The major responsibility of soccer referees is to regulate the match by making accurate decisions about play. For precise decision making, optimal positioning on the playing field seems to be crucial, because the lowest error rate was recorded when referees judged incidents from a distance between 11 and 15 m (13). The risk of error increased when referees were at a distance greater than 11–15 m (13). To reduce their distance from foul play incidents, referees must increase their amount of high-intensity running (i.e., increase their running speed to faster than 15 km·h−1) (8). However, the amount of high-intensity running has been shown to decrease in the initial 15 minutes of the second half of soccer matches compared with the same phase in the first half (7,10). In addition, a previous study indicates that sprint and jump performance decreases after traditional, passive halftime practice (16). These reductions of sprint, jump performance and high-intensity running in the initial phase of the second half in soccer referees have been suggested to be due to lack of physical preparation before the second half (15). For example, muscle temperature, which is associated with sprint performance (5), decreased by 1.5° C in an assistant referee at halftime when a traditional passive half-time practice was performed (9).
Several studies have suggested that halftime rewarm-up (RW) exercise of low-to-moderate intensity in the later part of halftime maintained the soccer players' sprint and jump performance from the initial phase of the second half (5,11,12,16,19). Edholm et al. have suggested that a continuous halftime RW exercise for 7 minutes at 70% of the maximum heart rate (HRmax) attenuated reductions in sprint and jump performances compared with a traditional passive halftime practice in professional soccer matches (5). However, a continuous halftime RW exercise for 7 minutes may not be realistic for referees, because they undergo tactical debriefing, medical and/or nutritional treatments, and personal preparation during the halftime period (18). In addition, soccer referees have a few obligations in the last part of halftime period (e.g., preparing their substitute before the second half and/or instructing the players to return to the field for beginning the second half on time). In practical applications, halftime RW with intermittent exercise can be performed alongside their other duties during the rest period. However, to our knowledge, there has been no study regarding the effects of halftime RW with intermittent exercise on subsequent exercise performance.
Therefore, the purpose of this study was to examine the effects of halftime RW with intermittent exercise on subsequent exercise performance. We hypothesized that halftime RW would increase the subsequent exercise performance of soccer referees.
Materials and Methods
Experimental Approach to the Problem
To examine the effect of halftime RW with intermittent exercise on subsequent exercise performance, participants completed 2 experimental trials (Control and RW trials) in a randomized order. First, participants performed the Loughborough Intermittent Shuttle Test (LIST), an exercise that mimics the actual first half of soccer matches, followed by a 15-minute halftime period. Then, participants performed an exercise performance test (the Yo-Yo Intermittent Recovery Test level 1 [Yo-Yo IR1]). All trials were separated by at least 6 days. At least 6 days before the first trial, participants performed a trial to familiarize themselves with the experiment. In addition, the HRmax was determined during this familiarization trial. The maximal heart rate was recorded at completion of the final Yo-Yo IR1 exercise.
Ten male referees who trained (i.e., more than an hour per session) more than 4 days per week participated in this study. All referees had either second, third, or fourth class registered official licenses from the Japan Football Association. Their age, height, and body mass were 22 ± 1 year (range 19–24 years), 173.6 ± 5.8 cm, and 67.2 ± 6.4 kg (mean ± SD), respectively. All referees were described the experimental procedures and possible discomfort associated with participating in the study before their written consent for participating. The study was approved by the Ethics Committee of the Tokyo Gakugei University (Approval number: 132).
Participants were instructed not to alter their regular exercise training, diet, and lifestyle habits on the day before each trial. Moreover, participants refrained from drinking alcohol for 48 hours and caffeine for 12 hours before each trial. Participants recorded all meals and drinks consumed the day before each trial. Participants replicated their dietary intake from the first trial in subsequent trials, ensuring that meals were standardized across trials. A questionnaire was used to verify these procedures before each trial. The 2 trials commenced at the same time of day (between 7 am and 8 am). This study noted similar weather conditions during the 2 trials. The mean temperature, globe temperature, wet-bulb globe temperature, and humidity were 14.5 ± 3.7° C, 14.3 ± 5.2° C, 11.2 ± 3.8° C, and 53.7 ± 7.7% (mean ± SD), respectively.
The study protocol is shown in Figure 1. The 2 trials consisted of the LIST, halftime, and Yo-Yo IR1 periods. After an 8-hour overnight fast (no food or drink except water), participants were asked to visit the laboratory. After resting for 10 minutes, participants performed the LIST (17). The movement pattern of the LIST comprised 3 × 20 m at walking speed (6 km·h−1), 20-m maximal running sprint, 4-second recovery, 3 × 20 m at jogging speed (12 km·h−1), and 2 × 20 m at running speed (16 km·h−1). This cycle was repeated 10 times (≈12 minutes) during each of 3 exercise blocks. The exercise blocks were separated by 3-minute rest periods. The intensity of the LIST reflects the physiological demand of soccer referees during the first half of matches (8). In the halftime period, participants either rested on a chair (Control) or performed a halftime RW exercise. The halftime RW protocol consisted of 2.15 minutes of seated rest on a chair as brief passive recovery, followed by 2.15 minutes of running at 70% of the HRmax, which were repeated for a total of 13 minutes. The halftime RW protocol began 1 minute after the start of the halftime period and completed 1 minute before beginning the Yo-Yo IR1 period. After the halftime period, participants performed the Yo-Yo IR1. Yo-Yo Intermittent Recovery Test level 1 performance is closely related to the high-intensity running that soccer referees undergo during matches (7). Participants ran to the 20-m marker and back to the start, which had to be completed by the time of second signal. Then, participants had a 10-second break, during which they walked around the 5-m marker. If participants arrived at the start marker before the next signal, they waited there until the next signal. If participants failed twice to reach the start marker before the second signal, the test was terminated. Participants consumed water ad libitum during the familiarization trial. Participants were allowed to consume water only during the 3-minute rest periods between the exercise blocks of the LIST and during halftime period. The ingested volume and pattern were replicated in the main experiment trials.
For plasma glucose, serum-free fatty acids (FFAs), serum triglycerides (TG), and plasma creatine kinase (CK) measurements, blood samples were obtained from a vein in the participants' right arms using 5-ml syringes after each period (after 10-minute rest: pre, LIST: post1, halftime: post2, and Yo-Yo IR1 periods: post3). Blood lactate concentration was determined using capillary blood from a fingertip. The blood lactate concentration was immediately determined using a lactate analyzer (Lactate Pro 2; Arkray, Kyoto, Japan) after each period and every 2.15 minutes throughout the halftime period (at 2.15 minutes: T1, 4.3 minutes: T2, 6.45 minutes: T3, 8.6 minutes: T4, and 10.75 minutes: T5). The rating of perceived exertion ([RPE]; scale 6–20) (4) was assessed after each period and every 2.15 minutes throughout the halftime period. Heart rate (HR) was monitored throughout the experiment trial using an HR monitor (Polar RS800CX; Polar Electro, Kempele, Finland). The mean HR was calculated during the 10-minute rest, LIST, halftime, and Yo-Yo IR1 periods. The maximal heart rate was calculated at completion of the final Yo-Yo IR1 exercise.
A sample size was estimated based on a previous study that investigated the effects of halftime RW on endurance performance (12). To detect improvements in exercise performance with 80% power at an alpha level of 5%, we determined that a sample size of ≥5 participants was required. All values are shown as mean ± SD. Statistics were computed using SPSS computer software (Version 18.0; SPSS Japan, Inc., Tokyo, Japan). Yo-Yo Intermittent Recovery Test level 1 performance and HRmax were compared using a paired t-test. Plasma glucose, serum FFA, serum TG, plasma CK, blood lactate concentrations, RPE, and the mean HR were compared using repeated-measures 2-factor (trial × time) analysis of variance. When a significant interaction was detected, the values were subsequently analyzed using a Bonferroni multiple comparisons test. Statistical significance was set at P ≤0.05.
Yo-Yo Intermittent Recovery Test Performance
The Yo-Yo IR1 performances of the control and RW trials were 2,904 ± 421 and 3,095 ± 326 m, respectively. The Yo-Yo IR1 performance was longer in the RW trial than in the control trial (P ≤ 0.05, Figure 2).
Blood Metabolite Response
The plasma glucose, serum FFA, serum TG, and blood lactate concentrations during each trial are shown in Figure 3. There was a main effect of time (P ≤ 0.05) for the plasma glucose, serum FFA, serum TG, and blood lactate concentrations. There was no main effect on trial and trial × time interaction for the plasma glucose, serum FFA, serum TG, and blood lactate concentrations.
The plasma CK concentrations during each trial are shown in Figure 4A. There was a main effect on time (P ≤ 0.05). There was no main effect on trial and trial × time interaction for the plasma CK. The RPEs during each trial are shown in Figure 4B. There was a main effect on trial (P ≤ 0.05), time (P ≤ 0.05), and trial × time interaction (P ≤ 0.05). Post hoc tests revealed no significant between-trial differences for RPE at pre, post1, T1, and post3. However, RPE was higher in the RW trial than in the control trial at T2, T3, T4, T5, and post2 (P ≤ 0.05). In the control trial, RPE was lower at T2, T3, T4, T5, and post2 than it was in post1 (P ≤ 0.05).
The mean HR in each period is shown in Table 1. There was a main effect on time (P ≤ 0.05) and trial × time interaction (P ≤ 0.05). Post hoc tests revealed significant between-trial differences for the mean HR in the halftime period, which was lower in the control trial than in the RW trial (P ≤ 0.05). The mean HR during the running exercise in RW protocol was 128 ± 3 b·min−1. No significant differences between trials were observed for the 10-minute rest, LIST, and Yo-Yo IR1 periods. Furthermore, there were no significant differences between trials for the HRmax during the Yo-Yo IR1 period (191 ± 6 b·min−1 for the control trial and 192 ± 7 b·min−1 for the RW trial).
The purpose of this study was to examine the effect of halftime RW with intermittent exercise on the subsequent exercise performance of soccer referees. The present findings demonstrate that halftime RW with intermittent exercise increased subsequent Yo-Yo IR1 performance, which is an important indicator of soccer referees' performance (7). This study is the first to demonstrate that the effect of halftime RW with intermittent exercise in soccer referees.
Previous studies have examined whether halftime RW with continuous exercise affects soccer player's subsequent exercise performance in the initial phase of the second half of matches (5,11,12,16,19). Only 1 study has focused on the effect of halftime RW on endurance performance, but it did not investigate explosive performance (12). The present study showed that halftime RW with continuous exercise subsequently increased the Bangsbo field test performance (i.e., soccer-specific endurance performance) compared with seated rest (12). Although the Bangsbo field test was performed twice in that study, interceded by a 15-minute halftime period (12), the duration of the Bangsbo test (16.5 minutes) might not have been long enough to mimic the first half of soccer matches and the physical activity demanded of soccer referees. In the present study, the intensity and duration of the LIST exercise mimicked the first half of soccer matches. Time-motion analysis of referees in soccer matches showed that 22, 40, 31, and 7% of time was spent in standing (0 km·h−1), walking (≤6 km·h−1), low-intensity (<15 km·h−1, including sideways running and backward running), and high-intensity exercises (≥15 km·h−1), respectively (8). The LIST exercise used in this study was similar to these percentages (i.e., standing: 19%, walking: 44%, low-intensity exercise: 22%, and high-intensity exercise: 15%) compared with actual soccer matches (8). Also, the total distance covered, 5.4 km, and duration, 40 minutes, were similar to those observed in the first half of actual soccer matches (i.e., 5.1 km and 45 minutes, respectively) (8). Furthermore, blood metabolites (plasma glucose and blood lactate concentrations) after the LIST exercise and mean HR during the LIST exercise in this study were similar to those observed in a study that examined the physiological responses of soccer referees during the first half of soccer matches (8). Thus, the present findings may have better external validity and provide important insights for practical applications.
Potential mechanisms have been proposed to explain the improved effect of halftime RW on subsequent exercise performance (5,11,12,16,19). These include (a) maintaining muscle temperature during the halftime period and (b) elevating baseline oxygen consumption before the second half. Preventing the reduction in muscle temperature during the halftime period is important for maintaining explosive exercise performance (5). By contrast, the elevation in baseline oxygen consumption is important for improving exercise performance determined by an endurance exercise protocol (2,3). Less of the initial work would be fueled anaerobically by elevating baseline oxygen consumption, reserving more anaerobic capacity for the later tasks (2,3). Initial sparing of anaerobic capacity would increase time to exhaustion and improve performance (2,3). This study investigated the effects of halftime RW on blood metabolism, which have not been investigated in any previous study (5,11,12,16,19). Carbohydrates and lipids are principal substrates required to perform intense intermittent exercise, including soccer (1). Although it is not clear if halftime RW affects oxygen consumption as this study did not measure this factor, the halftime RW in this study did not affect plasma glucose, blood lactate, serum FFA, and TG concentrations. Although a previous study has suggested that the repetition of moderate exercise can contribute to greater exercise-induced lipid oxidation, a better lipid profile after the halftime period was not observed in this study. This may be due to the duration of exercise may be not long enough to shift from carbohydrate to lipid oxidation (6).
Previous studies have not examined if halftime RW induced additional fatigue (5,11,12,16,19). If halftime RW leads to fatigue during halftime and the second half, it may not be an appropriate conditioning strategy for soccer referees. This study demonstrated that halftime RW with intermittent exercise did not increase plasma CK and RPE during the halftime period (i.e., we compared post1 and post2 in the same trial). Similarly, the plasma glucose concentration did not change. These findings suggest that halftime RW with intermittent exercise may not induce additional fatigue compared with seated rest during the halftime and Yo-Yo IR1 periods.
In conclusion, halftime RW with intermittent exercise increased the subsequent exercise performance of soccer referees compared by seated rest, determined with the use of the Yo-Yo IR1. To the best of our knowledge, this study is the first to demonstrate that halftime RW with intermittent exercise improves subsequent exercise performance. The present findings add valuable information to the literature and imply that halftime RW with intermittent exercise may be an effective conditioning strategy for soccer referees.
This study demonstrates 2 primary insights that can be applied to soccer referees. First, the intermittent protocol of halftime RW is an effective method for improving subsequent exercise performance determined by the use of Yo-Yo IR1. It is assumed that intermittent protocol of halftime RW is easier for practical application than halftime RW with continuous exercise, as it allows soccer referees to perform their other responsibilities during the rest period. In addition, for the application of halftime RW, inside-based protocol of halftime RW may be required for soccer referees. Soccer referees may not be allowed to use the field for reasons such as pitch protection, any ongoing activity at the pitch, and safeguarding during halftime. Therefore, it may be necessary for referees to have access to a treadmill, cycle ergometer, or enough space for halftime RW in the stadium. Second, halftime RW with intermittent exercise may not induce additional physiological and psychological fatigue compared with seated rest. Reduction in the amount of high-intensity running also occurs toward the end of the match because of accumulated fatigue (14). Therefore, it is important to examine whether halftime RW causes additional fatigue. Our findings demonstrate that halftime RW with intermittent exercise does not affect physiological and psychological measurements compared with seated rest. Thus, the findings of this study are recommended for the halftime conditioning strategy of soccer referees.
The authors thank the participants for their cooperation. No external research funding was obtained for this study. The authors have no conflict of interest directly relevant to the content of the study. The results of this study do not constitute endorsement of the equipment by the authors or the National Strength and Conditioning Association.
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