on-court endurance and performance testing in

ON-COURT ENDURANCE AND PERFORMANCE TESTING IN COMPETITIVE MALE TENNIS PLAYERS ERNEST BAIGET,1,2 JAIME FERNAŃDEZ-FERNAŃDEZ,3 XAVIER IGLESIAS,2 LISI´MACO VALLEJO,2 FERRAN A. RODRI´GUEZ2

AND

1

Department of Physical Activity Sciences, University of Vic, Barcelona, Spain; 2Research Group on Sport Sciences, National Institute of Physical Education-Barcelona, University of Barcelona, Spain; and 3Department of Training and Exercise Science; Faculty of Sports Science, Ruhr-University Bochum, Bochum, Germany

ABSTRACT

INTRODUCTION

Baiget, E, Fernańdez-Fernańdez, J, Iglesias, X, Vallejo, L, and Rodrı´guez, FA. On-court endurance and performance testing in competitive male tennis players. J Strength Cond Res 28(1): 256–264, 2014—The aims of this study were (a) to establish a specific endurance test procedure for competitive tennis players, combining performance, physiological and technical parameters and (b) to determine the relationship between these parameters and their competitive levels. Thirty-eight competitive male tennis players (age, 18.2 6 1.3 years; height, 180 6 0.08 cm; body mass, 72.7 6 8.6 kg; mean 6 SD) performed a specific endurance field test. Performance (level achieved), physiological (heart rate, maximum oxygen uptake (V_ O2max), and ventilatory thresholds (VT1, VT2), and technical parameters (technical effectiveness [TE]) were assessed. Bivariate and multivariate models for predicting performance level were developed. Technical effectiveness was 63.1 6 9.1%, with 3 identified phases throughout the test (adaptation, maximum effectiveness, and steady decline). Low to moderate correlations were found between performance (final stage), physiological (VT1, VT2) and TE, and competitive performance (r = 0.35–0.61; p = 0.038–0.000). Technical effectiveness explained 37% of variability in competitive performance (r = 0.61; p = 0.001). Using TE combined with VT2 or predictability increased explaining approximately 55% (p , 0.05) of the variance in competitive performance. The present study showed the usefulness of a field test including physiological and performance elements in high-caliber tennis players, and VT2 values combined with TE were good predictors of tennis performance.

KEY WORDS specific endurance tennis test, aerobic capacity, technical effectiveness, competitive performance

Address correspondence to Ernest Baiget, [email protected]. 28(1)/256–264 Journal of Strength and Conditioning Research ! 2013 National Strength and Conditioning Association

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ennis is a complex sport requiring a mixture of physical, technical/tactical, and psychological skills (8). From a physiological perspective, during long and fast rallies, tennis elicits average heart rates (HRs) of 70–80% of maximum (HRmax) and peak values around 100% of HRmax. Average oxygen uptake (V_ O2max) values correspond to approximately 50–60% of maximum, and with HR, during intensive rallies, peak values can reach values above 80% of maximal oxygen uptake (V_ O2max) (8,16). During match play, energy demands alternate between metabolic systems for bouts of high-intensity work during points (intramuscular phosphates and glycolysis) and low-intensity work during rest intervals (oxidative metabolism) (14,26). Oxidative metabolism helps to replenish energy sources during the course of a match (8,16,25), and values above 60 ml$kg21$min21 reported in elite male players seem to confirm the importance of possessing a medium to high aerobic capacity for tennis play (8,16). Testing the aerobic fitness of tennis players can involve evaluation through both laboratory and field tests. Incremental exercise tests conducted in the laboratory (e.g., direct measure_ O2max while running to exhaustion on a treadmill) ment of V are commonly used for tennis players to provide useful markers to evaluate physical fitness, identify key training areas, and characterize training effects (12). However, during treadmill testing, the mode of exercise (continuous activity) cannot simulate the intermittent demands of tennis and does not reflect the specific intra- and intermuscular activity of both upper and lower limbs with respect to hitting and footwork (i.e., accelerations, decelerations, and changes of direction) (7,11). To date, several tennis-specific test procedures have been used to determine the exercise capacity and technical performance of athletes with acceptable accuracy under standardized conditions (4,11,12,23,30). Data obtained in these previous studies provide information about physiological (i.e., HRmax) and performance (i.e., maximum level achieved) measurements, but the underlying testing criteria are difficult to standardize (rhythm, direction, bounce, and stroke simulation). In this regard, identifying the physiological determinants of performance is crucial for the profiling

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Journal of Strength and Conditioning Research of athletes, prescribing and monitoring training, and predicting performance (20). Thus, far information about the profile of competitive tennis players (i.e., playing or competing at International Tennis Federation or Association of Tennis Professionals [ATP] events) and their relationship with competitive performance is scarce. Contradictory results can be found in the literature, probably because of the different populations analyzed (e.g., children vs. adults) or clinical variables used (e.g., dynamic, isometric measurements) (13,17,21). Further knowledge of the relationship between the most important physical attributes (i.e., endurance), and the ranking of tennis players, could help assist in determining the relative importance of such measures and thus provide more suitable optimal training programs (13). Therefore, the aims of this study were (a) to examine performance (stage achieved following the test), physiological (ventilatory thresholds [VTs] and HR measures), and technical (technical effectiveness [TE]) parameters in competitive tennis players using a specific endurance test procedure and (b) to determine a prediction model using the relationship between these parameters and their competitive levels.

METHODS Experimental Approach to the Problem

This study was designed to evaluate physiological, performance, and technical parameters in competitive tennis players and develop a prediction model based on that information. A group of high-level tennis players were tested between February and April, in noncompetitive periods. The experimental design was divided into 2 parts: (a) test-retest reliability of the field test and effect of cardiorespiratory monitoring and (b) test administration. All tests were performed on an outdoor tennis court (i.e., GreenSet surface, GreenSet Worldwide S.L., Barcelona, Spain). Temperature ranged from 18 to 238 C with a stable

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environmental and wind conditions (i.e., air velocity , 2 m$s21, relative humidity 54.4–61.0% [Kestrel 4000 Pocket Weather Tracker; Nielsen Kellerman, Boothwyn, PA, USA]). Measurements began after an 18-minute standardized warm-up, which consisted of 10 minutes of jogging around the court, dynamic flexibility, forward, sideways, and backward running, and acceleration runs; 5 minutes consisting of ground strokes (players were asked to hit the balls to the center of the court); and 3 minutes of test familiarization, performing the test protocol at the lowest work load (frequency of balls ejected from the ball machine [Ballf ] = 9 shots per minute). To reduce the interference of uncontrolled variables, all the subjects were instructed to maintain their habitual lifestyle and normal dietary intake before and during the study. The subjects were told not to exercise on the day before a test and to consume their last (caffeine free) meal at least 3 hours before the scheduled test time. Subjects

Thirty-eight competitive male tennis players (age, 18.2 6 1.3 years; height, 180 6 0.08 cm; body mass, 72.7 6 8.6 kg; mean 6 SD), with an International Tennis Number (ITN) ranging from 1 (elite) to 4 (advanced) (ITN 1= 8 players; ITN 2 = 10 players; ITN 3 = 9 players; ITN 4 = 11 players), and corresponding to an ATP ranking between positions 600 and 1,000, volunteered to participate in this study. The mean training background of the players was 6.6 6 2.0 years, which focused on tennis-specific training (i.e., technical and tactical skills), aerobic and anaerobic training (i.e., on- and off-court exercises), and strength training. Before participation, all subjects were provided with written informed consent and the experimental procedures and potential risks were explained. Parental written informed consent was obtained for the 5 subjects under 18 years age. The research committee of the local university approved the study.

Figure 1. Schematic setting for the specific endurance field test.

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TABLE 1. Performance, physiological, and technical parameters (mean 6 SD) obtained during the 3 field tests performed (T1, T2, and T3) (n = 12). T1 Test duration (min:s) Final stage (stage no.) HRmax (b$min21) TE (% of successful hits)

11:42 5.7 196.8 66.4

6 6 6 6

T2 01:00 0.5 9.5 8.6

12:03 5.8 194.0 65.0

6 6 6 6

T3 00:55 0.5 9.3 5.5

13:06 6.4 192.4 65.0

6 6 6 6

ANOVA (p)

T1 vs. T2

T2 vs. T3

0.003 0.002 0.494 0.855

NS NS

0.003 0.003

00:53 0.4 9.4 6.9

NS = non significant; TE = technical effectiveness.

Procedures

Field Testing. A modified version of the test previously published by Smekal et al. (23) was used. During development, a literature- and data-based (4,24,30) consensus for the test protocol was found (i.e., running distance, footwork, stroke activity). The test protocol was approved in close discussion with high-level coaches (i.e., professional coaches, certified with the highest level by the Spanish Tennis Federation [RFET], and with a minimum of 10-year experience), although do not simulate the great variety of playing situations during a tennis match, and includes some elements of tennis, is performed on a tennis court, and is combined with analysis of stroke precision. The original field test proposed by Smekal et al. (23) (conducted with a ball machine) began with a Ballf of 12 shots per minute, which was increased by 2 shots per minute every 3 minutes. The modifications from the original test were (a) the inclusion of shorter stages (2 instead of 3 minutes) and (b) the test beginning with a lower Ballf (9 instead of 12 shots per minute). These changes were implemented to limit the duration of the test within limits favoring the determination of main cardiorespiratory parameters (i.e., V_ O2max and VTs). Taking into account that ball velocity (Ballv) was constant (68.6 6 1.9 km$h21), the protocol (length of stages and Ballf ) was designed to fulfill the accepted criteria for exercise testing (29), with a test duration between 8 and 20 minutes. Balls were alternately pro-

jected by a ball machine (Pop-Lob Airmatic 104; Bagneux, France) to the right and the left corners of the baseline. Participants had to hit alternating forehands and backhands cross court or down the line in a prescribed pattern (i.e., drive, topspin). The landing point for the balls was chosen about 2 m in front of the baseline. Subjects had to hit the balls alternating between backhand and forehand (Figure 1). Slice strokes were not allowed because we assumed that they might influence player movement to the ball and therefore physiological responses and test reliability. The test began with a Ballf of 9 shots per minute, which was increased by 2 shots per minute every 2 minutes. The test ended at the player’s request or stopped by the researchers if the player was no longer able to fulfill the test criteria (i.e., the player was no longer able to perform strokes with acceptable stroke technique and precision, determined by the experienced researchers, through subjective observation). In this regard, we acknowledge that variability may exist with the testing protocol based on the coaches at hand. In addition to the physiological measurements (ventilatory gas exchange and HR), an objective evaluation of the TE was carried out. Technical effectiveness was calculated based on the percentage of the hits and errors, and 2 performance criteria were defined as follows: (a) precision: the ball returned by the player had to bounce inside the target (i.e., 3.1 by 4.5 m square located 1 m from the service line and 1 m

TABLE 2. Test-retest reproducibility (T1 vs. T2) and effect of using the portable gas analyzer on reproducibility (T2 vs. T3) (n = 12).*† T1 vs. T2 r (p) Test duration (min:s) Final stage (stage no.) HRmax (b$min21) TE (% of successful hits)

0.85 0.85 0.91 0.80

(0.001) (0.000) (0.000) (0.002)

ICC 0.84 0.85 0.91 0.72

(0.000) (0.000) (0.000) (0.003)

T2 vs. T3 CV (%) 3.1 2.0 1.4 4.7

r (p) 0.86 0.72 0.92 0.95

(0.000) (0.008) (0.000) (0.000)

ICC 0.86 0.71 0.92 0.93

(0.000) (0.004) (0.000) (0.000)

CV (%) 6.0 7.4 1.0 1.7

*CV = coefficient of variation; ICC = intraclass correlation; TE = technical effectiveness. †Simple correlation (r), ICC, and CV are shown.

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Figure 2. Oxygen uptake (A), minute ventilation (B), HR (C), and stage during the different test periods (warm-up, exercise and recovery). Values are 1-minute mean (6SD) (n = 38). HR = heart rate.

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TABLE 3. Performance and physiological variables (mean 6 SD) corresponding to and the VTs (VT1 and VT2) (n = 38).* VT1 Duration (min:s) Stage (no.) ml$min21 ml$kg21$min21 ml$min21 V_ E (L$min21) HR (b$min21) R

05:57 2.7 2,679 36.8 2,349 65.0 153.3 0.87

6 6 6 6 6 6 6 6

01:32 0.8 436 4.2 447 8.3 11.4 0.1

Percentage of maximum 46.8 43.7 64.3 64.6 50.7 48.7 79.1 79.9

6 6 6 6 6 6 6 6

11.7 11.0 4.1 4.5 7.7 7.2 4.5 15.7

VT2 09:43 4.6 3,506 48.2 3,419 93.2 177.0 0.98

6 6 6 6 6 6 6 6

01:31 0.8 535 5.4 605 10.9 9.1 0.12

Percentage of maximum 75.8 74.6 84.7 84.7 73.6 69.7 91.8 86.5

6 6 6 6 6 6 6 6

10.3 11.0 3.4 3.4 7.4 8.4 2.6 6.8

12:54 6.2 4,194 57.1 4,685 134.9 190.1 1.12

6 6 6 6 6 6 6 6

01:49 0.9 653 6.0 892 17.3 8.5 0.19

*HR = heart rate; VT = ventilatory threshold.

over the prolongation of the center service line) and (b) power: once the ball was bounced inside the target zone, it had to go over the power line (located between 5 m from the center of the baseline and 4 m from the side line), before bouncing for a second time. The ball machine was manually calibrated before each test, and the device’s reliability was assessed by manual timing (mean coefficient of variation [CV] of Ballf = 3.5 6 0.9%) and using a radar device (Stalker ATS 4.02; Radar Sales, Plymouth, MN, USA) (mean Ballv = 68.6 6 1.9 km$h21; CV of 2.7%). A minimum of 40 new tennis balls (Babolat Team Spain) were used for each test.

the test (recovery phase). Expired air was analyzed continuously for gas volume (Triple digital-V1 turbine), oxygen concentration (zirconium analyzer), and carbon dioxide concentration (infrared analyzer) using a portable gas analyzer (K4 b2; Cosmed, Rome, Italy). The portable measurement unit was carried by the player in the same way during all tests. Heart rate monitoring (Polar, Kempele, Finland) was used alongside the Cosmed K4b2 system. Gas and volume calibration of the measurement device was done in the morning of each test session. Room air calibration occurred before each test. Ventilatory threshold (VT) detection was done by analyzing the points of change in slope or breaks in linearity of Physiological Measurements. Ventilatory gas exchange and HR ventilatory parameters (1). Two VTs were determined accordwere continuously recorded, beginning 2 minutes before the ing to the model proposed by Skinner and MacLellan (22): familiarization phase and finishing 5 minutes after the end of VT1 or first VT and VT2 or second VT (Wasserman’s respiratory compensation point). VT1 was determined using the criteria of an increase in the ventilatory equivalent for oxygen (VE/ V_ O2) with no increase in the ventilatory equivalent for carbon dioxide (VE/VCO2) and the departure from linearity of VE (minute ventilation), whereas VT2 corresponded to _ O2, an increase in both VE/V _ and VE/VCO2. VO2max was determined by the observation of a “plateau” or leveling off in or when the increase in 2 successive periods was less than 150 ml$min21 (31). HRmax was considered as the highest value reached during the Figure 3. Evolution of TE evaluated as a percentage of hits, based on the periods of the specific endurance field final minute of the test, and thetest. Phases were defined as follows: 1 = adaptation; 2 = maximum effectiveness; 3 = steady decline. TE = technical effectiveness. oretical HRmax was taken as 208 2 0.7 3 age (28).

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Journal of Strength and Conditioning Research Performance and Technical Measurements. The main performance measurements were the total duration of the test, until the player felt exhausted or failed to hit the ball twice in a row, and the final stage achieved, with a precision of 0.5 periods (i.e., including the last completed minutes of exercise during the final stage). Technical scores (i.e., hits-errors) were registered at 30-second intervals by an experienced coach, and data were processed to derive the overall (i.e., whole test average) TE% of the test, which was defined as the percentage of correct hits. Test-Retest Reliability. Reliability was evaluated comparing 2 consecutive test series (T1 and T2) performed within 48 hours. Twelve players (age, 17.2 6 1.0 years; height, 176 6 0.1 cm; body mass, 71.8 6 9.0 kg; mean 6 SD) performed the 2 tests under similar conditions, without using the portable gas analyzer. Test duration, final stage, HRmax, and TE were recorded. Seven days after T2, a third test (T3) was conducted with the same subjects wearing the portable gas analyzer to examine the effect of wearing the equipment on test performance and technical parameters. All 3 tests were carried out within 9 days. Statistical Analyses

Mean values (6SD) were calculated for all variables. The normality of variable distribution was assessed by the KolmogorovSmirnov test. The relationship between quantitative variables was established with a linear correlation analysis, by calculating the Pearson’s linear correlation coefficient (r), followed by a linear regression analysis. The multivariate analysis was carried out using a multiple regression model (stepwise method), with competitive level (ITN) as the predicted variable and all physiological and TE variables as predictors. Significance was tested at the 95% confidence level (p , a # 0.05). Testing reliability was assessed by calculating simple (r) and intraclass correlation coefficients (ICC) and the CV% to determine the degree of stability of the values between 2 tests (T1 vs. T2 and T2 vs. T3). One-way analysis of variance with pairwise multiple post hoc comparisons (Tukey’s) was used to test the significance of the differences. Significance was tested at the 95% confidence level using the Bonferroni’s correction (p , 0.008). All statistical analyses were performed using SPSS for Windows 15.0 (SPSS, Inc., Chicago, IL, USA).

RESULTS Test-Retest Reliability

No significant differences between T1 and T2 were found for all the variables analyzed (p , 0.008) (Table 1). When evaluating the test-retest reproducibility (T1 vs. T2), indices showed good consistency in the measurement of physiological (i.e., HRmax) and performance parameters (test duration and final stage) and lower values for the TE parameter (Table 2). Results comparing the effect of wearing the portable gas analyzer on test performance are also presented in Table 1 (T2 vs. T3). There were no significant differences

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TABLE 4. Correlation coefficient between the competitive level of tennis players (ITN) and performance, physiological, and TE parameters during the field test (n = 38).* Competitive level (ITN) Variables

r

p

Final stage (stage no.) V_ O2max ml$kg21$min21 VT1 (ml$kg21$min21) VT2 (ml$kg21$min21) TE (% of successful hits)

0.50 0.55 0.35 0.55 0.61

0.001 0.001 0.038 0.001 0.000

*ITN = International Tennis Number; TE = technical effectiveness; VT = ventilatory threshold; V_ O2max = maximal oxygen uptake.

between physiological and technical parameters. Comparing performance parameters in T2 and T3, significantly higher values were obtained for final stage and test duration (p = 0.003) (Table 1). All the variables reached good consistency index levels (Table 2). Physiological Measurements. Figure 2 shows the increase of V_ O2 VE and HR during the different test periods and stages. Table 3 shows the values of the physiological and load parameters corresponding to the intensity in which V_ O2max and the VTs (VT1 and VT2) were attained. Test Performance. Test duration was 13:39 6 01:36 minutes: seconds, corresponding to a final stage of 6.61 6 0.82. For TE, a total of 197.7 6 30.8 hits were made per test, of which 63.1 6 9.1% were considered to be successful. In general, the evolution of TE was inversely proportional to the testing load. Figure 3 shows the test evolution. Based on TE results, 3 phases were identified. A first phase (first and second stages) in which, although the load was low, the best percentage of successes was not achieved; a second phase (third and fourth stages), with the players achieving the best TE results; and a third phase (from the fifth to the ninth stages), in which the TE started to decrease progressively. Correlation Between Test Performance and Competitive Level. A significant relationship was found between all the measured parameters and the players’ ITN, with Pearson’s r coefficients ranging from low to moderate (r = 0.35–0.61). Two stages of multiple regression analysis were carried out. In the first stage, the model only took into account the TE, explaining the 37% in ITN variability (r = 0.61; p = 0.001). The final model using the stepwise method showed that the major predictors of performance level were TE and VT2 and explains 56% of the variance of ITN (r = 0.75; p = 0.001). TE and V_ O2max explains 53% (r = 0.73; P = 0.006) of ITN variability. VOLUME 28 | NUMBER 1 | JANUARY 2014 |

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On-Court Endurance Testing in Tennis DISCUSSION Recent efforts have been made to develop field tests for tennis to determine the endurance capacity (7,11,12) or technical performance (i.e., stroke precision, accuracy) (23,27,29,30) of athletes with acceptable accuracy under standardized conditions. The present study showed the usefulness of a field test _ O2max and VT) and perin determining physiological (i.e., %V formance (i.e., percentage of successful hits in the test or TE measurements in high-performance tennis players. Results also showed that there was a moderate relationship between _ O2max the competitive level, the physiological parameters (V and VT2, r = 0.55), and the final stage of the test (r = 0.50). Moreover, VT2 and V_ O2max values combined with TE seem to be good predictors of tennis performance, explaining 53% and 56% of the variance in performance level, respectively. Using this type of field test provides a better measurement of physical capacity combined with technical performance and may be routinely used to accurately prescribe aerobic exercise in a context appropriate to the sport of tennis (12,23). Although the proposed field test used is based on one test previously published and validated by Smekal et al. (23), one of the aims of the present study was to show the reliability and reproducibility of the derived modification (i.e., inclusion of shorter stages and beginning with a lower Ballf ). Test-retest results showed no significant differences among 2 subsequent test administrations in any of the parameters studied. Moreover, data showed high reproducibility in the physiological and performance parameters analyzed (ICC . 0.80 and CV , 4%). Results are in line with those reported by several studies developing field tests for tennis players (11,12,23). Although some values (i.e., ICC = 0.72) were similar to those reported by previous research (23,30), percentage of hits, defined as TE earlier in the article, was the only parameter that showed lower repeatability, possibly because of its complex nature. This is probably dependent not only on physical parameters but also on technical demands related to stroke performance (i.e., grip, ball-hitting point). Regarding the effect of using the portable gas analyzer on the test performance, results show good consistency in all the physiological (i.e., HRmax) and performance parameters (TE) analyzed. No significant effects can be attributed to wearing the portable analyzer except for a slightly longer duration of the test and final stage achieved. The somewhat longer duration of the final stage could be explained by the extra motivation elicited by using a high technology device during the test (i.e., Hawthorne effect) (6), and a possible learning effect, as the order of the tests was not randomized. _ O2max values Physiological responses to the test showed V 21 21 of 57.1 6 6.0 ml$kg $min , which are in the range of those obtained in previous studies, reporting V_ O2max values between 55 and 65 ml$kg21$min21 (11,12,23). Although the critical phases of play during tennis matches (i.e. serve, primary strokes, quick changes of direction, and short accelerations) are likely to be metabolically dependent on anaerobic

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pathways of energy supply, these are superimposed on a background of largely aerobic submaximal activities (8). In this regard, Smekal et al. (25) reported peak V_ O2 values during a simulated match play representing approximately 80% of V_ O2max. This confirms that medium to high aerobic power would be essential to successfully sustain an elevated level of technical, tactical, physiological, and psychological capacity during several hours of tennis (8,16,25). Regarding submaximal intensities achieved during the field test, intensity at VT2 (approximately 92% of HRmax and approximately 85% of V_ O2max) was in line with those values reported by previous research (12,24,25). Only limited data are available regarding submaximal parameters (i.e., VT values) in tennis players because it has traditionally been considered the gold standard (12), but there is increasing evidence that the VT may be a better predictor for submaximal endurance performance (5,12). This would be especially important in tennis where the performance is multifaceted, involving technical, tactical, psychological, and physiological factors. In the present study, VT evaluation provided valuable information on a player’s endurance in competition and it seems that V_ O2max and VT2 showed the same performance predictive ability in the players analyzed, with moderate correlation values (i.e., 0.55) (Table 4). Moreover, a moderate relationship (r = 0.50–0.55) was found between the competitive level, the physiological parameters (i.e., V_ O2max and VT2), and the final stage achieved during the test, suggesting a link between aerobic fitness and tennisspecific performance. Although successful performance cannot be defined by one predominant physical attribute, tennis demands a complex interaction of several physical components and metabolic pathways (8,10). Among these physical components, it is recognized that aerobic fitness (i.e., V_ O2max) is an important component of tennis performance and enables the player not only to repeatedly generate explosive actions (e.g., strokes and on-court movements) but also to ensure fast recovery between rallies, especially during long matches (12,15,25). Three differentiated phases were identified in the evolution of the TE (Figure 3). A first phase (first and second stages of the test) in which, despite the fact that the testing load was lowest (9 and 11 shots per minute), the TE level was not the highest (i.e., percentage of successful hits around 62–67%). It is possible that these 2 initial stages corresponded to a first phase of task adaptation. A second phase (third and fourth stages of the test) of maximum TE was also observed, in which the moderate intensity (13 and 15 shots per minute) did not affect the technical performance, and players were able to achieve an optimal percentage of successful hits (i.e., approximately 70%) with a proper technique. Finally, a third and final phase was identified (from the fifth to the ninth of the test), in which there was a progressive decrease in TE (i.e., until approximately 35%), because of the high intensity of the testing load and the accumulated fatigue from previous periods. The overall

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Journal of Strength and Conditioning Research values found during the test (i.e., approximately 65% of successful hits) showed a high degree of precision, based on the large number of hits performed per test (i.e., approximately 200). These results are in line with those obtained in previous research carried out with competitive male tennis players (23,27,30) reporting error percentages between 30 and 40% during other tennis-specific tests. There was a low to moderate bivariate relationship between the evaluated parameters and the competitive level of the players (r = 0.35–0.55; p = 0.038–0.001), which is in line with previous research and supports the multifactorial nature of tennis performance (2,13,23,29,30). If TE is taken as the single predictive parameter, results show that it accounts for just 37% of the performance level (i.e., ITN). However, the multivariate analyses used to predict players’ competitive performance showed that a large part of the variability in their competitive level can be explained by the TE and physiological (VT2 and V_ O2max) parameters taken together (i.e., TE and VT2: 56%; TE and V_ O2max: 53%). These results suggest that the parameters analyzed in the present study showed a moderate ability to predict tennis performance, largely is basically because of the multifactorial nature of tennis. Performance during match situations requires a fine interaction between the tactical, technical, psychological, and physical components (16). It is possible that the predictive capacity found is more attributable to an improved training ability (i.e., physiological conditioning and technical ability under fatigue) rather than overall performance in tennis. The rest of the variance is likely to be explained by the tactical/technical (i.e., positioning on the court, stroke quality) and psychological skills (i.e., concentration) of the players. In conclusion, the present study showed the usefulness of _ O2max and VT) and a field test including physiological (i.e., %V performance (i.e., percentage of successful hits in the test or TE) measurements in high-performance tennis players. Parameters obtained in the test demonstrate a moderate predictive capacity of performance in tennis, which can be explained by improving training ability (physiological conditioning and technical ability under fatigue), and highlight the relative importance of the aerobic fitness in tennis performance. To improve the predictive capacity of the model, it would be necessary to introduce tactical and psychological variables. In this regard, the hypotheses that the predictive precision of this model could increase in lower level players, in which the technical and conditional performance could affect the final performance more than in high-level players, warrants further investigation.

PRACTICAL APPLICATIONS Performance in tennis is multifactorial, requiring a mixture of physical, technical/tactical, and psychological skills. The inclusion of field testing with the analyses of physiological (i.e., oxygen uptake, VT) and performance (i.e., percentage of successful hits) variables seems to provide a more specific

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measurement of physical performance and may be routinely used to accurately prescribe aerobic exercise in a context appropriate to the sport of tennis. Results of the present study showed a moderate correlation between parameters analyzed during the test and tennis performance (i.e., ITN level). In this regard, coaches and sport scientists can use the test not only to measure fitness during different periods of the season but also to prescribe or administer training “regimes.” Thus, test parameters, such as Ballf or final stage, can be used as target training areas. For example, if the training aim is focused on the technique stabilization under specific conditions, coaches can use medium intensities, where the best percentage of successful hits was achieved (i.e., stages 3– 4; 13–15 shots per minute). On the other hand, if the training aim is focused on physical conditioning, coaches can use moderate to high intensities (stage 5–6; 17–19 shots per minute) prescribing on-court high-intensity interval training, close to the levels (i.e., 4 sets of 2 minutes at levels 5–6 [17–19 shots per minute], with 90-second rest), to improve cardiorespiratory fitness in competitive tennis players. In this regard, the complementary use of high-intensity interval exercises (e.g., work and rest intervals ranging from 15 seconds to 4 minutes; 90–100% velocity at the level of V_ O2max; HR values approximately 90% HRmax; work to rest ratios of 1:1–4:1) should also be considered as training alternatives (3,9,18,19,32). Moreover, it would be interesting to use the submaximum and maximum test levels for training purposes. Thus, the combination of different training strategies (i.e., high-intensity aerobic interval training) warrants further investigation.

ACKNOWLEDGMENTS The authors thank all the players and coaches for their enthusiastic participation. They would also like to thank Sańchez-Casal Academy, Bruguera Top Team Tennis Academy, Escola Balear de l’Esport, and Centre Internacional de Tennis of the Catalan Tennis Federation. The authors gratefully acknowledge the technical assistance of Pedro Zierof and Valery Kryvaruchka during the experiments.

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