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A Systematic Review of the Physical, Physiological, Nutritional and Anthropometric Profiles of Soccer Referees

Sports Medicine - Open volume 9, Article number: 72 (2023) Cite this article

Abstract

Background

The importance of soccer referees is widely accepted by international soccer organizations and governing bodies, although there is little research summarizing and appraising the literature on soccer referees. The aim of this study was to systematically review the information related to physical demands, nutrition and physiological profiling of soccer referees.

Methods

Conforming with Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines, searches of three electronic databases (Web of Sciences, PubMed and Scopus) were conducted on 24 April 2022. The following search terms were used: (Soccer OR football AND refer*) AND (physical OR physiolo* OR load* OR “body composition” OR “fat mass” OR “fat free mass” OR “body size” OR “nutrition*” OR “nutritional assessment” OR “nutritional intake” OR “macronutrient*” OR “micronutrient*”). The inclusion criteria of the manuscripts written in English were that articles with male and/or female soccer referees and included relevant data concerning performance, physical testing, nutrition, body composition, body size and/or physiology of soccer referees. The tools developed by the National Institute of Health were used to assess risk of bias according to the study design.

Results

In total, 110 manuscripts were included in the present review. Match activities of soccer referees and assistant referees are not comparable. Variation in performance is influenced by competitive level and competitive schedules. Additionally, match performance is often used to validate field protocols. The associations between match indicators and field protocols were not constant across the included studies, particularly in short-maximal tests. An age decline in physical performance was not conclusive. Body size did not discriminate between referees and assistant referees, as well as referees of different competitive levels. Few studies focused on nutritional guidelines among referees, particularly exercise energy expenditure. Energy intake was comparable across studies, but referees did not follow the daily dietary recommendations. Aerobic output was frequently assessed, and it was not comparable to the values for soccer players.

Conclusions

Although there are decreases in the running profiles of older referees, they maintain the same distances from the ball, and thus, age per se should not be used to define participation at the international level. The assessment of physical fitness warrants future consideration given the levels of fatigue that are apparent at the end of matches. In order to attain ideal levels of body composition, future studies need to provide guidelines for daily energy expenditure and nutritional intake.

Trial registration: The protocol was published in INPLASY (International Platform of Registered Systematic Review and Meta-Analysis Protocols) with the registration number 202280052 and https://doi.org/10.37766/inplasy2022.8.005.

Key points

  • Age is not a useful indicator of match performance in soccer referees.

  • Referees and assistant referees demonstrate distinct activity profiles during matches.

  • Field protocols designed for soccer referees are not strongly associated with match performance.

  • Nutritional guidelines are generalized from soccer players to referees; however, bespoke studies regarding nutritional guidelines should be developed for soccer referees.

Background

According to official data from the Federation Internationale de Football (FIFA), in 2006, 843,000 referees and assistant referees were involved in soccer [1]. The Union of European Football Associations (UEFA), English Football League, Premier League and The Football Association created special programs in order to guide training, development and mentoring of soccer referees and assistant referees [2, 3]. Therefore, the importance of soccer referees is widely recognized by international soccer organizations and governing bodies [4, 5]. The FIFA International Refereeing List is a global publication of referees and assistants qualified to officiate at international level [6, 7]. In order to qualify as a FIFA referee, field protocols must be completed [7, 8]. The results of these tests are compared to reference values according to sex and competitive level [7]. The application of these protocols is often generalizable within national institutions [9].

A recent review examined the validity and reliability of fitness tests in European elite soccer referees [10]. This study only included elite soccer referees from Europe and the criteria used to investigate the validity of field protocols were widely variable (i.e. match performance, discrimination by competitive level or age groups and comparisons with concurrent field protocols or physiological outputs). However, other studies that associated match performance indicators [11,12,13,14], age [15, 16] and concurrent protocols or physiological outputs [17, 18] with physical tests were not included in the previous review.

An additional question among soccer referees is the age defined by FIFA for forced retirement (i.e. 45 years old). Instead, additional physical tests, medical examinations and technical assessments could be required by FIFA based on an individual analysis of individual referees ≥ 45 years old [7]. However, for example, in Portugal, the national organization [19] defined a narrower age range age for international-level referees (25–37 years) and assistants (31–39 years). Research suggests that older referees tend to cover less total distance and high-speed distance than younger referees, but differences between groups were negligible for distance from the ball [20]. This study suggests that older referees maintained the pacing of the game and perhaps demonstrate tactical superiority to “keep up with play”. A meta-analysis that described match activities in European and South American soccer referees reported that referees covered, on average, 10 km per game with approximately 37% of the distance covered was spent running at lower intensities [21]. Considerable levels of heterogeneity were noted across the studies thereby, the results should not be globally extrapolated for all soccer referees [21]. Nevertheless, referee activities during the matches are associated with team strategies (i.e. tactical positioning on the field) [22, 23] and this issue was neglected in the analyses of the mentioned review [21].

Interestingly, size, body composition and nutritional guidelines which are related to match indicators and physical conditioning are often overlooked as criteria for soccer official selection. Height was negatively associated with disciplinary behaviour, with shorter soccer referees producing more yellow cards [24]. From 2001 to 2012, a significant reduction in body fat percentage was observed in elite Spanish soccer referees [25]. A recent study presented the nutritional data of FIFA soccer referees selected 2012–2013 World Cup [26], with such findings requiring further attention. The guidelines developed for soccer players might be applied to soccer referees should there be a lack of understanding of nutritional requirements among soccer referees. Therefore, further understanding of the current nutritional intakes and habits as well as the energy requirements of soccer referees is needed before bespoke nutritional guidelines can be developed.

Given the discrepancies across studies included in the review that examined the external loads of soccer referees [21], and considering the importance of field protocols in the selection process of international referees, a clear appraisal of the current soccer referee literature is needed for the scientific and applied communities to enhance the scope and application of future research. The aim of this study was to systematically review the information related to the physical demands, nutrition and physiological profiling of soccer referees.

Methods

The current review was conducted according to Preferred Reporting Items for Systematic Review (PRISMA 2020 statement) following Cochrane Guidelines [27, 28].

Eligibility Criteria

Empirical research published in peer-review journals was analysed. The publications included in the present review met the following inclusion criteria: (1) male and/or female soccer referees and/or assistant referees; (2) all types of interventions or exposures; and (3) relevant data concerning body size, body composition, physical performance, physiological profiles and nutrition. Given the interest in examining size and composition, manuscripts that described sample characteristics, even though covering different topics (e.g. injuries), were also considered in the present study. Only English publications were assessed.

Information Source and Search Strategy

Three electronic databases (Web of Sciences, PubMed and Scopus) were searched on 24 April 2022 using the following search terms: (Soccer OR football AND refer*) AND (physical OR physiolo* OR load* OR “body composition” OR “fat mass” OR “fat free mass” OR “body size” OR “nutrition*” OR “nutritional assessment” OR “nutritional intake” OR “macronutrient*” OR “micronutrient*”).

Selection Process

Reference managing software (EndNoteTMX9, Clarivate Analytics, Philadelphia, PA, USA) was used to export the papers. Duplicates were manually removed and two independent authors (DVM/HS) screened according to the title and abstract. Full-text manuscripts were assessed to check if they met eligibility criteria. A third independent reviewer (AR) was consulted to solve the disputes between authors.

Data Collection Process and Data Items

Eligible studies were individually examined and extracted by two authors (DVM/HS). Relevant information, including, sampling characteristics, country, purpose of the study, significant results and practical applications, was organized on an adapted template of Cochrane Consumers and Communication Review Group [29]. The studies were, subsequently, categorized into four different topics: size and body composition, performance, physiological variables and nutrition. Few studies included female referees; hence, results were not separately presented by sex. Corresponding authors were contacted where relevant data were not presented within the manuscript.

Risk of Bias

Three different tools, according to study design, were used to analyse the risk of bias of manuscripts in the present review. The Quality Assessment Tool for Observational Cohort and Cross-Sectional Studies [30] includes fourteen items. These criteria related to the research question, study population, groups recruited from the same population and uniform eligibility criteria, sample size justification, exposure assessed prior to outcome of measurement, sufficient timeframe to observe an effect, different levels of the exposure effect, exposures measurement, repeated exposure assessment, outcomes measurement, blinding of outcomes assessors, follow-up rate and statistical analysis. A global assessment of the publication was undertaken and each article was categorized as good, fair or poor. The Quality Assessment Tool for before and after (pre–post) studies with no control group [31] contained twelve questions about study purpose, inclusion criteria and population, eligibility of participants, sample size, description of intervention, data quality about dependent variables, blinding process, follow-up rates, data analysis, multiple outcome measures, inter and intra-individual variability in addition to a global assessment (good, fair or poor). Finally, the bias of studies with a control group was obtained using a specific tool—Quality Assessment of Controlled Intervention Studies [32]. The tool considers fourteen questions about randomization design, allocation of participants, blinding process, sample characteristics at baseline, dropout, adherence, confounding interventions, measurements of the outcome, power calculation, pre-determined outcomes and intention-to-treat effects. The overall assessment of study quality (good, fair, poor) was assessed via discussions among researches. Two independent authors (DVM/HS) familiarized with the tools completed the bias assessment for each study and disagreements were solved by a third reviewer (AR).

Results

Study Selection

The initial search of Web of Science, PubMed and Scopus identified 3133 manuscripts. Duplicates (n = 1208) were removed, and subsequently, 1925 titles and abstracts were screened, resulting in the removal of 1744 papers. The full texts of 181 manuscripts were examined in more detail; 71 of which were omitted as eligibility criteria were not satisfied. Many reasons were identified to exclude studies in the present review, including studies with American Football referees (n = 1) or school soccer referees (n = 1), studies combining males and females (n = 1), studies lacking relevance to discipline of soccer referees (n = 1), studies assessing psychological aspects (n = 2), studies using questionnaires to collect data (n = 2), studies about decision making (n = 3), studies that evaluated visual function abilities (n = 3) and/or studies that investigated health-related issues (n = 6). An additional 25 records were excluded due to not being original papers and 26 were not written in English (Fig. 1). Finally, the 110 full texts that met the inclusion criteria were read.

Fig. 1
figure 1

Flow chart of the study selection procedures used in the present review article

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Study Characteristics

The sample characteristics and purpose of each study are summarized in Additional file 1: Table S1. It was decided that the presentation of results should be categorized according to the main purpose of each study. Therefore, as shown in Fig. 2, four topics were considered: match indicators, physical testing, nutrition and physiology. Internal and external load was frequently associated with field-based protocols [12, 13, 22, 33,34,35,36,37,38,39,40,41]; therefore, studies that examine the relationship between match performance and physical tests are discussed in the 'Physical Testing' section.

Fig. 2
figure 2

Scope of the literature on soccer referees

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Some studies involved exploring the effects of age variation in physical assessments [15, 16, 42,43,44] and examined variation by competitive levels in soccer referees [43, 45, 46]. Three studies analysed differences in physical performance among adult referees grouped into different age groups [15, 16, 42]. Variations in linear straight sprinting tests, agility tests and YOYO protocol were compared between referees aged ≤ 35 or > 35 years old [43]. The effects of age on field tests were also assessed by examining the correlation coefficient between variables [44]. Comparisons by competitive level were mainly detailed in Italian and Spanish referees [45, 46]. Relationships between different field protocols or data quality concerning physical tests were often described among referees [8, 17, 46,47,48]. Other studies highlighted seasonal variation in physical tests focused on YOYO protocol and velocity [16, 49]. Interventional studies involving tests were applied on match day [50,51,52,53] and training sessions [13, 54,55,56,57]. The effects of match load in soccer referees were evaluated on linear velocity [51], jumping performance [52] and isokinetic strength [53]. Three studies confirmed the importance of planning and implementing high-intensity interval training protocols in soccer referees [54, 55, 57]. One randomized cross-over trial examined the re-warm up effects on the YOYO protocol after the application of an intermittent test—The Loughborough Intermittent Shuttle test [58].

Nutrition studies were related to about body size [24, 59, 60], body composition [60,61,62,63,64,65,66,67], energy expenditure [68,69,70,71], energy, macronutrients and supplementation intake [72,73,74,75,76]. Daily energy or exercise expenditure was estimated by mathematical equations in two studies  [68, 70] or using training watches [71]. Different methods of body composition were also used to obtain fat mass percentage (i.e. skinfolds, bioimpedance and dual-energy X-ray absorptiometry) [40, 61,62,63,64, 66, 67, 77].

Studies investigating the physiology in soccer referees described heart characteristics [78,79,80], maximal oxygen uptake [17, 81,82,83,84,85] and compared strength in referees across different professional levels [86, 87].

Eight studies reported body size or body composition [88,89,90,91,92,93,94,95]; thereby, the full text was read, and sample characteristics were extracted for the present review. The main results and aims of the aforementioned studies are summarized in Additional file 2: Table S2.

Risk of Bias

The risk of bias was examined using specific tools according to study design—Quality Assessment Tool for Observational Cohort and Cross-Sectional Studies [30], the Quality Assessment Tool for Before–After (Pre–Post) Studies With No Control Group [31] and Quality Assessment of Controlled Intervention Studies [32] as shown in Additional file 3: Table S3. In general, cross-sectional, cohort and pre–post-studies were classified as good or fair even though sample size estimation was lacking. Few observational studies included more than one observation. None of the pre–post-studies considered intra-individual variability to determine the effects at the group level. Two interventions with a control group were considered in the present analysis [58, 96]. The latter did not clarify if the sample was randomized and how the allocation of participants was conducted.

Results of Individual Studies

Match Indicators

The match indicators among soccer referees are described in Table 1. Although intermittent activity during a soccer match was noted in referees [34] and assistant referees [36], they varied in distances covered and activity profiles [13, 89, 97]. Contrasting match activities of referees and assistant referees in the American Cup showed that referees covered 4.4 km more and greater distance in accelerations than assistant referees [97]. The total distance and distance in high-intensity running covered by referees were 10.3 km and 1.9 km, respectively. Corresponding data showed that assistant referees covered, on average, 6.8 km during a match and 1.0 km at high intensities [89]. Match performance in the studies was determined by a considerable number of factors: total distance covered during a game [98], activities of the players [5, 99], distance covered by the ball [22], variability between matches [100], competitive level [5, 101,102,103,104], age [20] and competitive schedule [105]. Distance from the ball and infringements was used to examine the capacity of referees to maintain the pace of the game [20, 34, 89, 100, 106]. The distance from the offside line was frequently reported to investigate the performance of assistant referees [23, 36, 107]. Assistant referees maintained the distance from the offside line across the entire match [36, 107]. Only two studies examined female referees [108] and assistant referees [107]. Female referees covered, on average, 10 km per match and spent 7% and 21% of the time walking and jogging, respectively [108]. In contrast, assistant referees covered 5.6 km per game and sideways movements were the most representative activity during the match [107].

Table 1 Studies focused on match indicators
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Physical Testing

The validity of field protocols was tested by comparing physical tests with internal or external match load (Table 2). Although many physical tests have been adopted among soccer referees (Additional file 4: Table S4), the relationship with match performance is inconclusive. The application of linear sprint protocols showed trivial associations with high-intensity actions during official matches [35]. Among Spanish referees from the 3rd Division, the maximum velocity derived from a straight-line sprint test (3 × 30 m; mean value: 29.5 km h−1) was higher than the maximum velocity recorded in the matches (referees: 23.9 km h−1; assistant referees: 25.7 km h−1) [38]. The best sprint from the 6 × 40-m sprint test was strongly associated with high-intensity match activities (i.e. high-intensity running and sprinting distance). The fastest sprint explained 58% and 56% of variance in high-intensity running and sprinting distance, respectively. The mean time from the 6 × 40-m sprint test was also largely associated (r =  − 0.77) with high-intensity activities [37]. Inconsistent results were verified in 28 participants (9 referees and 19 assistant referees from Norway). Repeated sprint ability protocol (mean sprint time for 5 × 30 m) was correlated with high-intensity running in assistant referees. However, no associations were noted for 6 × 40-m mean sprint times and match performance in soccer referees [39].

Table 2 Studies focused on testing and interventions
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Contradictory results regarding the associations of the match performance with distance covered in long-term field protocols were also obtained. The YOYO intermittent recovery protocol was associated with high-intensity match activities in referees from different competitive levels [33, 34, 40]. Non-significant associations between the 12-min running test and total distance covered (r = 0.24) or time spent in high intensity (r = 0.35) in assistant referees were found [36]. Moreover, the 12-min running test did not discriminate referees of different competitive levels but, top-level referees covered 514 m more and 602 m more during YOYO protocol than those in third or fourth divisions, respectively [45]. The change of direction ability test was reproducible in referees [46] and also differentiated national referees from regional referees [125]. The intermittent endurance protocol was reproducible for assistant referees [8].

The effect of age on physical performance is not conclusive (Table 2). Studies found a negative relationship between age and the 12-min running test, countermovement jump [15], 40-m sprint test and distance covered on the YOYO protocol [16]. In contrast with the previous results, two papers reported that younger and older referees attained similar velocities at 4 mmol L−1 of blood lactate [15] and an age-associated variation was not found in maximal oxygen uptake, the 12-min running test or the 200-m sprint test [42].

High-intensity intermittent protocols improved physical performance in soccer referees [34, 54, 57]. In addition, a re-warm up intervention demonstrated considerable improvements in the endurance performance by YOYO test [58]. Strength, jump and velocity performance were reduced after a soccer match [51, 53].

Nutrition

This topic included the articles focused on body size, body composition and nutrition. Height was associated with competitive level and authority [59, 60] and match statistics (i.e. the number of yellow cards, red cards and penalties given per match) [24]. Although few studies reported fat mass, an overall conclusion that emerged from the included studies was the inverse relationship between age and fat mass percentage [61,62,63]. Brazilian referees aged 20–30 years had, on average, 16.9% body fat percentage, while older groups tended to present values of fat mass percentage around 19% [61]. Lower values of fat mass percentage obtained by bioimpedance analysis were found for Spanish referees (mean: 7.8%), and younger officials presented less fatness than their peers [62]. Referees tended to present lower values of fat mass compared with assistant referees [62, 64, 67]. Based on dual-energy X-ray absorptiometry, fat mass assessment for referees and assistance referees was 15.6% and 17.8%, respectively [64]. Mean heights and weights obtained from the studies included in the current review were plotted against age. The data suggested a reasonable degree of heterogeneity in the distributions of height and weight among referees of different age groups (Fig. 3a and b). Using of height and weight to discriminate between soccer referees by competitive level was not evident [44,45,46, 87, 101, 104, 125] (Fig. 1c and d). As shown in Fig. 4, the majority of data points referring to fat mass percentages were below 15% (21 of 34 data points).

Fig. 3
figure 3

Heights (a) and weights (b) of soccer referees plotted against age; heights (c); and weights (d) by competitive level

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Fig. 4
figure 4

Fat mass percentage estimated from different methodologies plotted against age in soccer referees

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Total energy intake was comparable in three studies (Table 3); however, macronutrient intake did not meet the nutritional recommendations [72,73,74]. Only three studies estimated the energy expenditure during training sessions [71] and matches [61, 68].

Table 3 Studies focused on nutrition topic
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Physiology

Different physiological variables were analysed among soccer referees—heart dimensions [17, 78,79,80], maximal oxygen uptake [17, 81,82,83,84,85] and strength [86, 87]. Two separate studies conducted in Brazil suggested that maximal oxygen uptake was lower in referees than soccer players [81, 85] (Table 4). Only one study compared aerobic fitness in male and female referees [83]. When normalized for weight, maximal oxygen uptake was significantly lower in females (48.1 ml kg−1 min−1) than male referees (51.9 ml kg−1 min−1) [83]. However, it did not discriminate referees and assistant referees [84].

Table 4 Studies focused on physiological outputs
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Discussion

The purpose of this systematic review was to synthesize the literature on match indicators, physical testing, nutrition and physiological profiling of soccer referees and assistant referees. There were no effects of age on match performance. Therefore, age may not present an acceptable indicator to determine international-level participation across soccer referees. Match performance is not similar among referees and assistant referees and it is also influenced by many factors (e.g. competitive level and competitive schedule). Consequently, match activities should not be exclusively used to corroborate physical tests. Height and weight did not discriminate referees, while 15% fat mass seems to present an acceptable value for referees.

Age is used as a criterion for international-level participation, with 25 and 23 years being the minimal age for a candidate for the annual FIFA Refereeing International List [7]. The candidates are selected by the member associations within FIFA, with the Portuguese Soccer Federation, for instance, adopting the age range of 25–37 years for referees and 31–39 years for assistant referees [128]. The upper age limits imposed by the governing bodies may require attention since the present review showed contradictory findings associated with age variation in match performance [54, 103]. Among 22 English Premier League referees followed between 2003 and 2007, younger referees (31–36 years) covered 907 m more in total distance and 266 m more in high-intensity running than older referees (43–48 years). However, differences between age groups were negligible for optimal positioning (i.e. distances from the ball and fouls) [54]. This suggests that although younger referees might cover greater overall and high-speed distance, older referees might not be disadvantaged based on the subsequent adjustments to positional sense, which might be gained through increased experience. Nevertheless, match performance is also influenced by other potential variables, such as competitive level [102,103,104] or competitive schedule [105]. These aspects might explain the variability between studies in match performance among referees and assistant referees.

Allowing for variation in match performance across studies, two important conclusions emerged from the present systematic review: referees and assistant referees differed on total distance covered, high-intensity actions, heart rate, type of movements and ratings of perceived effort [43, 89, 97, 112, 115, 120, 122]. Soccer referees covered higher total distance and high-intensity running [89] and also demonstrated higher ratings of perceived effort, maximum heart rate and mean heart rate than assistant referees [122]. Consequently, specific training programs should be developed for referees and assistant referees. Unfortunately, few studies have focused on training interventions among referees and assistant referees. In the present review, four intervention studies focused on referees [34, 54, 55, 57], while one study included assistant referees [56]. In the 2001–2002 season, the Belgian Football Federation introduced weekly training for referees comprising four purposes for each training session, including high intensity, speed endurance, speed training and recovery. The high-intensity intermittent training exercises performed on the pitch and track sessions were detailed in the original paper (p. 56S). After 16 weeks of intervention, high-intensity intermittent exercises improved the distance covered during the YOYO test [54]. In a separate study, high-intensity interval training and speed were effective strategies to improve the number of repetitions on 40 × 75 m FIFA protocol and distance covered on the COOPER test in 25 Turkish amateur referees [57]. Another study in assistant referees showed that 5 months of training with a linesman flag improved the sprint performance [56]. Therefore, the current lack of synergy between referees and assistant referees in relation to match demands and training practices means that both of these populations should be separately considered. Interventional studies in soccer referees and assistant referees, more specifically the training programs applied by the national organizations [128, 129], need future consideration.

Evidence is lacking as to whether the physical tests are aligned with the demands of the game. For example, the change of direction ability test (CODA) is proposed by FIFA to assess the ability of assistant referees' change of direction [7]. However, a trivial association between match performance and the time to complete the CODA test has been reported. Additionally, data quality about the methodological protocol used was not reported [70]. Furthermore, FIFA adopted the repeated sprint ability test (6 × 40 m) as an indicator of short-maximal outputs [7], but non-significant associations with mean heart rate during the match were found in 11 elite soccer referees [22]. FIFA also proposed a dynamic YOYO protocol that claims scientific validation. The assessment of physical capacity in soccer referees requires particular attention since studies using field protocols investigated match-related fatigue in soccer referees [43, 52, 53]. Evidence suggests that post-match 15-m and 30-m sprint performance [51] and isometric strength of non-dominant limb [53] were significantly lower compared with pre-match assessments; however, reductions were not observed in jump ability performance [52]. In a sample of 8 referees and 16 assistant referees, blood lactate increased by 1.7 mmol l−1 and 0.4 mmol l−1, respectively. Although the limited data indicated that referees finished the match with fatigue, muscle glycogen needs to be evaluated [130].

Fat mass had a negative impact on distance covered during the YOYO test and repeated sprint ability test [77]. Referees had lower values of fat mass percentage than assistant referees [62,63,64, 67], and younger referees tended to possess lower body fat than older referees [61,62,63]. The included studies tended to form a consensus that 15% fat mass is an acceptable value for referees. The Portuguese Soccer Federation provides bonuses to referees that achieve desirable fat mass percentage estimated from four skinfolds (biceps, triceps, subscapular and supra-iliac) [128]. However, possessing favourable body composition is not a prerequisite for international referees [7]. Additionally, the equation derived from 43 elite international soccer referees used two skinfolds (iliac crest and abdominal) and could be used to predict fat mass percentage in referees [66], but further research is needed before this predictive model can be adopted. It is also recommended that values are adjusted for referees and assistant referees given the difference in movement demands, and subsequent energy expenditure and physiological profiles likely influencing body composition.

Height was documented as an important characteristic to discriminate referees and assistant referees [60]. It was associated with authority and effectiveness during the match [59] as well as match performance [24]. However, data points of height and weight of the studies combined in the present review did not suggest a trend to distinguish between referees and assistant referees. Moreover, body size does not appear to discriminate referees of different competitive levels.

Energy and macronutrient intake in the three studies [72,73,74] suggested that energy intake ranged from 2409 kcal day−1 to 2819 kcal day−1. Additionally, for two of these studies, the Portuguese [72] and Spanish soccer referees [73] did not meet the recommendations for carbohydrate intake. Although the recommendations of soccer players have been generalized to referees [26], this point needs future consideration. In contrast with soccer players [131], no studies have determined the energy expenditure using the doubly labelled water method in referees, despite being considered the gold standard for measuring daily energy expenditure. Consequently, nutrition guidelines developed for soccer players should not be generalized to referees before detailed analyses are conducted.

Comparison of soccer referees and players also showed difference in maximal oxygen uptake [81, 85]. The referees attained lower values of maximal oxygen uptake but were typically older than the soccer players. Therefore, physiological parameters of aerobic fitness need to be frequently analysed among referees in order to ensure that they are able to reach aerobic levels comparable to soccer players [81]. Aerobic fitness also discriminated male and female soccer referees. Using an incremental running test, the maximal oxygen uptake in elite male referees was 3.98 l min−1, while elite female referees attained 2.94 l min−1 [83]. This study highlights the importance of aerobic training among female referees, especially if they are appointed for competitions in male soccer. This is even more important given that 6 female referees (3 referees and 3 assistant referees) were included on the list of 36 referees who participated in the World Cup hosted in Qatar 2022. It is also projected that there will be a higher number of female referees in male soccer moving forward. However, the literature in female referees is limited, and further research is required.

Although a considerable number of studies were extracted in the current review, a possible limitation is the inclusion of studies written solely in English. Additionally, studies investigating decision making, psychological aspects or visual parameters were not considered, but are key components for refereeing performance [132, 133]. The studies included in the present review also present limitations that should be acknowledged. Most of the studies were observational in design, which does not assess a cause–effect relationship. Furthermore, a negligible number of studies adequately determined the sample size, and the longitudinal studies ignored intra-individual variability.

Conclusions

Older referees maintained the same distances from the ball, despite demonstrating reductions in running output. Therefore, age appears to be a poor criterion for classifying soccer referees. Match performance is affected by many factors and thus should not be exclusively used to validate field protocols. Height and weight were not useful in differentiating between soccer referees of different levels. The assessment of physical fitness warrants further attention given that there are high levels of fatigue that are present during the latter stages of matches. In order to attain ideal levels of body composition, future studies need to provide guidelines for daily energy expenditure and nutritional intake.

Availability of Data and Materials

The authors confirm that the data supporting the findings of this study are available within the article and its additional files.

Abbreviations

UEFA:

Union of European Football Association

PRISMA:

Preferred reporting items for systematic review

CODA:

Change of direction ability

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Authors and Affiliations

  1. University of Coimbra, Research Unit for Sport and Physical Activity, Faculty of Sport Sciences and Physical Education, Coimbra, Portugal

    Diogo V. Martinho & Hugo Sarmento

  2. Laboratory of Robotics and Engineering Systems, Interactive Technologies Institute, Funchal, Portugal

    Diogo V. Martinho & Élvio R. Gouveia

  3. Department of Sport and Exercise Science, Manchester Metropolitan University, Manchester, UK

    Adam Field

  4. CIDEFES, Centro de Investigação em Desporto, Educação Física e Exercício e Saúde, Universidade Lusófona, Lisbon, Portugal

    André Rebelo

  5. COD, Center of Sports Optimization, Sporting Clube de Portugal, Lisbon, Portugal

    André Rebelo

  6. Department of Physical Education and Sport, University of Madeira, Funchal, Portugal

    Élvio R. Gouveia

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Contributions

DVM and HS conceptualized the manuscript; DVM, AF and HS wrote the original draft; DVM, AR and HS evaluated the bias; and DVM, AF, AR, ERG and HS reviewed and edited the manuscript. All authors have read and agreed to the published version of the manuscript.

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Correspondence to Diogo V. Martinho.

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Supplementary Information

Additional file 1: Table S1.

Summary of study characteristics (sample, country, body size and composition) and purpose.

Additional file 2: Table S2.

Studies that reported height, weight or fat mass.

Additional file 3.

Risk of bias for each study using the tools proposed by the National Institutes of Health.

Additional file 4: Table S4

. Tests often used among soccer referees.

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Martinho, D.V., Field, A., Rebelo, A. et al. A Systematic Review of the Physical, Physiological, Nutritional and Anthropometric Profiles of Soccer Referees. Sports Med - Open 9, 72 (2023). https://doi.org/10.1186/s40798-023-00610-7

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