Referees in sports games have the responsibility to ensure that teams and players always abide by the sport-specific rules and intervene when a team or a player infringed a rule (Reilly and Gregson, 2006; Rullang et al., 2017). Therefore, a central task in officiating sports games is to make decisions about technical, offensive and defensive infringements (Bar-Eli et al., 2011; Plessner et al., 2009). As referees' decisions can ultimately impact a game's outcome (Larkin et al., 2014; MacMahon et al., 2015), it is indispensable that referees' decision-making (RDM) is on a high level (Helsen et al., 2019; MacMahon et al., 2015). This includes that referees are able to apply the sport-specific rules during the entire match; that is, they need to have a well-developed physical fitness in order to keep up with the match dynamics when officiating (Kittel et al., 2019; MacMahon et al., 2015; Plessner and MacMahon, 2013; Reilly and Gregson, 2006). Mascarenhas et al. (2005) emphasize the relevance of both RDM and physical fitness for appropriate officiating when identifying five cornerstones of referees' performance: (1) personality and game management, (2) physical fitness, positioning and mechanics, (3) knowledge and application of the law, (4) contextual judgement and (5) psychological characteristics of excellence. However, albeit highlighting the important role of both RDM and physical fitness, the authors' model does not take into account potential relationships between cornerstones, RDM and physical fitness in particular whereby the central parameter of RDM is the correctness of the decisions made by the referees (Schweizer et al., 2011).
From this point of view, the relationship between RDM and physical load is of particular interest. Referees' decisions are a perceptual-cognitive process (Helsen et al., 2019; Plessner and Haar, 2006; Schmidt et al., 2019), which might be impaired through high physical stress (Chang et al., 2012; Schmidt et al., 2019; Tomporowski, 2003). High physical stress can, for example, lead to an excessive enrichment of nor-epinephrine, which can result in a reduction of neuronal activity in the prefrontal cortex and ultimately cause a reduction in attention and executive functions (Arnsten, 2009; Schmidt et al., 2019). Thus, physical load may impair referees' attentional control and, as a consequence, their decisions (Helsen et al., 2019; Schmidt et al., 2019).
Since decision-making is considered as an important task in officiating (Kittel et al., 2019), it is necessary to understand the relationship between physical load and RDM. Here we review the evidence on the relationship between physical load and RDM in sports games.
The scoping review was performed according to the Preferred Reporting Items for Systematic Reviews guideline (PRISMA; Moher et al., 2009).
Information sources and search strategy
The systematic search was carried out on September 4, 2019, by using the following databases: Web of Science (all databases), Scopus, PubMed, SURF and SPONET. Searches in SURF and SPONET were conducted in English as well as in German. The option 'advanced searches' was chosen for searches in Web of Science, PubMed and Scopus with regard to the articles' title, abstract and keywords. "All Field" searches were executed in SURF and SPONET. The development and selection of the final keywords used was a process consisting of several search attempts and corresponding adjustments, which were discussed with all authors to improve the quality of the keywords and the search. In the end, the following search terms are used: "(referee* OR umpir* OR officiating) AND ("decision-making" OR performance OR judg*) AND (stress OR strain OR effort OR load OR exertion OR pressure)". The general German search term was: "(Schiedsrichte*) AND (Entscheid* OR Leist* OR Bewert*) AND (Stress OR Belastung OR Anstrengung OR Beanspruchung OR Erschopfung OR Druck)".
Eligibility criteria and study selection
To be included in the review, articles must strictly fulfil the following inclusion criteria. Articles have to be primary studies and investigate the relationship between physical load and RDM. They must also refer to sports games and be written in English or German. We set no limitation to the year of publication. Besides, articles were excluded when meeting defined exclusion criteria: articles are reviews, abstracts, project descriptions, conference papers, interviews, theoretical papers or dissertations. Studies referring to athletes, studies without reference to sport games or articles not written in English or German were also excluded.
In accordance with the PRISMA approach (Moher et al., 2009), additional articles were appended in the selection process after the database search was conducted. Afterwards, duplicates were excluded and titles, abstracts and the written languages were screened and excluded when not meeting inclusion criteria. To ensure that no relevant article was excluded, critical articles were kept up and were discussed by the authors. Finally, full-texts were analysed and discussed before exclusion The remaining articles were integrated into the scoping review (see PRISMA flowchart in Figure 1).
For each study selected for inclusion in the review, the following parameters were extracted:
* Study characteristics: sport, sample, referee type, expertise (level of officiating, experience), sex, age and a brief report of the measurement
* Study results categorized by physical parameters and physical load (external or internal load), a short description of the results and its statistical key figures
As outcomes and reports of the studies were not uniform, data were processed for a scoping review. This type of review article aims at a first examination of the existing evidence on a research subject especially for elaborating future research (Tod, 2019)
Risk of bias
All studies included in this review were assessed with regard to risk of bias by the authors. To this end, we used the Mixed Methods Appraisal Tool (MMAT; Hong et al., 2018) to assess the methodological quality of included studies. Criteria and the assessment of the risk of bias of included studies are illustrated in Table 1.
With reference to the quality assessment with the MMAT, we follow the MMAT publishers' recommendation to not report a numeric score to better inform about the quality of the studies and to avoid misunderstandings by discussing meanings of the ratings (Hong et al., 2018).
A total of n = 2,250 potential articles were identified through the literature search. Of those, n = 2,224 articles were found in the database search and n = 26 additional articles were included in the selection process, as required by PRISMA. Additional articles were obtained by screening references from relevant articles. After excluding duplicates, n = 1,854 articles remained for further analyses. Subsequently, these articles were screened with reference to their written language, title and abstract. As a result, n = 1,842 articles were excluded, because they did not fulfil the inclusion criteria. The remaining n = 12 articles underwent a full-text analysis against the eligibility criteria. This led to further exclusion of n = 1 article. Finally, n = 11 articles were included in the qualitative analysis (see Figure 1).
Sports games and referee type
Most of the included studies were carried out in soccer (n = 6) followed by Australian football (n = 3), rugby (n = 1) and futsal (n = 1). The participants' referee type comprised umpires (Australian football), referees (soccer, futsal and rugby) and assistant referees (soccer). Participants officiated at least on an advanced level (n = 24), but most participants were experts (n = 179). Detailed study characteristics are listed in Table 1.
Included studies differ in the selected approach of analysing the relationship between physical load and RDM. In eight studies, video-analysis of recorded matches were conducted and thus RDM was analysed on-field (Ahmed et al., 2017; Catteeuw et al., 2010; Elsworthy et al., 2014a; Emmonds et al., 2015; Gomez-Carmona and Pino-Ortega, 2016; Mallo et al., 2012; Mascarenhas et al., 2009; Oudejans et al., 2005). With the exception of the study by Catteeuw et al. (2010), selected video-sequences were assessed by experts (Ahmed et al., 2017; Elsworthy et al., 2014a; Emmonds et al., 2015; Gomez-Carmona and Pino-Ortega, 2016; Mallo et al., 2012; Mascarenhas et al., 2009; Oudejans et al., 2005).
Three studies used video-based decision tests conducted in the laboratory (Paradis et al., 2015; Samuel et al., 2019) or in the half times of real matches (Larkin et al., 2014)...