Recently, a number of studies have been published on soccer refereeing examining the match activities of elite-level field (FR) and assistant referees (AR) (6,11,13). These studies have addressed the interaction of factors such as physical fitness (7,12,14), competition standard (14,19), fatigue over match periods (9,21), and the effect of ball displacement and team's activities on refereeing (15,16,21). In general, a top-class FR covers 11–12 km of which approximately 10% is performed at high intensity (i.e., >18 km·h−1). Like soccer players, FR and AR experience a number of short accelerations (ACs) at maximal or near maximal intensities that could represent an important source of muscle strain and energy expenditure but that are not considered with traditional kinematic analysis (17). Therefore, this analysis is needed for a more precise characterization of referees' activities because the frequency of AC performed by officials is not known, including the relationship between AC frequency and fatigue over the match.
Match demands considered as distance covered in selected activities has been reported to be affected by team standard and competitive level (6,14,21). In this regard, a comparative study (18) showed that game style may affect match physical performance in elite soccer teams. More specifically, Rienzi et al. (18) reported lower match work-rate in American teams when compared with English Premier League teams. Similarly, it may be expected that officials could experience different demands according to teams' game style because player and referees' activities are related (21). Therefore, it may be speculated that American elite referees might display a match physical performance different to that of their European counterparts. Unfortunately, no studies are currently available on elite-level American soccer referees. This could be of great interest in order to develop specific training strategies (6) and assist in the selection of officials for top-level international matches played in other continents. Thus, America's Cup represents a unique opportunity for studying the performance of top-class American soccer referees and to confirm the physiological strain of these referees and compare with that exhibited by European referees in similar high-level competitions.
During the match, the FR has full responsibility of the game with 2 AR helping him mainly for taking control of the off-side line. The AR's physical performances have been investigated in a number of recent investigations. For instance, Krustrup et al. (11) showed that AR fitness may affect match assignments (e.g., distance from off-side line) and match activities (e.g., distance covered at high intensity). Moreover, Helsen and Bultynck (10), studying European FR and AR during the UEFA Tournament, reported remarkable differences in cardiovascular stress between the 2 officials categories. From these and other previous studies (12,13), it may be suggested that AR exhibit a different pattern of match-related activities and a lower total distance. However, to the best of our knowledge, no study has simultaneously compared match activities and cardiovascular stress between FR and AR during the same tournament. This comparison would be unique as prior studies of FR and AR have only considered cardiovascular stress, a measure reported to be only marginally related to match activities (14).
Therefore, the aim of this study was to examine the physical and physiological demands of top-class soccer FR and their AR (i.e., officials trio) during the football America's Cup. We hypothesized that AR would exhibit lower cardiovascular stress and match-related activities in comparison with FR. Additionally, it was expected that American officials would exhibit lower match demands when compared with their European counterparts.
Experimental Approach to the Problem
The FR and their AR were equipped with a global positioning system (GPS) unit and a heart rate (HR) monitor for simultaneous recording of match activities (i.e., standing, walking, slow running, medium-intensity running, high-intensity running, and maximal-speed running), and exercise HR during matches of 2007 America's Cup. The referees' displacement at different velocities and accelerations and the cardiovascular stress experienced during the matches were subsequently analyzed with respect to 5- and 15-minute periods, halves, and the whole match.
Top-level FR (age: 40 ± 2.2 years; height: 1.75 ± 0.42 m; weight: 68.0 ± 6.8 kg, n = 7) and their AR (age: 36 ± 5.2 years, height: 1.74 ± 0.35 m, weight: 76.1 ± 6.3 kg, n = 7) were randomly selected from all the officials participating (i.e., 13 FR and 12 AR) in the 2007 America's Cup from June 26th to July 15th. All officials trained 3 to 5 sessions per week depending on the competition schedule and were required to successfully undertake the field tests determined by FIFA (14) just before competition. They were hosted in the same hotel during the tournament, and therefore, their nutritional habits and timetable were similar and consistent across the study. After receiving a detailed explanation of the procedures of the study, FR and AR provided informed written consent. The study received the approval of the Ethics Committee of the University of Granada for experimentation with human participants.
Officials' match activities were monitored using GPS Technology (SPI Elite, GPSports Systems, Australia). During the matches AR and FR wore a GPS device (1 Hz) equipped with a triaxial accelerometer (100 Hz) in a custom back-pack that was fitted with an adjustable neoprene harness. The GPS device was turned on 45 minutes before matches, and a minimum of 4 satellites were detected. Motion activities were recorded during the warm-up to ensure a functioning GPS unit. The validity and the reliability of this system have been previously reported with a coefficient of variation of 1.7% (4). Additionally, a recent study (1) has shown the appropriateness of GPS technology for the evaluation of subelite soccer referees' activities. Despite the absence of a gold standard method (3) to confirm GPS validity, the use of GPS in the current study was deemed appropriate considering its good reliability (4), the incorporation of accelerometry, and use in prior studies of players and referees (1,3).
Subsequently, the data recorded were exported to the proprietary software (Team AMS software V 18.104.22.168, GPSports) for analyses. Based on previous studies (6,8), the following categories were considered for match-related activities: standing (0–0.4 km·h−1), walking forward (0.5–4 km·h−1), low-intensity run (4.1–8 km·h−1), medium-intensity run (8.1–13 km·h−1), high-intensity run (13.1–18 km·h−1), and maximal-speed run (>18.1 km·h−1). Individual peak running velocities and the number of high-intensity accelerations (i.e., ACs) were also recorded. According to the acceleration-power relationship described by Osgnach et al. (17), we assumed a value of 1.5 m·s−2 within a velocity range of 8–18 km·h−1, as the threshold for identifying ACs occurrence.
For the evaluation of cardiovascular stress, a telemetric portable HR monitor (Polar T31, Polar Electro Oy, Kempele, Finlandia) was employed. Subsequently, time spent at spent at various exercise intensities (<65% HRmax; 65–75% HRmax; 76–85% HRmax; 86–95% HRmax; and >95% HRmax) were considered for analysis (10). Additionally, a new index, the performance efficiency (Effindex) was examined because it integrates mean velocity (i.e., external load) with respect to the relative cardiovascular stress (i.e., internal load) over a determined period of time into a single parameter. The Effindex (arbitrary units) was calculated (velocity in meters per minute per percent maximum HR [%HRmax]) for every 15-minute period.
Variables are shown as mean (SD). All the considered match-related variables were analyzed over 5- and 15-minute time intervals, and also after halves and the entire match. Between groups comparisons were performed using a 2-way mixed analysis of variance (ANOVA) design. Where appropriate, an ANOVA for repeated measures was performed. Post hoc differences were assessed with pairwise comparisons with Bonferroni correction. Pearson's product moment correlation coefficient was applied for verification of the relationships between selected parameters. The significance was set at 5%.
The FR covered a mean total distance of 10,197 (952) m, whereas AR covered 5,819 (381) m (p < 0.05). The absolute and the relative (percent) values for each activity category in both FR and AR are summarized in Table 1. Briefly, officials did not exhibit any difference between halves for any match-related activity except for medium-intensity runs distance for FR. The distance covered for every match-related activity was significantly different between officials' categories with FR exhibiting longer distances for all walking and running activities. No differences were detected between halves when comparing 15-minute intervals.
Groups' activities frequency and distance covered in the ACs category every 15 minutes of play are presented in Table 2. There were no significant differences between FR and AR for the number of ACs over the 15-minute intervals (p > 0.05), with FR exhibiting systematically a greater distance covered during these actions (p < 0.05). For FR, the number of ACs was significantly reduced during the second half (p = 0.027), with a tendency detected (p = 0.081) in the distance covered during these actions. In contrast, there was no significant difference between halves for AR (p > 0.05).
The number of ACs was significantly correlated with the number of maximal-speed activities (r = 0.501; p = 0.001; and r = 0.537; p = 0.007; for FR and AR, respectively). Similarly, the distance covered during ACs and maximal-speed running were also significantly correlated (r = 0.606; p = 0.000; and r = 0.502; p = 0.012; for FR and AR, respectively). Moreover, a significant correlation was found between the number of ACs of FR and the number of ACs of their AR recorded every 5 minutes over the whole match (r = 0.336; p = 0.03). Furthermore, the distance covered during ACs by FR and their AR was also significantly correlated (r = 0.367; p = 0.017).
Regarding cardiovascular stress, the mean HR was of 85.6% (4) and 75.3% (5.6) of HRmax for FR and their AR, respectively (p < 0.05). Considering the entire match, a significant correlation (r = 0.457; p = 0.057) between the mean %HRmax and the distance covered by the FR in every 5-minute interval (Figure 1) was identified. Furthermore, the Effindex was significantly worse at the end of both halves for FR, and only at the end of the second half for AR (Figure 2).
This is the first study that examined simultaneously match-related activities and cardiovascular stress of top-class American Officials (i.e., officiating trios) in the same tournament. The main findings of this study were the association of FR and AR match activities, and the reduced number of ACs during the second half for FR without any difference observed for high-intensity actions between halves.
As expected, the total distance and the number of actions in different running categories were found to be different between FR and AR. Additionally, the profile of such actions for every category was similar to that reported in previous studies (13,11,14). In contrast, there were no differences between halves nor between 15-minute periods for the match-related activities analyzed in both FR and AR (p > 0.05), except for medium-intensity runs in FR (p = 0.049). Therefore, the expected effect of fatigue in high-intensity and maximal-speed runs was not evident in this study. Given the similar total distance covered and the same match-related activities profile as reported in previous studies (11,14), this could imply that the referees of our study were of high fitness during the investigated tournament. In fact, they were required to successfully complete the FIFA battery tests just prior to the tournament confirming their readiness for competition. Alternatively, the match tempo may have influenced these variables (8); however, we do not know the magnitude of such influence. Nevertheless, it should be pointed out that the main limitation of this study was that the nonorthodox activities (e.g., side or back running) could not be discriminated with the current methodology. Therefore, it is not known how this kind of activities were impaired as a consequence of fatigue over the entire match. However, side and back run is used by referees at low or medium intensities to adjust the proper position before or during playing transitions. Therefore, their relative importance may not be as important as other physical capabilities such as sprinting or ACs.
The mean total distance covered by top-class American referees during America's Cup was similar to that reported in previous studies of international tournaments (i.e., ∼10 km) (11,14). However, the total distance was lower than the values exhibited by Italian and English elite referees during their respective national leagues (i.e., >11 km) (7,19,20,21). Similarly, ARs covered a similar total distance to that during the FIFA Confederations Cup (15), but lower than that reported in the National Danish League (12). Previously, it has been suggested that factors like physical fitness, competition standard or the total distance covered by the ball could influence total distance covered by both FR and their AR (7,12,14,15,21). Although American top-level referees should demonstrate a high physical fitness level similar to that of their European counterparts, it may be suggested that matches during international tournaments may result in lower demands with regard to total distance when compared with national leagues competition. This could be a consequence of the tactical skills performed by the national teams, with a more conservative game style during the qualifying rounds. Further, the association between player and referee match activities (21) may also help explain the lower total distance covered by officials during the America Cup. Therefore, the game style related to the nature of the competition, and not to the geographical origin of the teams, may be the major factor that accounted for the differences exhibited between the current and previous studies. Future studies are needed for appropriately testing this hypothesis.
This is the first study analyzing ACs in refereeing. The ACs analysis was based on previous studies that showed the high energy demand of these actions in soccer players (17), and the significant effect of fatigue on the number of ACs over the match in Australian football players (2). In this study, FR performed a significantly higher number of ACs compared with their AR (p < 0.05). This could be a consequence of the large playing area that FR have to cover compared with their AR. Previous studies reported that FR perform about 17 sprints of maximal or near maximal velocity (11) over the entire match. This study showed that elite American soccer referees perform on average approximately 70 ACs, which represent a substantially greater larger number of fatiguing and energy demanding actions. This high frequency of ACs highlights the importance of tracking ACs to assist officials in their preparation and match evaluation.
Interestingly, the activity frequency and the distance covered during ACs were associated with maximal intensity activities. This reinforces the importance of this ability for refereeing as a prerequisite for achieving high running velocities. Moreover, the correlation between the number and the distance covered by FR and their AR during ACs suggests a key role of this ability during refereeing. It may be expected that this synchronization between officials and assistants ACs reflects the current demands of the game. Additionally, while there were not differences in high-intensity activities between halves for both officials, a significant difference in the number of ACs between halves for FR was noted. This suggests that the deterioration of this ability over the course of the match could impair the capacity of referees to be near infringements. Nevertheless, this ACs reduction as the match progresses may also be a consequence of teams' game style and the effect of fatigue on players. Significantly lower distance covered at medium intensities during the second half for FR (p = 0.049) could have contributed to the ACs reduction; however, no correlation was detected between these parameters. Therefore, further studies addressing refereeing style, match activities, and the distance from infringements are warranted. More importantly, the current results reinforce acceleration capacity as an important fitness component of soccer referees.
The mean HR and the HR profile of FR and their AR was similar to that of previous studies with elite-level and top-class soccer referees (6,10,11). This reinforces the validity of HR profile for a simple evaluation of referees' cardiovascular stress during matches that is independent of competitive level (5). Furthermore, the reported correlation between the mean %HRmax and the distance covered during the same 5-minute interval for FR (r = 0.457; p = 0.057) suggests a relationship between cardiovascular stress and match-related activities when a short interval period is considered (14,15). This association may be influenced by the time delay that exists between the change in match activity and the subsequent HR alteration, or the impact of psychological stress (5). Nevertheless, the current and previous results suggest that the HR profile could be used as a simple and valid tool for the evaluation of match demands in referees of a similar level. Interestingly, the new Effindex was demonstrated to be more sensitive than the match categories analysis for detecting a deterioration in referees' performance across 15-minute time intervals. This new index that associates the mean velocity per mean relative HR during a determined period (e.g., 15 minutes) expresses how efficiently one can run with a given cardiovascular stress. Although we did not find any difference in match categories among 15-minute periods, the Effindex was demonstrated to be sensitive enough for detecting a lower running efficiency at the end of both halves for FR and in the last 15 minutes of the match for AR (Figure 2). Consequently, we would suggest the employment of this new index for comparisons of referees' and athletes' performances because this method has been demonstrated to be more sensitive to the effect of fatigue on referees compared with traditional match-related activities.
It should be noted that the current results were limited to a small number of officials examined during America's Cup. Although the current results warrant further confirmation with greater sample sizes, it is important to consider that the America's Cup provided an unique opportunity for studying the majority of the best American soccer referees (i.e., ∼60% of total officials available).
From these results, Top-Class American FR and AR experienced a work load during the America's Cup, which was similar to that previously reported during other European tournaments. Additionally, correlations between the distance covered and the %HRmax experienced by FR during the same 5-minute period indicated a high work load for FR that is impaired in the second half for FR (i.e., lower Effindex). The high number of ACs observed, the effect of fatigue on these actions over the match, and the synchronization of both FR and AR suggest that the ACs should be further considered for the evaluation and preparation of referees.
Field and assistant American soccer referees may follow similar recommendations to their European counterparts with regard to physical preparation as they exhibited a similar work load in an international tournament. However, accelerations should be further considered as an important part of the physical training programs of both FRs and ARs. The new Effindex could be considered an appropriate tool for detecting changes on referees' and athletes´ performance over the entire match.
The authors would like to thank referees for participation on this study. They also want to recognize the useful comments and the English revision of Dr. Anthony Leicht. The results of this study do not constitute endorsement of the product by the authors or the National Strength and Conditioning Association.
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