elite futsal refereeing: activity profile and physiological demands

ELITE FUTSAL REFEREEING: ACTIVITY PROFILE PHYSIOLOGICAL DEMANDS

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ANTOŃIO N. REBELO,1 ANTOŃIO A. ASCENSAÕ,2,3 JOSE´ F. MAGALHA˜ES,2,3 RASMUS BISCHOFF,4 MADS BENDIKSEN,4 AND PETER KRUSTRUP4 1

Soccer Department, Faculty of Sport, University of Porto, Porto, Portugal; 2Centre for Research in Physical Activity, Health and Leisure, Porto, Portugal; 3Department of Sports Biology, Faculty of Sport, University of Porto, Porto, Portugal; and 4Institute of Exercise and Sport Sciences, Department of Human Physiology, University of Copenhagen, Copenhagen, Denmark

ABSTRACT Rebelo, AN, Ascensaõ, AA, Magalha˜es, JF, Bischoff, R, Bendiksen, M, and Krustrup, P. Elite futsal refereeing: activity profile and physiological demands. J Strength Cond Res 25(4): 980–987, 2011—The purpose of this study was to determine the physiological demands and to establish the relationship between activity profile and endurance capacity of futsal referees. Eighteen elite futsal referees (33.0 6 5.1 years, 173 6 5 cm, and 73.2 6 8.4 kg) were studied. Video filming (n = 18) and heart rate (HR) recordings were performed throughout games. Blood lactate (n = 14) was determined at rest and after the game. Endurance capacity was determined with the Yo-Yo IE2. The number of activity changes was as high as 1,395 6 218 (6SD). Total distance covered, high-intensity running (HIR), sprinting (SPR), and sideways running were 5.89 6 0.56, 0.96 6 0.29, 0.09 6 0.07, and 0.91 6 0.46 km, respectively. The number of HIR and SPR bouts was 129 6 41 and 9 6 8, respectively, with a mean duration of ;1.4 seconds. Blood lactate content was 1.0 6 0.3 and 1.5 6 0.5 mM before and after the game. The amount of HIR performed during the match correlated significantly (r = 0.77; p , 0.05) with the Yo-Yo IE2 performance. Considering the data obtained in the present study, the use of match-specific intermittent fitness tests to evaluate futsal referees seems to be required.

KEY WORDS heart rate, blood lactate, high-intensity running, Yo-Yo IE2 test, sideways running

INTRODUCTION

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he responsibility of the referees of the so-called invasion sports (i.e., handball, basketball, and soccer) to implement the rules of the game and to guarantee that the players abide by the regulations require that the referees are obligated to keep up

Address correspondence to Antońio N. Rebelo, [email protected]. 25(4)/980–987 Journal of Strength and Conditioning Research Ó 2011 National Strength and Conditioning Association

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with play to be in a good position to notice infringements. Moreover, the success of refereeing in these sports is related, at least partially, to the ability of the referees to meet the physical and physiological demands imposed during the match play (10,22,24). Several studies dealing with soccer refereeing have been conducted to evaluate the physiological impact of the game and the referees’ motor pattern activity profile (9,10,15). Furthermore, these data allowed the design of physical tests providing relevant knowledge of referees’ preparedness, contributing to improve training programs, and strategies undertaken to meet refereeing demands (11,15). However, despite some similarities with soccer refereeing, several futsal-specific features, including pitch size, number and position of the referees, as well as game rules, likely impose distinct physical and physiological strains. This Fe´de´ration Internationale de Football Association (soccer) (FIFA)– regulated sport has become a popular indoor alternative to football, played on all the continents of the world in over 100 countries by more than 12 million players. As an example, futsal is now played at about 1,100 boys and girls’ clubs throughout the U.S.A. The major differences between rules of soccer and futsal consist in the local of practice (indoor), in the pitch size (20 3 40 m), in the number of players of each team (goalkeeper plus 4 field players), and in the total duration of the match (two 30-minute halves of effective playing time). As in other indoor team sports, 2 field referees are responsible to implement the rules of futsal. As was referred above, soccer and futsal are regulated by FIFA. However, contrarily to soccer players and referees (6,8,12), the scientific knowledge regarding refereeing physical and physiological demands of competitive futsal matches is lacking. An important issue of analysis is to which extent the activity profile and the physiological strain of the referees are related to their physical capacities. For instance, previous data showed that in soccer players (15,18), soccer referees (9), and soccer assistant referees (16), the amount of high-intensity running (HIR) and the total distance (TD) covered by the practitioners during the game were significantly correlated both with aerobic performance and with the maximal ability

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Journal of Strength and Conditioning Research to exercise intermittently (measured using Yo-Yo intermittent tests or repeated sprint tests). Therefore, the aims of the present study were to describe and to analyze the activity profile of elite futsal referees during competitive games and to establish the relationship between activity profile categories and the intermittent endurance performance of the referees.

METHODS

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Procedures

Time-motion, HR, and lactate analyses were performed during a total of 9 official futsal matches in the Portuguese top league, in which both referees were analyzed. The 10 international referees also accomplished an intermittent endurance test. The study was conducted in the middle of the 2005–2006 season, at the fourth and fifth months of the competitive period (December and January). During the study, subjects followed the weekly training routines and refrained to be involved in any other type of exercise.

Experimental Approach to the Problem

To describe and analyze the activity profile of first league Portuguese futsal referees during competitive games and to establish the relationship between activity profile categories and the intermittent endurance performance, 18 referees participated in this study. With this study, we pretend to have more knowledge contributing to improve training programs to meet refereeing demands and to analyze to which extent the activity profile and the physiological strain of the referees are related to the endurance in intermittent exercise. The activity profile in competitive games was analyzed using time-motion methodology. Heart rate was recorded throughout entire match. Time-motion and HR data were analyzed in periods of 10 minutes as a strategy to elucidate the time course of workload during the game and to diagnose eventually the occurrence of fatigue. Blood samples were collected at rest, at halftime, and after official matches to determine blood lactate concentration. In an attempt to analyze possible associations between match workload and the intermittent endurance capacity, 10 referees also performed the Yo-Yo intermittent endurance test—level 2 (Yo-Yo IE2).

Subjects

Eighteen elite Portuguese futsal referees (33.0 6 5.1 years, 173 6 5 cm, 73.2 6 8.4 kg, and 15.7 6 5.4% fat mass) participated in this study. The participants had at least 5 years of experience, in the top Portuguese futsal league and 10 of them were international referees. Prior to the match analyses, the referees refrained from strenuous exercise and alcohol for 48 hours and from tobacco and caffeine for 12 hours. The referees were instructed to consume a carbohydrate-rich diet over the last 24 hours prior to the match and that they should consume a light meal consisting of cereals, bread, and fruit approximately 2 hours prior to the game. The referees were allowed to drink water ad libitum during the game and the water intake was recorded. The referees were informed about the experimental procedures and possible constrains associated with the setup before providing their informed consent to participate. The experimental protocol was approved by the Ethical Committee of Faculty of Sport Sciences, University of Porto, Portugal, and followed the Declaration of Helsinki of the World Medical Association for research with humans.

Activity Profile Analysis. Time-motion analysis is a very useful tool to describe the workload of match play for field players (6,23) and referees (15). Each of the analyzed referees were video filmed close-up throughout the game by a VHS movie camera (NV-M50; Panasonic, Hamburg, Germany) placed at the opposite side of the pitch at the level of the halfway line, at a height of about 15 m, and at an approximate distance of 15 m of the touching line. The videotapes were later replayed for computerized recording of the referees’ movement patterns according to previous works using time-motion (6,15). The used motor pattern categories included TD covered, standing (0 kmh21), walking (6 kmh21), jogging (8 kmh21), low-speed running (12 kmh21), moderate-speed running (15 kmh21), high-speed running (18 kmh21), sprinting (SPR) (25 kmh21), backwards running (BR), and sideways running (SR). These categories and the corresponding mean speeds were determined after detailed analysis of the videotapes using glass premarkers previously established as reference. High-intensity running corresponded to the sum of the TD performed under moderatespeed running, high-speed running, and SPR. The percentage of total time, number of occurrences and duration, and TD covered were recorded for all the analyzed categories and for both match halves. The number of stops and turns performed from both forwards and sideways running (SR) was also measured. Considering the muscular effort involved in movement deceleration following high-speed running actions, only stops and turns preceded by high-intensity forwards running and SR were considered for analysis. The coefficient of variance in test-retest time-motion analyses has previously been shown to be in the range of 1–5% in each of the locomotor categories for soccer players (6) and referees (15). Specifically, the study by Krustrup and Bangsbo (15) revealed that the intraindividual difference in test-retest analyses of the TD covered by soccer referees during competitive games was 0.2 km (coefficient of variation [CV] 1%) and that the CV was 2, 5, 3, 3, and 2% for walking, low-intensity running, HIR, backwards running, and SR, respectively. In the study by Bangsbo et al. (6), it was furthermore shown that the variation in the results obtained by 2 independent observers was never more than 4% in any of the abovementioned locomotor categories. The 18 time-motion analyses performed in the present study were all carried out by the same experienced observer. For data analysis, total VOLUME 25 | NUMBER 4 | APRIL 2011 |

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Physical Demands of Futsal Referees During Games match time was divided in 10-minute periods. Considering that futsal games comprise two 30 minutes of effective playing time, but total match lasts more than 70 minutes, 4 periods of 10 minutes in each part were established. The last periods of each part (30+ and 70+) correspond to the interval time from 30 and 70 minutes, respectively, to the end of each part.

days after the match and preceded by a 10-minute warm-up. The Yo-Yo IE2 consisted of repeated 2 3 20-m runs with a progressively increased speed until exhaustion. The speed running is low in the beginning and progressively increases as dictated by an audible cue played from a CD. Subjects must be at one end of a 20-m base every time a cue is played. The test allows 5 seconds of recovery after every 20-m shuttle run. The second time that subjects are unable to maintain the speed running under the given signals, the test is over, and the Yo-Yo IE2 test performance is considered as the TD covered. The test was performed in running lanes 2 m wide and 20 m long, marked by cones. To familiarize the referees with the test procedures, a pretest was performed 1 week before the test.

Heart Rate and Blood Lactate. Heart rate (HR) was continuously monitorized during all matches for further evaluation of exercise intensity over the match (between first and second halves and in 10-minute periods). Heart rate was recorded at 5-second intervals throughout each game using a Polar Vantage NV HR monitor (Polar, Kempele, Finland). The HR monitor was attached to the referees 45 minutes before the kickoff. The highest HR value obtained during a Yo-Yo intermittent endurance level 2 test (see below) was considered as HRmax (13). To obtain an indication of the involvement of anaerobic metabolism for energy production, blood lactate concentrations were analyzed in 3 moments of the game. Blood samples (25 mL) were collected from the ear lobe at rest, at halftime, and 1–2 minutes after the end of the game and were immediately analyzed in duplicate to determine blood lactate concentration using an electroenzymatic device (YSI 1500 S; Yellow Spring Instruments, Yellow Springs, OH). A high (30 mM) and a low (5 mM) standard lactate references were daily used for previous calibration of the device according to the instructions of the manufacturer.

Statistical Analysis

The data are presented as mean 6 SD unless otherwise stated. Changes in match activities and HR within each 10-minute period of the game were evaluated by 1-way analysis of variance with repeated measures. When a significant interaction was detected, data were subsequently analyzed using a Newman-Keuls post hoc test. Differences in match activities and physiological responses to match play during the first and second halves of the game were determined using a paired t-test. The correlation levels between the performance on Yo-Yo intermittent endurance test and the high-speed running and SPR were determined using the Pearson’s correlation coefficient (1). Intraclass correlation estimates for measured variables ranged from 0.8 to 0.9. The level of significance was set at 5%.

Intermittent Exercise Capacity. Within the 4 weeks of data collection, 10 referees (internationals) undertook an incremental and intermittent shuttle running field test until exhaustion, the so-called Yo-Yo intermittent endurance test—level 2 (Yo-Yo IE2), following the instructions of the test. Yo-Yo IE2 is considered a valid field test to evaluate endurance performance (2,14). The test was performed 3

RESULTS Activity Profile and Distances Covered

The number of occurrences, mean duration, and percentage of total time in each of the 9 activity categories are summarized in Table 1. Briefly, the total number of match activities recorded was 1,395 6 218 over ;80 minutes,

TABLE 1. Activity profile of futsal referees during the game.*† Low Moderate High Standing Walking Jogging speed speed speed SPR Fraction of total time (%) Number of repetitions (n) Distance covered (m) Duration (s)

45.0 4.5 321 84 0 0 6.9 1.3

34.1 5.8 488 76 2,674 488 3.4 1.0

4.7 1.9 154 62 489 195 1.4 0.3

4.6 2.1 149 52 719 305 1.4 0.2

3.3 1.1 94 31 618 205 1.7 0.5

BR

SR

Sum

1.1 0.3 1.0 5.8 100.0 0.4 0.2 1.0 3.0 — 35 9 23 122 1,395 16 8 21 59 218 259 87 135 911 5,892 99 82 128 461 564 1.5 1.2 2.1 2.2 3.4 0.3 0.4 0.3 0.4 0.6

*Data are presented as mean 6 SD (n = 18). †BR = backwards running; SPR = sprinting; SR = sideways running.

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Figure 1. Distance covered by futsal referees in the match in 10-minute periods. Data are presented as mean 6 SD (n = 18). *Significantly different from 0 to 10 minutes (p , 0.05).

corresponding to a change in activity every 3.5 seconds on average (Table 1). The number of high-intensity activities was 138 6 41 (range: 92–245) with a mean duration of 1.5 seconds. The number of SR bouts was 122 6 59 (27–245) with a mean duration of 2.2 seconds. The TD covered was 5.89 6 0.56 (4.99–6.87) km of which HIR and SR accounted for 0.96 6 0.29 (0.59–1.69) and 0.91 6 0.46 (0.20–1.84) km, respectively. The distance covered by SPR was 0.09 6 0.07 (0.01–0.25) km. The distance covered by SR was higher in the first half compared with the second half (0.54 6 0.26 vs. 0.37 6 0.23 km), whereas no significant differences were observed in TD covered and HIR between the halves (p , 0.05). The TD covered in the first 10-minute period of the game was higher (p , 0.05) than that in the 30+, 50–60, 60–70, and 70+ minutes periods (Figure 1). No

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Figure 3. Distance covered in sideways running by futsal referees in 10-minute periods of the game. Data are presented as mean 6 SD (n = 18). *Significantly different from 0 to 10 minutes (p , 0.05).

differences (p , 0.05) were found between any periods of the second half. The distance covered by HIR in the first 10-minute period was higher (p , 0.05) than that in the last periods of the first and of the second halves (Figure 2). No significant differences (p , 0.05) were found between any periods of the second half. The distance covered by SR was higher (p , 0.05) in the first 10-minute period compared with all other periods of the game (Figure 3). The amount of HIR and SR decreased by 24 and 56%, respectively, from the first 10-minute period to the period from 60 to 70 minutes (p , 0.05). The number of stops and turns from forwards running and SR are given in Table 2. Briefly, the total number of stops was 28 6 6 (18–41) and the number of turns was 14 6 8

TABLE 2. Turns and stops performed by the futsal referees in the match. Stops From forwards running From SR Total Turns From forwards running From SR Total Stops and turns From forwards running From SR Total Figure 2. Distance covered by futsal referees in high-intensity running in the match in 10-minute periods. Data are presented as mean 6 SD (n = 18). *Significantly different from 0 to 10 minutes (p , 0.05).

21.3 6 5.1 7.0 6 3.0 28.3 6 5.9 8.1 6 4.9 6.2 6 4.2 14.3 6 8.0 29.4 6 7.4 13.2 6 6.4 42.6 6 11.4

SR = sideways running. Data are presented as mean 6 SD (n = 18).

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Figure 4. Correlation between the distance covered in the Yo-Yo intermittent endurance level 2 test and the distance covered at high-intensity running during the match (n = 10; p , 0.05).

(7–34). A significantly higher number of turns were observed in the first half compared with the second half (8 6 4 vs. 6 6 4; p , 0.05). Yo-Yo IE2 Performance

The performance of the Yo-Yo IE2 was 975 6 237 (560– 1,240) m. The Yo-Yo IE2 performance correlated with the amount of HIR during the match (r2 = 0.52; p , 0.05; Figure 4). The Yo-Yo IE2 test performance also correlated with the TD covered during a match (r2 = 0.40; p , 0.05), whereas it was not correlated to the relative aerobic loading (% of HRmax) during a match (r2 = 0.02; p . 0.05).

Figure 6. Peak and mean heart rate of futsal referees during the match. Data are presented as mean 6 SD (n = 18).

bmin21, which corresponds to 86 6 8% of HRmax. No differences (p , 0.05) were observed between the first and second half for mean HR (149 6 14 and 143 6 13 bmin21) and peak HR (173 6 14 and 172 6 13 bmin21). The mean and peak HR were not significantly different (p , 0.05) between 10-minute periods of the game, although it tended to become lower toward the end of the game (Figure 6). The referees had HR between 70 and 90% of HRmax for 78 6 6% of the total match time (Figure 7). Moreover, the referees exercised about 10% of the match time above 90% of their HRmax. The time spent with HR below 60% of HRmax was very low. Blood Lactate During a Game

Heart Rate During a Game

An example of a futsal referees’ HR response is given in Figure 5. The mean HR during matches was 146 6 13 (123–169) bmin21 corresponding to 76 6 6% of HRmax. The peak HR during the match was 167 6 14 (152–194)

Figure 5. Representative heart rate pattern from a futsal referee during the warm-up and throughout the match, interspersed by the breaktime.

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Blood lactate was 1.0 6 0.3 (0.8–1.5), 2.0 6 0.8 (1.3–3.2), and 1.5 6 0.5 (0.9–2.3) mmol/L before, at halftime, and after the game, respectively (Figure 8).

Figure 7. Percentage of match time spent at different interval percentages of referees’ maximal heart rate. Data are presented as mean 6 SD (n = 10).

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Figure 8. Blood lactate before, at halftime, and after futsal matches (n = 18).

DISCUSSION Futsal is becoming a popular indoor alternative to soccer, with specific rules and millions of players and fans worldwide. Given the growing competitive level of the game observed in the last decade, increasing physical and physiological demands have been imposed to futsal referees. However, in contrast to soccer, and to the best of our knowledge, this is the first study that analyzed the activity profile of futsal referees during competitive matches as well as the physiological demands imposed by the game. Given the differences in match duration and pitch size between soccer and futsal, time-motion analysis expectedly showed that futsal referees covered a lower TD than both top-level soccer referees and assistant referees (5.9 vs. 10.3 and 7.3 km, respectively) (6,16). For the examined elite futsal referees, the TD and the distance covered with SR and HIR decreased markedly from the first to the last 10-minute period, indicating that elite futsal referees experience fatigue toward the end of the game. In contrast to what has been observed for soccer players (19), we did not find a statistically significant decrease in running performance in the first 10-minute period of the second half, although the analyzed referees may well have had cold muscles after a 15-minute long passive half-time period (16,20). However, it should be emphasized that the statistical power of this comparison was only 0.55 and that further analysis is necessary to elucidate whether the running performance is deteriorated for futsal referees in the initial period of the second half. As for soccer match officials, the activity profile of futsal referees is characterized by long periods of activity performed at low-intensity exercise interspersed with brief periods of high-intensity exercise. Thus, futsal referees perform numerous bouts of high-intensity runs as well as several near-

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maximal and maximal sprints, although it should be noted that the duration of the intense periods are shorter than for soccer referees and assistant referees (;1.5 vs. ;2.1 seconds). Similar to the movement pattern previously reported for soccer assistant referees (16), our data also demonstrate that futsal referees accomplished long distances in sideway running (;1 km). Actually, in contrast to soccer assistant referees, instead of jogging, SR represented the runningbased activity with the highest fraction of the total match time. This might be considered an important energetic demand because Reilly and Bowen (21) have shown that SR implies higher energy expenditure than forward running. Furthermore, our data showed that futsal referees performed a considerable number of stops (;30) and turns (;15), although fewer than the values found by Bloomfield et al. (7) in professional soccer players from FA Premier League soccer (90–100 turns of 90–180°). Combined with the finding of a high number of activity changes (;1,400), these data emphasize that unorthodox movement, accelerations, and decelerations are an important part of the activity profile of futsal referees. As this type of actions, particularly decelerations, imply eccentric muscle contractions, it is likely that SR bouts, stops, and turns contribute to additional mechanical demand and are important physical stress contributors for muscle fatigue in futsal refereeing (4). Although conducted with soccer players, a very recent study from Bloomfield et al. (7) found a high frequency of turns within match play. The authors observed that these 500–700 turns per match were performed at distinct degrees of arc movement (0–360°) and were distributed according to the position of the players. Based on the results of this study, the authors questioned the ecological validity (only through physiological measurements and closely related to the physiological load imposed through match play) of some ‘‘gold standard’’ soccer-specific fitness tests with respect to motion types, directions, turns, and intensities of the physical demands. The data obtained in the present study showed a specific motor pattern of futsal referees during the match, with an elevated percentage of SR. Considering these results, the design of new or adapted fitness tests to evaluate futsal referees should not be excluded. It is also noteworthy that, alike soccer assistant referees (16), futsal referees exercise intermittently at moderate to high HR throughout the match (78 6 6 vs. 73 6 2% of individual HRmax, respectively). However, a more detailed analysis of the data revealed that futsal referees exercise at a higher intensity than soccer assistant referees (;50 vs. 31% of match time above 80% of individual HRmax, respectively). Taken together, time-motion and HR analyses suggest that futsal refereeing relies mostly on the aerobic metabolism for energy production. In fact, the aerobic pathways for energy production is under moderate stress during some parts of the match but is heavily taxed during substantial periods of the match time. Unexpectedly, the present study revealed that blood lactate at the end of the game was only slightly higher than resting VOLUME 25 | NUMBER 4 | APRIL 2011 |

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Physical Demands of Futsal Referees During Games levels. However, it cannot be excluded that higher blood lactate values were present earlier in the game, where more HIR and SR were performed. Actually, as can be depicted from Figures 2 and 3, both HIR and SR were significantly reduced toward the end of each half, which would certainly contribute to the low blood lactate values found and suggests the underestimation of lactate values during the match. Thus, investigations conducted in various team sports have reported an individual relationship between blood lactate concentration and the amount of HIR performed in the last 5-minute period prior to sampling (6,17). Accordingly, Krustrup and Bangsbo (15) found an individual relationship between blood lactate concentration and mean HR during the last 5 minutes before sampling in top-class soccer referees during competitive matches. Nevertheless, the average blood lactate values obtained after the investigated futsal games (1.5 mM) are markedly lower than values obtained at the end of matches for soccer referees (5.1 mM) and soccer assistant referees (4.8 mM), indicating a smaller reliance on glycolysis. A finding that may well be related to the fact that the sprints are shorter (;1.2 vs. ;2 seconds) for futsal referees than for soccer assistant referees and that futsal referees rarely sprint over more than 15 m. In fact, it is well known that the creatine phosphate breakdown accounts for most of the anaerobic energy turnover in the very early phase of intense muscle activity (3). Furthermore, the number of sprints was observed to be fewer for futsal referees than for soccer assistant referees (9 vs. 20), also when accounting for the difference in playing time. In conclusion, the activity of futsal referees is generally characterized by intermittent moderate to high intensity with numerous very brief bouts of fast speed and SR interspersed by long periods of low-intensity recovery. Moreover, most of the energy production is afforded by the aerobic metabolism and with considerable reliance on creatine phosphate breakdown for anaerobic adenosine triphosphate resynthesis, which may be due to the specificity of the futsal pitch size and the physical demands related to the ball and player movements. The present data reinforces the importance of sprint performance and the ability to recover between intense exercise periods for futsal refereeing. Therefore, sprint and match-specific intermittent exercise training strategies may be useful to improve the physical performance of futsal referees.

PRACTICAL APPLICATIONS Characterization of the external (time-motion) and internal (HR and blood lactate) workload imposed during competitions is an essential descriptive step preceding the design of conditioning training and physical evaluation of futsal players and referees. To the best of authors’ knowledge, this is the first study providing data on time-motion analysis of futsal referees. The data presented in this article will help practitioners and coaches to better design training programs and specific physical tests for elite futsal referees. According to the obtained time-motion and physiological

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variables, both physical evaluation and training for futsal referees must combine low- and high-intensity forwards running and sideways running. For training purposes and given the relationship between intermittent endurance capacity and distance covered at high intensity during the match, attention must be devoted to prolonged intermittent exercise aerobic high-intensity training (5). Considering the observed frequency of stops and turns and the mechanical and metabolic implications of eccentric muscle contractions, the training programs and physical tests must also include these types of actions. To summarize, the present study provides evidence that both training and testing of elite futsal referees should include intermittent low- and high-intensity forwards running and sideways running as well as specific futsal actions such as stops and turns.

ACKNOWLEDGMENTS This study was supported by the Portuguese Football Association and the Danish Football Association. We are greatly indebted to the referee Pedro Paraty and to the President of the Refereeing Association Mr. Carlos Esteves. Antońio Ascensaõ is founded by a Post Doctoral fellowship from the Portuguese Foundation for Science and Technology (SFRH/BPD/42525/2007).

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