Introduction

Sprint Interval Training (SIT), characterized by short bursts of high-intensity exercise interspersed with brief recovery periods, has gained attention for its ability to elicit rapid improvements in anaerobic capacity and metabolic adaptations1. It challenges athletes to perform at or near maximal effort, promoting gains in power and speed. While traditional endurance training (ET) involving prolonged moderate-intensity exercise often emphasizes aerobic endurance to enhance overall stamina2, it fails to address the specific demands of intermittent sports like tennis or basketball that require bursts of speed and power. SIT, on the other hand, can mimic the physiological demands of these intermittent sports, helping athletes better prepare for the rigors of competition3. Additionally, SIT has been associated with time-efficient workouts, making it an attractive option for athletes with busy schedules4.

Previous research has shown that various physiological parameters had a strong correlation with tennis performance5,6. In particular, tennis is characterized by a series of intense, short-duration efforts that demand rapid bursts of power, agility, and speed. These efforts include explosive serves, quick sprints to reach the ball, and forceful groundstrokes, all of which heavily rely on the anaerobic energy pathways7. The anaerobic lactic system becomes dominant during extended rallies and intense baseline exchanges, where energy is required at a high rate but can lead to the accumulation of lactate and fatigue8. Therefore, a well-developed anaerobic energy system not only enables tennis players to execute powerful strokes and swift movements but also helps in delaying the onset of fatigue and sustaining peak performance during crucial match moments. Studies have shown that improving anaerobic capacity can lead to enhanced sprinting abilities, quicker court coverage, and better overall match performance9. Given these findings, it becomes evident that a tennis player’s success hinges on the efficient utilization and development of the anaerobic energy system, making it an indispensable component of training and performance enhancement in the sport.

Kilit et al.10 proposed that HIIT is more suitable for young tennis players to enhance speed regulation, particularly in improving agility and technical skills. Despite the potential advantages mentioned earlier, the use of SIT among competitive tennis athletes has received limited attention in research. Existing studies have primarily focused on cardiovascular adaptations and metabolic changes among general or obese populations4,11, or on endurance athletes and other intermittent team sports like soccer, basketball, volleyball, and field hockey12,13,14. Furthermore, most SIT studies have been conducted in laboratory settings, with a lack of field studies in the literature15,16. Practical considerations such as the cost and time associated with specialized equipment like cycle ergometers and treadmills pose further challenges17, and traditional laboratory assessments (e.g. Wingate Anaerobic Test) may lack specificity for evaluating lower body power to reflect the field-based tennis performance. These limitations highlight the need for research that addresses these gaps and offers more practical conditioning protocols for tennis players to enhance performance.

To address these research and practical limitations, this study aims to investigate the effects of a low-volume, court-based SIT protocol on anaerobic capacity and sport-specific performance in competitive tennis players. By focusing on brief, high-intensity court sprints that simulate real match-play situations, this protocol eliminates the need for identifying metabolic parameters and power output for training prescription15. The findings of this study are expected to provide valuable insights into more effective and practical conditioning strategies for enhancing performance in tennis.

Methods

Participants

Twenty-four competitive collegiate tennis players volunteered to participate in the study during the preparatory season. To maintain consistency among the participants’ characteristics and account for the limited availability of female athletes from the tennis academy, the study focused exclusively on male tennis players. The sample size of the study was determined using a priori power analysis (utilizing G*Power version 3.1.9.7, University of Dusseldorf). The analysis considered several parameters, including the effect size (ES) index (0.40) assuming a large partial eta-squared (0.14), α error probability (0.05), power (0.90), number of groups (2) and measurements (3), and correlation among repeated measurements (0.5). Based on these parameters and the potential dropout rate due to training and other factors, a total of 24 participants were recruited in this study.

The participants had a weekly training volume of 20 h, which consisted of 3 h of technical and tactical tennis practice and 1 h of physical conditioning based on traditional ET each day on weekdays while all tennis players were right-handed. The inclusion criteria for participants are as follows: all participants were in good health and had no severe injuries during the last 6 months before the study and had a minimum of 4 years of systematic tennis training experience. Participants were fully informed of the experimental procedures, benefits, and risks associated with the study before giving their written informed consent to participate. The tests were conducted at least 48 h after a competitive match or heavy training session. The participants participated in all the training sessions as well as pre-and post-tests. The study was approved by the Research Ethics Committee of Beijing Sport University (Approval number: 2023210H) and all procedures were conducted following the Declaration of Helsinki.

Design

A longitudinal and randomized controlled experimental design was used to investigate the effect of a 6-week court-based SIT intervention on the anaerobic ability and performance parameters of tennis players. A 2-group, repeated measures (pre-test and post-test) design was used. Participants were randomly allocated into the SIT and ET groups (SIT: n = 12, ET: n = 12) using stratified block randomization. The demographic data of the participants are presented in Table 1 and no significant differences were observed among the groups in terms of competition level, biometric training characteristics, anaerobic parameters, and anaerobic-specific performance before intervention. Besides the intervention, both groups maintained the same technical-tactical training agreed upon by the tennis academy. There were no reports of missed sessions or injuries during the intervention period and this study was conducted during the off-season.

Table 1 Physical characteristics of tennis players included in the analysis (baseline).
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Methodology

The experimental flow is shown in Fig. 1. Prior to the start of the intervention, all participants were required to perform the Wingate test and blood lactate tests as well as two field-based assessments including the tennis-specific repeated sprint ability (RSA) test, and the YO-YO Intermittent Recovery Test Level 2 (YOYO-IR2). For each training and testing session, the participants followed a 10-min standardized general warm-up and 10-min cool-down protocol including jogging, skipping, dynamic warm-up, and stretching. Participants refrained from intensive exercise for a minimum of 48 h before the testing sessions and each session was separated by 48 h. The research took place during periods of preparatory training period that did not involve competition, and all measurements were conducted in the morning, typically between 7:30 a.m. and 8:30 a.m.

Figure 1
figure 1

Schematic diagram of the experimental process (ET, endurance training group; SIT, sprint interval training group; ITN, international tennis number; COD, change of direction).

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Laboratory anaerobic measurements

The Wingate Anaerobic Test was used as a basic anaerobic test18. Each player performed warm-up exercises for 5–10 min before each test, followed by a ride for 30 s at maximum speed against weight-related resistance (7.5%kg body weight using a Monark dynamometer). The following parameters were then determined: (a) the maximum power value (peak power, PP) during the test, which is assumed to correspond to the maximum anaerobic power; (b) the total work (average power, AP), which used as an indicator of anaerobic capacity; (c) the rate of power drop (fatigue index, FI); and (d) the time required to reach peak power (time to peak, TTP).

To analyze the anaerobic recovery speed of athletes after increasing the load and to evaluate their recovery ability after the Wingate test, blood samples were collected for rest (before testing with players being seated) and 0, 1, 3, 5, 7, 10, and 15 min immediately after the increasing load test via a volume of 20 µl of fingertip blood. The EKF Biosens-line automatic blood lactate analyzer (EKF-diagnostic GmbH, Barleben, Germany) was used to measure blood lactate, with the results being later recorded with the elimination rate of blood lactic acid (BLAer) being calculated using the following formula:

$${\text{Bla}}_{{{\text{er}}}} = \left( {{\text{Bla}}_{{{\text{max}}}} - {\text{Bla}}_{{{15}}} } \right)/\left( {{15} - {\text{t}}} \right),$$

where Blamax is the peak blood lactate value, Bla15 is the blood lactate value of 15 min, and t is the time of peak blood lactate production.

Tennis-specific RSA test

The player begins at the center mark on the baseline. Upon the ‘go’ command of the investigator, the player started sprinting followed by the direction of Fig. 2. The player should keep an eye on the opponent and the ball at the other end all the time. The player repeated the above exercise a total of five times with a recovery period of 20 s between each repetition19.

Figure 2
figure 2

Schematic diagram of the tennis-specific RSA test.

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It has been suggested that anaerobic power scores are major determinants of overall RSA performance (e.g., repeated mean power or speed). Therefore, the total accumulated sprint time instead of the decrement scores is used to evaluate the anaerobic ability8.

YO-YO intermittent recovery test level 2

YoYo-IR2 test was used as a reliable method (CV = 9.6%) to verify the ability of trained athletes to perform repeated bouts of high-intensity interval runs with high anaerobic energy contribution20. The YoYo-IR2 test provides a simple and valid way to obtain important information about an individual’s capacity to recover from repeated intense exercise and to examine training effects on performance21. After the completion of the standard dynamic warm-up, participants sprinted 2 × 20 m at progressively increasing speeds and jogged around a marker placed 5 m behind the finish line for 10 s during every 40 m shuttle (controlled by audio signals). The test ended when the participant chose to terminate it, or when the subject was unable to complete the shuttle run in time on two consecutive occasions. The final running distance was recorded for analysis.

Interventions

Participants commenced the training protocol 48 h after the pre-test. The training involved three sessions per week on alternate days (i.e., Monday, Wednesday, and Friday) for 6 weeks. ET consisted of 45 min of continuous treadmill running at a velocity corresponding to 75% VO2max. Before and after each ET session, a 5 µL blood sample was taken from the fingertip to determine the blood lactate concentration. The SIT protocol was modified from a previous study13 which involved three sets of high-intensity sprints interspersed with short recovery periods. Each interval run was 110 m in total distance and involved forward and backward sprints over distances ranging from 5 to 20 m with multiple changes of direction (COD) (Fig. 3). A set comprised three repetitions of a 110-m sprint with a 20-s recovery period between each sprint, followed by a 5-min recovery period between sets. Before the commencement of the SIT protocol and at the end of each run, a 5 µL blood sample was taken from the fingertip to determine the whole blood lactate concentration. Participants were verbally encouraged throughout both exercise protocols. All training sessions for both groups were supervised by an investigator with strength and conditioning experience. The Polar Team 2 System (Polar Electro Oy, Kemple, Finland) was used to monitor the heart rate of each player throughout each training session, with data later extracted from custom-specific software (Polar Team 2, Electro Oy, Kemple, Finland), to obtain maximum heart rate (HRmax), time spent in each HRmax% zone and training impulse (TRIMP). TRIMP takes into account the training duration and intensity at the same time and reflects the comprehensive effect of training on the internal and external load of the athlete’s body, as well as the load of medium and high-intensity training. The method to determine the athlete’s TRIMP in the current study is based on the formula proposed by Edwards, a weight factor of each heart rate zone is given whereas the TRIMP per each zone is acquired by multiplying the exercise time22. The HRmax of each player was established using the peak value recorded by the monitoring system during the training.

Figure 3
figure 3

Court-based SIT protocol.

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Statistical analysis

Experimental data were processed by the SPSS statistical software package (version 25.0, Chicago, IL, USA); all test results are reported as mean ± standard deviation (x ± s). The normality of the tests (Shapiro–Wilk test) results were checked before the subsequent analysis. Independent t-tests in biometrics, training characteristics, anaerobic and lactate parameters, and field-based RSA measurements were used to assess differences before training intervention (week 0) among the 2 groups. A 2-way repeated measures analysis of variance (ANOVA) was then used to compare the within (time, pre-test vs. post-test) and between-group (SIT vs. ET) difference to determine the effects of interventions on laboratory parameters, lactate clearance rate, field-based RSA measurements, and YoYo-IR2. Whenever a significant difference was detected for either of the main effects, a post hoc analysis using the least significant difference (LSD) method was conducted. The partial eta-squared (η2p) effect size measure was computed to assess the magnitude of the main and interaction effects in the analysis of variance. The η2p values of 0.01–0.05, 0.06–0.13, and ≥ 0.14 represent a small, medium, and large effect size, respectively. In addition, the effect size (ES) was calculated using Cohen’s d to quantify the magnitude of pre- and post-intervention change and to reflect the comparison of training effects within SIT and ET groups based on the following scales: < 0.2 trivial, 0.2–0.6 small, 0.6–1.2 moderate, 1.2–2.0 large and > 2.0 very large23.

Results

Laboratory anaerobic measurements

The anaerobic parameters with the Wingate ergometer test (PP, AP, FI, and TTP) at baseline and after 6 weeks of intervention are shown in Table 2.

Table 2 Effects of sprint interval training and endurance training on laboratory measurements and Court-based Anaerobic performance.
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There were no significant effects for FI, TTP, and BLaer whereas, a small increase in the PP and AP of SIT (ES = 0.33) and a trivial improvement of ET (ES = 0.12), a small increase in the FI of SIT (ES = 0.39) and ET (ES = 0.27), a trivial increase in the TTP of SIT (ES = 0.06) and ET (ES = 0.11), and a small improvement in the BLaer of SIT (ES = 0.41) and a trivial improvement of ET (ES = 0.13) for both SIT and ET, respectively, were observed. A significant group × time interaction (p < 0.001) was found in the mean power of the Wingate test (Table 2) while the main effect for time was also significant (p = 0.02).

Court-based anaerobic performance

There was a significant main effect for time in YoYo-IR2 performance (7.8% increase, p = 0.04, ES = 0.70), mean field-RSA time (5.1% decrease, p = 0.02, ES = 1.13), and sum field-RSA (5.2% decrease, p = 0.02, ES = 1.12) in SIT but no significant change in ET was observed. Moreover, there was no significant group × time interaction, for all court-based parameters in all the groups analyzed (SIT or ET) (p < 0.05).

The mean total TRIMP per training session of the players obtained in this study was 61.4 ± 2.4 for SIT and 147.3 ± 4.3 for ET, respectively. Figure 4 shows the percentage of time spent by the players in the different HR categories and corresponding TRIMP during the training sessions performed in the study. During the 6-weeks training, the mean sessions effective training time and TRIMP (in the 60–89% HRmax zone and total session) of the ET group were significantly higher than those in the SIT group (p < 0.001), while the mean sessions effective training time and TRIMP in the 90–100% HRmax zone of the SIT were significantly higher than the ET (p < 0.01).

Figure 4
figure 4

Percentage of time spent and TRIMP by players in the different heart rate (HR) categories during SIT and ET training sessions. (p < 0.05).

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Discussion

To the best of our knowledge, this study is the first to investigate the effects of court-based SIT on both laboratory and field-based anaerobic capacity outcomes in competitive tennis players. Our major finding is that the low-volume SIT utilizing a court-based, bidirectional sprinting protocol, led to comparable improvements in laboratory-based anaerobic capacity and potentially better tennis-specific field-based anaerobic RSA performance compared to the conventional equipment-based ET. Specifically, SIT showed superior improvements in the Wingate ergometer test (mean power) and tennis-specific anaerobic performance (Field-RSA mean and sum) than ET, and potentially a larger improvement in aerobic and anaerobic running performance (YoYo-IR2 distance). Importantly, these benefits were obtained in a much shorter time frame (i.e. ~ 90% less total exercise time or only 1/3 of the ET in terms of total training duration) than traditional ET and without the need for specialized equipment. Moreover, normally competitive athletes have diminishing returns from training whereas, in our study, court-based SIT training has demonstrated clear and more pronounced improvements than the ET group on competitive tennis players. Therefore, training remains essential for maintaining and optimizing performance at competitive and elite levels whereas court-based SIT can be considered as a good alternative to traditional endurance training for further performance breakthrough24.

Specifically, for the laboratory-based performance, the significant improvement in peak and mean power during the Wingate test signifies an enhancement in the anaerobic power of our participants. These anaerobic power indices are critical in supporting high-intensity, short-duration activities like sprinting and explosive movements. These findings align with previous research highlighting the effectiveness of high-intensity interval training (HIIT), including SIT, in improving anaerobic performance parameters. For instance, a study by Gibala et al.25 demonstrated substantial improvements in peak power output and mean power output after only six sessions of SIT in untrained individuals. Interestingly, although the Wingate test is anaerobic dominant and thought to have high involvement in both ATP-CP and anaerobic glycolysis, our SIT group did not show a significant improvement on the BLAer% and fatigue index%. Previous studies showed that SIT sessions require recovery durations of not less than 4 min to maximize anaerobic glycolytic energy contribution3. It is speculated that our SIT protocol with a very short recovery period did not maximize the involvement and improvement of the anaerobic glycolytic pathway. However, small but noteworthy increases (not statistically significant) were observed in anaerobic parameters, such as FI and TTP for both SIT and ET. These are partially in line with the similar improvements in previous studies using HIIT26,27, emphasizing the potential of court-based SIT in enhancing various aspects regarding anaerobic performance.

This study also explored the impact of training modalities on court-based anaerobic performance, since the validity of using cycling test for performance assessment in tennis has been questioned7. In the context of court-based anaerobic performance, the YoYo-IR2 performance showed a significant 7.8% increase in the SIT group, while no significant change was observed in the ET group. The YoYo-IR2 was thought to be valid in assessing both aerobic (e.g. VO2max) and anaerobic (e.g. Wingate fatigue index) capacity28,29. Krustrup et al.29 have shown a very high blood lactate level (approximately to 14–16 mmol/l) and the % of maximum heart rate (nearly 100%) during the later stages of the YoYo-IR2 test. As mentioned before, our SIT did not enhance the BLAer% and fatigue index significantly. Therefore, the observed improvement in YoYo-IR2 was mostly contributed by factors other than the anaerobic glycolytic system. Conversely, YoYo-IR2 distance was also shown to have moderate (r = 0.53) and significant correlation to the aerobic performance while the oxidative phosphorylation was proved to be critical on phosphocreatine and adenosine triphosphate (ATP) resynthesis29,30. Therefore, the enhanced repeated running performance on YoYo-IR2 could be more biased toward the aerobic pathways.

Apart from the YoYo-IR2, our tennis-specific RSA performance probably best reflected the court-based anaerobic performance providing insights on both tennis-specific moves and fatigability. Analog to the Wingate peak power, the Field-RSA best time could better express the anaerobic power scores resemble to the real tennis game conditions. Similarly, the overall tennis-specific RSA performance (i.e. Field-RSA mean and sum) resembles to the mean power output of the laboratory-based Wingate while the measure of fatigue is closely associated with the percentage decrement scores between the sprints31. Since the observed field-RSA performance enhancement in the SIT group can be attributed to the multiple factors (e.g. enhanced agility, better neuromuscular efficiency, and reduced fatigue), it is more challenging to identify the key performance determinants than the laboratory-based findings. Regarding the court-based peak anaerobic power, the Field-RSA best showed similar but not significant improvement in both SIT and ET groups. With the substantially lower training volume in the SIT group, the low volume court-based SIT was well maintained and potentially improved the tennis-specific anaerobic power. Since the average finishing time for a single sprint was approximately to 19 s while numerous movement directions were required, the Field-RSA best was thought to be dictated by the peak anaerobic power, acceleration, deceleration, movement efficiency, and partially the anaerobic glycolysis. In this regard, the clear improvement in the Wingate peak power (or lower limb peak power output) in the SIT group was not adequate to induce a significant enhancement in the tennis-specific anaerobic power. Therefore, conditioning coaches may supplement the court-based SIT with other modalities focusing on neuromuscular performance to maximize the improvement on Field-RSA best time. On the other hand, court-based SIT has induced a significant improvement in both Field-RSA mean and sum (but not in the ET group). The RSA test was thought to be vital to assess the fatigue index and maintenance of the sprinting power output on the subsequent sprints. Interestingly, both the laboratory-based fatigue index (by Wingate), BLAer%, and the Field-RSA % decrement were not significantly enhanced in both the SIT and ET group. Therefore, it is speculated that the court-based SIT training has enhanced the sprint time of most subsequent sprints on aspects other than fatigue resistance such as movement efficiency, sprinting or turning techniques, and the recovery of explosive power output on the subsequent RSA tasks due to the high resemblance of biomechanical demands. This outcome suggests that court-based SIT may be particularly beneficial for athletes who demanded for repeated short bursts of high-intensity moves on the court. This finding is also consistent with studies demonstrating the positive impact of HIIT on sports-specific performance7,32.

This study has several notable strengths, including the recruitment of high-level tennis players and the use of both laboratory and tennis-specific field tests, which significantly enhance the external validity of the findings. However, some limitations should be acknowledged. Firstly, our study design did not explore the moderating effects of specific variables within the SIT regimen, such as sprint length and the number of sprints per session. This decision was made to prioritize targeting high-end capacities (all-out efforts) and optimizing time efficiency during the training sessions. Given the individual differences of athletes’ fess profile and the specific demands of their playing style, future studies could investigate these variables to gain a more comprehensive understanding of the optimal parameters of SIT for anaerobic capacity and tennis-specific endurance in competitive tennis players. Furthermore, our 6-weeks intervention duration aligns with the typical length of a training block (mesocycle) in the periodization of competitive tennis players, providing clear training effects and practical implications for conditioning coaches in program planning. To capture the full potential effects on measured outcomes across different training blocks or macrocycles, future studies could extend the intervention over the entire macrocycle or compare the effectiveness of the two regimens in different periodization phases.

From a practical perspective, the results of this study suggest that incorporating SIT into the training regimen of tennis players can provide specific performance benefits beyond what is achieved through traditional ET methods. While tennis itself challenges multiple physiological systems concurrently, SIT may be a time-efficient option to target key anaerobic qualities like repeated sprint ability without overly stressing certain active and passive structures that are already heavily loaded from the sport-specific training. This highlights that coaches can strategically utilize SIT to specifically address performance weaknesses or recovery limitations, without necessarily having to rely on higher training volumes that may increase injury risk33.

Conclusion

In conclusion, our study demonstrates that low-volume court-based SIT is a promising approach for improving anaerobic capacity and sport-specific performance in competitive tennis players without the need for specialized equipment when compared to ET. As SIT participants mostly focused on the 80–100% HRmax zone while the total weekly effective training time and TRIMP were substantially lower than that of the ET group, coaches and athletes can use the time-efficient SIT to improve their fitness levels and enhance their on-court performance, giving them a competitive advantage in tennis matches. These findings contribute to the growing body of evidence supporting the efficacy of SIT-based training modalities in sports performance enhancement.