Soccer is characterized by its highly intermittent nature, requiring players to perform repeated bouts of maximal or near-maximal sprints, accelerations, decelerations, and changes of direction, interspersed with periods of lower-intensity recovery [1]. The ability to perform and sustain high-intensity running during matches is a critical determinant of both individual performance and overall team success [2,3]. Accordingly, both aerobic endurance and anaerobic power are essential physical attributes that strengthen optimal soccer performance [4,5]. The continued relevance of these physical components is underscored by recent data from top-tier leagues, which confirm that the volume and intensity of high-intensity running remain crucial performance indicators over the course of a season [6]. Traditional endurance training methods, such as continuous running or generic interval training, have been criticized for their limited specificity to the demands of soccer [4,7,8].

In response, training models such as Repeated Sprint Training (RST) and Speed Endurance Training (SET) have gained prominence as more soccer-specific conditioning strategies [4,9]. RST emphasizes the ability to perform short, maximal sprint efforts with brief recovery periods, primarily targeting neuromuscular and anaerobic alactic systems [10-12]. Conversely, SET involves longer high-intensity running bouts, aiming to enhance glycolytic energy system function and fatigue resistance [13-15]. While both methods are designed to improve Repeated Sprint Ability (RSA) and fatigue resistance, their distinct physiological demands mean their relative effectiveness for specific components remains a key area of inquiry [16].

While both approaches have demonstrated efficacy in improving physical fitness measures, direct comparisons of their effects, particularly on actual match performance parameters, remain limited. In fact, recent research comparing the effectiveness of high-intensity training modalities (e.g., small-sided game vs. high-intensity intermittent training) on performance outcomes often presents inconsistent or inconclusive results, highlighting the need for highly specific comparative studies [17,18]. From a theoretical viewpoint, the principle of training specificity suggests that the nature of the training stimulus must closely match the performance demands of competition to maximize adaptation [19]. Consequently, it is hypothesized that RST may better preserve high-intensity running performance during matches, whereas SET may be more effective for enhancing anaerobic performance under testing conditions.

Moreover, modern football periodization frameworks, such as Verheijen's weekly-based model, emphasize the strategic application of different training modalities based on the competitive calendar, advocating for the development of anaerobic capacity during preparatory phases and the maintenance of match-specific fitness qualities during congested schedules [20,21]. During the competitive season (in-season), the primary goal often shifts from maximum fitness development to the maintenance of physical qualities and managing training load relative to match demands [22]. As a result, selecting the most appropriate high-intensity training method during the in-season period becomes a strategic decision. Despite both RST and SET being effective, the training-induced physiological effects of each method suggest their selective application based on performance priorities. A direct comparison between these two methods, conducted in a realistic team environment, is therefore necessary to inform evidence-based conditioning strategies.

Therefore, the purpose of this study was to compare the effects of a 6-week RST program and a SET program on soccer-specific physical fitness and match performance metrics in university soccer players during the competitive season. It was hypothesized that both RST and SET would improve aerobic and anaerobic fitness parameters. Based on the principle of training specificity and previous research findings, it was anticipated that SET might lead to greater improvements in anaerobic power and repeated sprint ability, whereas RST could be more effective in preserving high-intensity running performance during actual match play. Given the limited sample size and applied setting involving a single university team, this research was conducted as a pilot study to inform future large-scale investigations.

This study compared the effects of RST and SET on soccer-specific physical fitness and match performance in university soccer players during the competitive season. The primary findings indicate that both RST and SET interventions significantly improved aerobic capacity, anaerobic power, and repeated sprint ability. However, distinct response patterns were observed in match performance variables. Specifically, SET resulted in greater improvements in anaerobic capacity and sprint maintenance, whereas RST was more effective in preserving high-intensity running performance during matches.

Both training groups exhibited significant improvements in aerobic fitness, as reflected by increased performance in the YYIRT-L2. This enhancement is consistent with previous studies demonstrating that various high-intensity interval training formats can effectively improve soccer-specific aerobic fitness [7,9]. This improvement observed after a relatively short 6-week period suggests that even during the competitive season, targeted interventions can meaningfully augment intermittent recovery capacity. Notably, the pre-test comparison of YYIRT-L2 performance showed no significant differences between the SET and RST group (p =.124), suggesting that the baseline aerobic fitness levels were comparable across groups. Therefore, the large average improvement observed in the SET group is more likely attributable to the physiological effects of the training intervention, rather than merely to lower initial fitness. In addition, the wide 95% confidence interval indicates considerable individual variability in training response, which may have influenced the group mean in a small sample setting.

Regarding anaerobic performance, the SET group demonstrated significant within-group improvements in peak power, average power, and fatigue index measured via the Wingate anaerobic test. Additionally, improvements in 20-meter sprint time and RSA decrement percentage (RSAdec) were observed in the SET group. However, these effects were not statistically different from the RST group, as indicated by non-significant group×time interactions. The substantial metabolic and muscular loads imposed by the SET protocol may have improved lactate tolerance and fatigue resistance, explaining the within-group improvements observed [13,14]. While these changes suggest a potential enhancement in the ability to tolerate successive high-intensity efforts, such effects were not clearly reflected in actual match-running performance in the present study. Although the SET repetitions were prescribed within a 23-29 second range, this corresponded to high-intensity running speeds (Zone 4) as defined in GPS-based classifications in professional soccer. These findings indicate that SET may be particularly effective in preparatory training phases aimed at developing anaerobic capacity and repeated sprint ability, even if its direct translation to in-game high-intensity performance is limited. In practical terms, a reduction in RSAdec reflects a smaller performance drop across repeated sprints, indicating improved fatigue resistance during successive high-intensity efforts. Likewise, faster 20-meter sprint times suggest enhanced acceleration capacity, which is critical for winning first balls, closing down opponents, and creating separation in match situations. Furthermore, improvements in fatigue index derived from the Wingate test imply better tolerance to anaerobic fatigue, which may help players sustain powerful actions throughout a game.

Match performance analysis revealed a notable distinction between the two training modalities. Although both groups maintained overall match activity levels, only the RST group preserved their high-intensity running performance (Zone 4 distance per minute) after the intervention, whereas the SET group exhibited a significant decline. This observation supports the principle of training specificity, whereby performance adaptations are highly dependent on the specific nature of the training stimulus [19]. RST, characterized by repeated short, maximal sprint efforts, appears to more effectively sustain the neuromuscular and metabolic qualities essential for maintaining high-intensity efforts during competitive matches [10-12]. In contrast, although SET improves anaerobic capacity and repeated sprint ability under controlled testing conditions [9,23], it may be less effective in preserving match-specific running performance demands when training is not closely aligned with match-play characteristics [11,19].

From a practical perspective, these findings offer important guidance for training periodization. RST emerges as a particularly effective strategy during congested competitive periods when maintaining match performance is a primary concern, offering fitness improvements without imposing excessive recovery demands. Conversely, SET may be strategically implemented during preseason or early-season phases, when maximizing anaerobic conditioning and enhancing sprint endurance are prioritized [11,20,21]. Accordingly, physical fitness coaches should consider tailoring high-intensity training interventions based on seasonal objectives and the specific performance characteristics they aim to develop [20,24]. For example, integrating RST sessions into in-season microcycles could help maintain players’ high-intensity running capacities, while incorporating SET during preparatory periods could optimize anaerobic and repeated sprint adaptations [11,21]. Such an approach is further reinforced by modern football periodization models, notably Verheijen's weekly-based framework, which emphasizes the need to develop anaerobic capacity during preparatory periods while prioritizing the maintenance of high-intensity running performance during congested competitive schedules [20]. Aligning high-intensity training modalities with these periodization principles may enhance both fitness outcomes and match readiness across the competitive calendar [11,25].

Several strengths of this study should be emphasized. Conducting a randomized controlled trial during the competitive season enhances the ecological validity of the findings, providing practical insights directly applicable to real-world team settings. The use of validated soccer-specific fitness assessments and Electronic Performance and Tracking System (EPTS) technology ensured robust and objective measurement of physical and match performance outcomes [26]. Nevertheless, the study is not without limitations. The relatively small sample size limits the generalizability of the results, although this is often unavoidable in studies involving competitive athletes. Additionally, potential confounding factors such as match conditions, tactical instructions, and player fatigue were not fully controlled. Moreover, individual physical activities outside of the supervised training sessions, including personal routines or informal workouts, were not strictly monitored. This represents a limitation that may have influenced players’ responses to the intervention. Future studies should aim to include larger samples, systematically monitor internal training loads (e.g., heart rate, blood lactate), and examine the long-term effects of different high-intensity training modalities across full competitive seasons, to better elucidate the sustainable impact of training interventions on soccer-specific performance. As a pilot study, the findings provide preliminary insights into the relative effects of RST and SET, although the results should be interpreted with caution due to the limited statistical power.

In conclusion, SET demonstrated significant within-group improvements in anaerobic performance and repeated sprint ability, while RST was more effective in preserving high-intensity running during matches. Although no significant interaction effects were observed, these within-group improvements suggest that each modality may offer distinct conditioning benefits depending on the training goal. Given the pilot nature of this study and its applied team setting, these findings offer preliminary guidance for selecting high-intensity training protocols across different phases of the season. Specifically, SET may be more suitable for preparatory phases aiming to enhance anaerobic capacity, while RST may help maintain match-specific performance during congested schedules. Future research with larger samples is needed to confirm these trends and refine evidence-based periodization strategies in soccer.