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.
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).
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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