learn/acquire basic skills in sports, which was not the focus of this review.It may also account for other differences in the performance contexts for competitive and non-competitive athletes that the psychological intervention targets to a larger extent as a function of competitive level (e.g., handling high-pressure situations).The log of rejected studies (Online resource file 2) shows that among the 603 excluded reports in the full-text assessments, 417 experiments/studies were excluded on the participant criterion.We naturally do not know how many were excluded because of underreporting.However, there may be a problematic low consensus of how/which sample characteristics should be reported in sport psychology [101].Therefore, we will extend our reasoning and declare our decision process regarding this criterion and the use of regional-and university-level athletes as the minimum of performance expertise.University athletes had to be on a university team and/or compete in intercollegiate competitions.University "sports clubs" and intramural participation were thus excluded.High school athletes were considered equal to junior level participants and were therefore excluded.When sample ages were reasonably young but not explicitly reported as competing on a junior or senior level, age standards of senior level participation of international sport federations were used when such guidelines existed (e.g., gymnasts under 15 years of age do not compete in senior level competitions according to the International Gymnastics Federation).The comparison of performance level between sports and countries is not clear-cut.This is aggravated by the available levels of expertise in different sports within a country, further complicated by international differences owing to the type of sport.However, performance level, referring to the geographical level of competition standards (e.g., local, regional, county, state, provincial, national, and international), has been used as a central criterion in taxonomy classification systems of athletic expertise [101,102] and therefore was used as a key criterion in this study.Other reviews in sport psychology have used similar criteria [20].As geographical definitions vary between countries, terms such as county, provincial, sectional, and state were considered comparable.Eligibility assessments were based on available sample information to verify the current performance level.However, it did not solely rely on terms such as "elite" because such terms have been shown to represent immense variation and inconsistency of actual performance level [101] and therefore are not possible to verify without additional information.For the same reason, the sole use of related terms, such as "semiprofessional" or "professional" was not considered without additional information (e.g., type of league or competitions that the athletes participated in) that could verify the competitive level.This also implies that "amateur" athletes could be included when competitive level was reported and eligible.The verification of performance level is necessary for feasible comparisons of study samples with athletes from different sports and countries and crucial in making valid conclusions about existing evidence.Hence, this is important for future research to consider as part of the methodological reporting of trials.Underreporting of study sample characteristics cannot be ruled out as a potential source of lost data for the current review.
This review emphasizes that eligible psychological interventions primarily consist of a psychological or psychotherapeutic focus.Psychologically related methods in which the independent variable primarily consisted of or directly focused on technical, instructional, tactical, and biological target factors were not considered.Naturally, psychological interventions and techniques commonly target sport behaviors as the very purpose of a sport psychology intervention may be to aid such behaviors.However, some techniques are so technically oriented around how to execute a certain motor behavior (e.g., "lift your knees high") that it becomes unclear whether the technique is still to be defined as a psychological or technical/instructional technique per se.There are, for instance, both self-talk techniques and attentional focus techniques that primarily focus on the technical execution of behavior.However, as the aim of the current article was to review the effect of psychological interventions and techniques, clarity of independent variables becomes a crucial issue to ensure that investigated effects are not due to, for example, the technical implication of a movement execution instruction.Although self-talk and attentional techniques that focus on technical execution may have a logical place in aiding certain sport behaviors, they did not meet the criteria for psychological intervention techniques in this review.As such, the current review does not contain full account of attentional and self-talk techniques.There are other reviews that use a wider framework for the inclusion of samples and interventions with both psychological and technically oriented independent variables for attentional focus interventions [11,88] and self-talk interventions [103].
A power analysis is an important part of planning a metaanalysis [84] but was not conducted prior to this systematic review.Power refers to the ability of an analysis to correctly reject the null hypothesis of no effect when a true effect exists.For random-effects meta-analyses, power can be affected by factors such as the number of included studies, sample size and the statistical heterogeneity of included effects [83,84].Power is a likely issue for the interpretation of the results found in this study for a few reasons.The significant effects found in meta-analyses I-III were accompanied by sensitivity analyses to investigate whether the removal of studies with certain characteristics, such as using an inferior study design (non-randomized trials) or subjective performance measures, affected the results.As studies were removed in the sensitivity analysis, power was probably reduced compared with the full meta-analyses that could be considered small to begin with.This weakens the interpretations of whether the non-significant results in the sensitivity analyses suggest a non-significant overall effect when considering only superior research designs (RCTs) or if it reflects a loss of power that fails to detect a true intervention effect that exists.Additionally, meta-analysis IV that compared attentional strategies (external vs internal focus) only had four effect sizes.Thus, the non-significant trend of favoring an external focus could be a result of low power.The number of studies in the meta-analyses is a limitation to the interpretation of the results for other reasons in addition to the issue of power.All five meta-analyses had too few studies (PST, k = 7, j = 11; MA, k = 6, j = 9; imagery, k = 5, j = 7; attentional, k = 4; regulatory focus instructions, k = 3, j = 6) to conduct meta-regression analyses to investigate moderating effects because of the risk of misleading results with fewer than ten studies [49].This makes it difficult to conclude to what extent certain study design features, risk of bias implications, or other experimental differences influence intervention effects.Furthermore, meta-analyses I (PST) and II (MA) consist of psychological intervention packages considered similar enough to synthesize as they share many common intervention procedures; however, there are still differences in the interventions between studies.Both PST and MA meta-analyses have different combinations of intervention procedures and protocols (see Table 2).Ideally, with additional trials, subgroups of intervention protocols could have been investigated or to what extent different intervention protocols moderated the average effect.Another limitation associated with small-numbered meta-analyses is that the basis for inspection of funnel plot asymmetry is rather poor, which weakens the interpretation of potential publication bias [104].
Another related limitation is that only 25 studies (j = 37) from the systematic review were quantitatively synthesized, which means that 86 studies (77.48%) were not.To be included in a meta-analysis, primary studies had to be similar across psychological intervention techniques, comparators, and experimental designs and present sufficient data to enable effect size calculations (and attempts to contact study authors were made when needed).These factors are the reasons why no further studies in the systematic review were included in a meta-analysis.However, some potentially useful data remain in the systematic review that together with a few additional studies could be included in further separate meta-analyses in the future for interventions such as hypnosis or some of the remaining PST single modal interventions (e.g., relaxation, self-talk, or goal setting), or for active comparisons between interventions (e.g., PST vs MA).The full list of studies in the systematic review not included in any of the meta-analyses can be found in Online resource file 4.

Conclusions
Significant moderate effects on athlete performance were found for PST, MA, and imagery interventions compared with controls.However, the results were no longer significant when non-randomized studies and subjective performance outcomes were removed from the syntheses.This suggests that the effects were not stable and that conclusions should be drawn with caution.Another two meta-analyses were also conducted.Attentional interventions comparing external and internal focus strategies showed no significant differences in athlete performance.Finally, prevention-and promotion-oriented regulatory focus performance instructions were compared and showed no significant differences.
In general, improved research methodology, reporting standards, and the provision of datasets in open science repositories are important to consider in future trials investigating the effect of psychological interventions on performance in athletes.Because of the limited number of studies in all five current meta-analyses, additional trials with robust methodology are needed for all types of psychological interventions with the aim of enhancing performance in athletes to further establish the effects found in this review and to enable additional investigations of moderating effects.

Fig. 1
Fig.1PRISMA (Preferred Reporting Items for Systematic reviews and Meta-Analyses) flow chart of the eligibility process[25].a Some reports (publications) included more than one study, therefore the

Fig. 2 Fig. 3
Fig.2Forest plot for meta-analysis I illustrating the effect of multimodal psychological skills training on sport performance outcomes in athletes in comparison to controls.Effect sizes are the standardized mean difference (SMD [Hedges' g]) with a 95% confidence interval (CI).Black squares are individual effect sizes and the square size rep-

Fig. 4
Fig.4 Forest plot for meta-analysis II illustrating the effect of mindfulness-and acceptance-based interventions on sport performance outcomes in athletes in comparison to controls.Effect sizes are the standardized mean difference (SMD [Hedges' g]) with a 95% confidence interval (CI).Black squares are individual effect sizes and the

Fig. 5
Fig. 5 plot for meta-analysis II: and acceptancebased interventions versus control.Black dots are individual effect sizes with the standardized mean difference (Hedges' g) on the x-axis and the standard error of the effect sizes on the y-axis.The dashed vertical line represents the summary effect size estimate in the metaanalysis model.Positive standardized mean difference values favor mindfulness-and acceptance-based interventions and negative standardized mean differences favor controls for the performance outcome.The plot does not account for the fact that some effects sizes are dependent because of being clustered within the same study

Fig.Fig. 9
Fig. Forest plot for meta-analysis IV illustrating the effect of applying either an external or internal attentional focus during sport performance in athletes.Effect sizes are the standardized mean change (SMC) using change score standardization with a 95% confidence interval (CI).Positive SMC values favor an internal focus and nega-

Table 1
Risk of bias assessment of meta-analysed studies
a Outcome a in the meta-analysis b Outcome b in the meta-analysis c Additional study characteristics exist that did not meet eligibility criteria d Conditions had other names in the article but are renamed to make the intervention content and comparison between studies clearer e Groups were combined