Intermittent high-intensity endurance and the ability to repeatedly sprint within relatively short time intervals (RSAs) are deemed relevant fitness prerequisites in competitive soccer players (28,35,41,46). Consequently, intermittent training and testing protocols have been proposed to improve soccer players fitness and guide talent selection (5,22,42). Recent studies reported that high-intensity intermittent endurance and RSA are both influenced by anaerobic and aerobic metabolism (25,27,43). Additionally, training studies showed that RSA training positively affects intermittent high-intensity performance (5).
Consequently, the information provided by field tests of intermittent high-intensity endurance and RSA may potentially provide redundant information (34). This topic is of relevance for soccer coaches and strength and conditioning professionals because intermittent endurance tests and RSA protocols are usually time consuming and sprint testing requires expensive devices (i.e., photocells) (14).
Furthermore, information about this issue may help sport science and strength and conditioning professionals in establishing performance oriented (i.e., soccer) test batteries to enhance fitness assessment and training prescription.
Another issue of importance in field testing protocols is the number of sprints required to induce significant performance decrements in RSA testing (14). Too short an RSA test would not induce the required fatigue state whereas extensive sprinting may induce pacing that can lead to redundant information and the likelihood of injury (14,40,43).
Therefore, the first aim of this study was to examine the possible relationships between intermittent high-intensity endurance and RSA testing in soccer players. The second aim was to investigate the occurrence of acute fatigue during the development of RSA testing to optimize field test protocols in soccer. We hypothesized that intermittent high-intensity endurance and RSA were mutually influential in soccer players (5).
Experimental Approach to the Problem
As a paradigm of intermittent high-intensity endurance, the Yo-Yo intermittent recovery test level 1 (Yo-Yo IR1) was used (25). Recent studies showed that the Yo-Yo IR1 is a valid and reliable field test to assess intermittent endurance in soccer players (2). Furthermore, Rampinini et al. (38) and Impellizzeri et al. (23) reported that a lower Yo-Yo IR1 performance was related to impairment in soccer-skill (i.e., short-passing ability) ability during experimental matches. Biopsy studies showed that Yo-Yo IR1 maximally stresses the aerobic pathway with an important involvement of anaerobic metabolism as exhaustion approaches (25). As a result, the Yo-Yo IR1 can be considered as the most relevant field test of soccer fitness (2).
Although several tests have been proposed to assess RSA in team sports (3,14,22,41,43,45,49), no gold standard protocol is currently available to test players in field conditions (16). However, there is an agreement that 30 m is regarded as the sprint distance in the RSA soccer protocols currently used (28,41). This distance is the longest usually performed during competitive soccer play, and it has been shown to be suitable to induce fatigue if repeated with short recovery cycles (14,26,28,31,48). In the present study, we therefore used as the RSA paradigm a 7 × 30-m sprint with 25-second recovery protocol, which is an extension of the Krustrup et al. (28) protocol. Sprint number was deliberately chosen higher than previously reported in other studies that addressed RSA in soccer and in team sports (i.e., 3-5 and 6, respectively) (28,45). This was decided with the aim of gaining information on the exact number of sprint bouts necessary to induce significant sprint performance decrements. However, the sprint number considered was similar to that previously suggested by Reilly et al. (41,42) and Wragg et al. (49) for talent selection in soccer and within the ranges recommended to avoid pacing but still inducing temporary fatigue (14,28,29).
Sprint performance over 30 m was reported to be affected by individual running strategies and abilities in soccer players (24,48). In the present study, to monitor variation in acceleration performance during the RSA test, split sprint times were assessed at 5 and 10 m of each 30-m bout (24,48). This was achieved using photocell beams set at 0, 5, 10, and 30 m from the start point.
The type of RSA data analysis is another concern of sport scientists because it has been reported that various methods (e.g., summation of time and percentage of decrement) are not directly interchangeable (33). No conclusive information is currently available as the best variable to be considered in soccer, because the available literature addressed field hockey and Australian Rules Football players (34,45). In the present study, the sum of sprinting scores over 5, 10, and 30 m was assumed as global RSA performance (total time [TT]) (14). To track changes on sprinting performance and consequently examine acute fatigue during repeated sprinting, the percent difference from the first and last sprints (%Diff) and the percent of the variation between the total best (7 × best sprint score) and actual total sprint performance (TT) were considered (18). Potential for information redundancy between Yo-Yo IR1 and RSA performance (TT, %Best, %Diff) were assessed using Pearson's correlation coefficient. Criterion performance validity was assessed to examine the differences in RSA performance between higher and lower Yo-Yo IR1 performers.
Twenty-three elite-level male soccer players (age 19 ± 1 years, height 181 ± 5.7 cm, body mass 73.2 ± 4.1 kg, %body fat 11 ± 2.4) were randomly chosen among members of a successful league 1 professional club of the Tunisian National Leagues (Club African, Tunis, Tunisia). The club is considered as 1 of the top 3 ranked teams in Tunisia during the last 15 years. All these subjects had competed at national and international levels for their respective age groups. Twelve subjects were members of the under-21 team, and the remaining 11 belonged to the under-19 team that ranked first during the 2007-2008 season. Players possessed at least 6 years of experience in soccer training and competitions and took part in national and international championships at the time of the investigation. Players trained 5-6 times a week (∼90 minutes per session) with a competitive match taking place during the weekend. Training sessions consisted mainly of technical and tactical skill development (80% of the training time). Physical conditioning was performed twice a week and was aimed toward anaerobic and aerobic performance development (1). Anaerobic training consisted of plyometrics and sprint training drills (1). Aerobic fitness was developed using small-sided games (37) and short- or long-interval running (21).
Testing procedures were performed during the last stage of the competitive season (February-March 2007). Written informed consent was received from all players after verbal and written explanation of the experimental design and potential risks of the study. The study was conducted according to the Declaration of Helsinki and the protocol was fully approved by the Clinical Research Ethics Committee and the Ethic Committee of the National Centre of Medicine and Science of Sports of Tunis before the commencement of the assessments. All players were fully accustomed with the procedures used in this research and were informed that they could withdraw from the study at any time without penalty.
Players were tested for RSA and Yo-Yo IR1 performance on separate days at least 1 week apart in a randomized and counterbalanced order. Each player was instructed and verbally encouraged to give a maximal effort during all tests. A standardized warm-up, consisting of jogging, dynamic stretching, and then a series of increasing intensity sprints was performed before testing. No static-stretching exercises were allowed before any test.
Before the RSA test, players performed 3 maximal 30-m sprints (with 5 and 10-m split times recorded) on an indoor synthetic track to assess their best individual sprint time (recovery 2-3 minutes passively). The run with the lowest 30-m time (and corresponding 5- and 10-m split times) was selected for analysis.
After a 15-minute rest, players performed the RSA test. They were requested to perform 7 30-m sprints providing all-out effort during each bout, decelerating as fast as possible 1 m after the finishing line. The recovery consisted of 25 seconds of self-paced jogging with the players positioning themselves on the starting line approximately 5 seconds before the next sprint. Sprint time over the first 30 m of the RSA test had to be not slower than 5% of the individual's best 30-m performance time (6,14,43,45) for the RSA test to be considered valid. Sprint times were recorded using photocell gates (Brower Timing Systems, Salt Lake City, UT, USA; accuracy of 0.01 seconds) placed 0.4 m above the ground. When ready, the subjects commenced the sprint from a standing start 0.5 m behind the first timing gate. Stance for the start was consistent for each subject.
The Yo-Yo IR1 was performed according to the procedures suggested by Krustrup et al. (25) and Castagna et al. (7). The reliability of Yo-Yo IR1 was established in a previous study (intraclass correlation coefficient [ICC] = 0.98, p < 0.0001 with a coefficient of variation [CV] of 3.5%) (9). The ICC and CV for the TT were 0.92 and 2.7%, respectively (n = 21).
Before using parametric tests, the condition of normal variation was verified using the Shapiro-Wilk W-test. Pearson's product-moment correlation coefficients were used to examine correlations between variables. The magnitude of the correlations was also determined using the modified scale by Hopkins (20): r < 0.1, trivial; 0.1-0.3, small; 0.3-0.5, moderate; 0.5-0.7, large; 0.7-0.9, very large; >0.9, nearly perfect; and 1 perfect. The RSA data were analyzed by 1-way repeated measures analysis of variance. Tukey's post hoc test of critical difference was used to locate significance between means.
Using the ‘median split’ technique, the soccer players were divided into 2 groups (best and worst) according to the median Yo-Yo IR1 distance covered (Yo-YoBest and Yo-Yoworst) (36,47). The player obtaining the median distance value was excluded to obtain 2 subgroups with the same number of participants (n = 11).
Independent 2-tailed Student's t-tests were used to determine any significant difference in RSA variables (Total sprint time, Sprint performance decrement) between Yo-YoBest and Yo-Yoworst. The effect size (ES) was calculated to assess meaningfulness of differences. Effect sizes of >0.8, between 0.8 and 0.5, between 0.5 and 0.2, and <0.2 were considered as large, moderate, small, and trivial, respectively (10). The results are expressed as mean ± SD and 95% confidence intervals. A significance level of p ≤ 0.05 was selected.
During the Yo-Yo IR1, players ran for 2,289 ± 409 m. Total time and total best time during the 7 × 30-m RSA test were 31.21 ± 1.13 and 30 ± 1.04 seconds, respectively. Sprint decrements were 4.3 ± 2.0 and 6.0 ± 2.5% for %Best and %Diff, respectively. Significant sprint time decrements occurred from the second 30-m sprint compared with the first sprint as reference (p < 0.05, Figure 1C). Sprint performance over 5 and 10 m showed a significant impairment compared with the first sprint from the and third bouts, respectively (Figure 1A, B, p < 0.05). There was no significant decrement for the seventh sprint compared with the fifth (3.4 ± 1.9%) or sixth (3.9 ± 2.0%) sprint (Figure 2) for the %Best. Using the %Diff, speed decrements were not significantly different (p > 0.05) among sprints 5-7 (Figure 2).
The distance covered during the Yo-Yo IR1 was significantly correlated with %Best RSA over 30 m (r = −0.44 moderate, p = 0.04). No significant relationship was found between Yo-Yo IR1 performance and TT (r = −0.40, p > 0.05).
The median of the Yo-Yo IR1 performance was of 2,320 m. Results obtained after the median split technique analysis are reported in Table 1. The Yo-YoBest group showed significantly better performances for RSA variables over the 30- and 10-m distances (p < 0.05). Interestingly, players in the Yo-YoBest group showed significant sprint performance decrements over 30 m from the fourth sprint onward (Figure 3C), whereas sprint time deterioration over 30 m occurred earlier (from the second sprint on) in the Yo-YoWorst group (p < 0.001, Figure 3C). Significant (p < 0.01) performance decrements were evident in the Yo-YoWorst group from the fourth bout for both the 5- and 10-m sprint splits (Figure 3A, B, respectively). Both (%Best and %Diff, n = 23) sprint decrements methods were significantly correlated over 30 m (r = 0.81 very large, p < 0.001), 10 m (r = 0.93 nearly perfect, p < 0.001), and 5 m (r = 0.87 very large, p < 0.001) split times. Performance over 30 m (i.e., pre-RSA test performance) was significantly associated with TT during the RSA test (r = 0.61 large, p = 0.02).
The main finding of the present study was the occurrence of only a moderate relationship (r = −0.44) between 2 soccer-relevant physical abilities such as intermittent high-intensity endurance (Yo-Yo IR1 distance) and the repeated sprint ability (7 × 30-m %Best). Indeed the present study results showed that only 19% of variance was shared between the Yo-Yo IR1 and RSA tests. The independence of these 2 measures of ability is further supported by the lack of relationship between Yo-Yo IR1 performance and TT: The sum of sprinting scores (TT) is considered the most reliable outcome of RSA tests (34,45). As a result, the original working hypothesis assumed for this study has not been affirmed.
The present study findings are in line with those previously reported for Australian rules football players, which found a low association between multistage fitness test performance and 6 × 30-m sprint test (r = −0.20, trivial) (34). Similar results were also reported by Krustrup et al. (25), who found no significant relationship between level 2 of the Yo-Yo IR and repeated sprint performance over 5 × 30 m with a 25-second active recovery (r = 0.26 small, p > 0.05). This suggests that RSA performance and intermittent high-intensity endurance should be regarded as semi-independent soccer performance-related variables. As a result, coaches and strength and conditioning professionals should consider these 2 physical ability variables separately to depict players' fitness performance profiles.
To further examine the effect of intermittent endurance fitness on RSA, players were separated according to their Yo-Yo IR1 results into 2 groups, depending on whether they ran over or under the median distance covered by the group, using the construct of Higher and Lower performers (36,47). The data showed that the resulting construct was able to discriminate for RSA performance; players belonging to the Yo-Yo IR1Best group demonstrated significantly better performances on RSA variables (Table 1 and Figure 3) than did the Yo-Yo IR1Worst. Interestingly, RSA variables were also significantly better in Yo-Yo IR1Best players when they were calculated over 10 m. Sprint sequence analyses across the considered distances (i.e., 5, 10, and 30 m) showed that Yo-Yo IR1Best players' sprint performance times deteriorated later than those of the Yo-Yo IR1Worst players. Split sprint performance was impaired only in the Yo-Yo IR1Worst group, with decrements occurring after only 4 sprint bouts in the 5- and 10-m splits (Figure 3A, B). These data warrant the construct validity of the Yo-Yo IR1 test in soccer with respect to RSA. Moreover, it suggests that performances in Yo-Yo IR1 over 2,320 m (median value) may be considered as being required in junior-level elite soccer players.
The Yo-Yo IR1 distance reported in this study (i.e., 2,289 ± 409 m) was lower than that (2,414 ± 456 m) reported by Mujika et al. (32) in senior elite-level professional male soccer players, but higher (2,092 ± 260 m) than gender-matched junior-level players playing in the Spanish national championship (32). Mohr et al. (30) and Krustrup et al. (25) found Yo-Yo IR1 performances similar to that of the present study players in professional male soccer players competing in the Italian and Danish championships. These similarities indicate that the level of fitness of the present study's soccer players was at the elite level and the study's main findings and conclusions (8).
Although RSA testing may be time consuming, as a single subject assessment and expensive (with photocell time triggers) is very often the only method of reliable measuring sprint performance. The present study's findings provide evidence for the inclusion of RSA protocols in soccer-specific fitness-testing batteries (5,22,28,35,41). In this regard, optimization of the number of sprint bouts may be of interest to reduce procedure time during RSA tests. Although several proposed RSA test procedures are to be found in Sports Science literature, no structured research has previously been carried out to throw light on this interesting issue (43). One research study suggested that an RSA test should not involve >8-10 sprint bouts when short recovery times (20-30 seconds) are used (14). The reasons that underpin this practical suggestion are based on the recurrently observed pacing strategy that is used by subjects when performing numerous repeated supposed all-out sprints (43). Although shortening sprint distance to form paradigms that do not exceed 30 m may discourage pacing, familiarization has been shown to be necessary to reduce pacing strategies (14,43,49). In soccer, RSA tests have logical validity because the 30-m sprint has been reported to be relevant to actual match play and to inducing the necessary temporary fatigue required to allow performance deterioration for RSA assessment (26,28,31). The present study was performed with the secondary aim of assessing the optimal number of bouts required to identify meaningful information on players' RSA performance. Analyses of the RSA data showed that using 30-m sprints interspersed with 25 seconds of self-administrated active recovery produced sprint performance decrements as early as the second sprint (n = 23, Figure 1C). This protocol resulted in sprint decrements that were not significantly different (p > 0.05 for %Diff and %Best, respectively) from that produced on the seventh 30-m sprint compared with the fifth and sixth bouts. This indicates that to assess a player's RSA profile, a 5-6 × 30- protocol could be used. This is in line with that previously reported by Spencer et al. (45) for field hockey players. Interestingly, sprint deterioration over 10 m paralleled (r = 0.79 very large, p < 0.01, n = 23) that reported for 30-m performance, showing that acceleration may be associated with or may induce fatigue over the repeated 30-m sprints (28). This is of relevance for soccer sprint-specific training because match analysis studies have shown sprinting in the range of 10-15 m as being the most frequent distances covered in a competitive game (4,12,13,36,39). In light of the present study's findings, it might be advisable to implement multiple sprint training drills in an attempt to promote acceleration performance conservation in those players that possess poor intermittent endurance performance (5,11).
Another debated issue in Sports Science is the best way to express RSA performance to provide meaningful information to the coach and strength and conditioning professionals (16-18,33,44). Recently, several studies have addressed the validity and reliability of the methods proposed so far to measure repeated sprint ability (17,18,33,45). Glaister et al. (18) showed that among the different methods used to measure RSA performance, the percentage decrement calculation originally proposed by Fitzsimons et al. (14) was the most valid and reliable way of quantifying sprint decrements in tests of multiple-sprint performance. In their article, Pyne et al. (34) highlighted that attention should be paid when expressing RSA results, because this is sensitive to the method used for performance analysis. However, contrary to the Pyne et al. (34) study, we found significant and strong correlations between the %Best and %Diff methods (r = 0.81 very large, p < 0.0001). The difference between the findings of Pyne et al. (34) and the present study may be that we used a longer RSA protocol (i.e., 6 vs. 7 × 30 m, respectively) and a longer recovery time between sprint bouts (i.e., 20 vs. 25 seconds, respectively). The resulting CVs for the 2 RSA measure methods in our study were 51 and 53% for the %Best and %Diff, respectively. These values were higher than those reported for the same measures by Pyne et al. (34) (i.e., 28.6 and 37.5%, respectively). Presumably, the difference in the degree of correlation between these variables could be found in score variability between the 2 studies (18,19,33).
Intermittent high-intensity endurance and RSA should be regarded as 2 different fitness attributes of soccer players. Consequently, distinct tests addressing these 2 physical abilities should be considered by coaches and strength and conditioning professionals to provide a detailed profile of players' fitness. In this context, performance in the Yo-Yo IR1 ≥2,320 m may be beneficial in promoting RSA in elite soccer players.
The RSA protocols involving 30 m with 25-second recovery should involve at least 5 sprint bouts as fatigue effects (sprint decrement percentage) is similar to that of longer protocols. This suggestion may also be extended to training prescription when striving to develop RSA with specific training protocols (15,16,43). In the present study, percentage measures of performance decrements were found to be significantly associated. However, methods that take into account all sprint bouts (%Best) are preferable (18). In light of the poor reliability of sprint performance methods, total sprint time should be considered for within-subject comparisons (45).
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