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 hours, which consisted of 3 hours of technical and tactical tennis practice and 1 hour 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 six 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.
Methodology
The experimental flow is shown in Figure 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-minute standardized general warm-up and 10-minute cool-down protocol including jogging, skipping, dynamic warm-up, and stretching. Participants refrained from intensive exercise for a minimum of 48 hours before the testing sessions and each session was separated by 48 hours. 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.
Laboratory Anaerobic Measurements
The Wingate Anaerobic Test was used as a basic anaerobic test 16 Each player performed warm-up exercises for 5–10 min before each test, followed by a ride for 30 seconds 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 microliters 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:
Blaer =(Blamax-Bla15)/(15-t),
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 Figure 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 seconds between each repetition.
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 ability 8.
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 contribution 17. 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 performance 18. 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 seconds 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 48h after the pre-test. The training involved three sessions per week on alternate days (i.e., Monday, Wednesday, and Friday) for six 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 study 11 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) (Figure 3). A set comprised three repetitions of a 110-meter sprint with a 20-second recovery period between each sprint, followed by a 5-minute 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 time 19. The HRmax of each player was established using the peak value recorded by the monitoring system during the training.
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 to 0.05, 0.06 to 0.13, and ³0.14 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 large 20.