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he aim of this study was to validate critical frequency specific test (critf) for the estimation of the aerobic endurance in table tennis players. Methods: Eight male international-level table tennis players participated of this study. Specific tests were applied by using a mechanical ball thrower to control the inten- sity of the exercise. The critf was determined by applying three or four series of exercises to exhaustion (Tlim). The critf was evaluated by using lactate steady state test (90, 100, and 106 % of critf intensity). The other specific test was an incremental protocol used to determine the anaerobic threshold (AnT BI) and the onset of blood lactate accumulation (OBLA) using a ball thrower. Results: The critf (39.87 ± 3.31 balls·min -1 ) was not significantly different among AnT BI (48.11 ± 7.36 balls·min -1 ) and OBLA 3.5 (49.36 ± 12.04 balls·min -1 ) frequencies and it was correlated with AnT BI parameter (r = 0.78). At frequencies of the 90 and 100% of critf a dynamic equilibrium was verified in lactate concentration between the eighth and twentieth minutes. However, this dynamic equilibrium was not found at 106% intensity. Conclusion: The data indicate that in table tennis the critf model can be used for measuring the aerobic endurance. Key words: Anaerobic threshold - Aerobic endurance - Blood lactate - table tennis. Introduction Racket sports are characterized by effort and rest periods. During these effort periods the movements of inferior limbs are rapid and powerful whereas, the movements of upper limbs are rapid only. Overall, the aerobic system is the predominant mechanism of resynthesis of

measuring the aerobic endurance. Key words: Anaerobic threshold - Aerobic endurance - Blood lactate - table tennis. Introduction Racket sports are characterized by effort and rest periods. During these effort periods the movements of inferior limbs are rapid and powerful whereas, the movements of upper limbs are rapid only. Overall, the aerobic system is the predominant mechanism of resynthesis of energy (ATP) in these sports, but in effort periods the phospha- genic system (ATP-PCr) is the main mechanism to resyn- thesis of ATP (Zagatto et al., 2008). The measurement of the aerobic and anaerobic capacities and power are very important in sports to verify the physical aptitude status of athletes and to determine ideal exercise prescription. Despite the fact that the ATP-PCr is the main mechanism to resynthesize of ATP in effort periods in rackets sports, the aerobic system is responsible for the recovery between effort periods (rally) and is also the main tool for the prescription of exercises (i.e., anaerobic threshold). The aerobic endurance, called, anaerobic threshold (AnT) or maximal lactate steady state (MLSS) by some investiga- tors, has been used as a main tool for the prescription of the intensity of the exercise, in both aerobic and anaerobic sports. However, despite the importance of verifying the aerobic endurance with precision, few studies have meas- ured this aerobic component using specific protocols for racket-sports. Nevertheless, the majority of investigations that applied specific tests were performed in tennis, bad- minton and squash (Chin et al., 1995; Girard et al., 2005; 2006; Smekal et al., 2000; Wonisch et al., 2003). How- ever, there have been few applications of specific proce- dures in table tennis (Morel and Zagatto, 2008; Zagatto and Gobatto, 2007; Zagatto et al., 2008). The use of blood lactate concentration ([Lac]) has been the main physiological parameter used for determin- ing aerobic endurance. However, the measurement of blood lactate currently requires invasive and expensive techniques to analyze the samples (Heck et al., 1985; MacIntosh et al., 2002). Monod and Scherrer (1965) pro- posed the critical power model (critP) as a non-invasive procedure to estimate the aerobic endurance by measuring

concentration ([Lac]) has been the main physiological parameter used for determin- ing aerobic endurance. However, the measurement of blood lactate currently requires invasive and expensive techniques to analyze the samples (Heck et al., 1985; MacIntosh et al., 2002). Monod and Scherrer (1965) pro- posed the critical power model (critP) as a non-invasive procedure to estimate the aerobic endurance by measuring the time of exercise until exhaustion. The critical power model has been described as a good procedure for meas- uring the aerobic endurance (Dekerle et al., 2002; Toubekis et al., 2006; Wakayoshi et al., 1993). Several investigations have been adapted to the original critical power model for other sports and ergometers, such as swimming (Dekerle et al., 2002; Di Prampero et al., 2008; Toubekis et al., 2006; Wakayoshi et al., 1993), cycle ergometer (Bishop et al., 1998; Pringle and Jones, 2002), running (Bosquet et al., 2006), kayaking (Clingeleffer et al., 1994) and recentily table tennis (Morel and Zagatto, 2008 ; Zagatto and Gobatto, 2007; Zagatto et al., 2008). Although the critical power model has been adapted for table tennis and so-called critical frequency (critf), the validity of this procedure adapted for table tennis has not yet been verified. Therefore, the purpose of this investiga- tion was to verify the validity of the critical power model adapted to table tennis (critical frequency test) to measure the aerobic endurance in a specific protocol using a me- chanical ball thrower. Methods Subjects Eight male table tennis players of international level (mean ± SD – age 18 ± 3 years, body mass 67.0 ± 10.7 kg, height 1.76 ± 0.10 meters, body fat 14.7 ± 7.1 %, and body mass index 21.7 ± 2.9 kg . m -2 ) participated in this study. The players were fully informed of the nature and Research article

Critical frequency test for table tennis 462 NEWGY PONG 50 - 6 0 cm Table Net Balls Figure 1. Illustration scheme of table tennis and the contact areas for the balls shot by the equipment NEWGY PONG 2000 in the specific tests for table tennis. possible risks of the investigation before giving their written informed consent. The experimental procedure was approved by the Ethics Committee of São Paulo State University, Brazil. Experimental design Three sport-specific protocols were applied (critical fre- quency, lactate steady state test and incremental test) simulating forehand offensive strokes with ball shots from a mechanical ball thrower (NEWGY-PONG 2000, Newgy, CANADA). Prior to each sport-specific test a 4 minute warm-up exercise at moderate intensity (35 balls·min -1 ) was performed by subjects. The tests started five minutes after the end of the warm-up period. Description and adaptation of the mechanical ball thrower The NEWGY-PONG 2000 (Newgy, Canada) mechanical ball thrower has adjustments from 0 to 10 for speed con- trol, lateral ball oscillation, and thrower frequency. Lat- eral ball oscillation was adjusted (setting 3) so that balls were shot systematically to different areas of the table tennis table (between the two extremities) so that the ball contacted the table between 50 and 60-cm away from the net (Figure 1). Ball speed was constant at to “setting 5”. Only ball frequency (exercise intensity) was changed for each effort (Zagatto et al., 2008). To minimize interference from learning before the sport-specific test, the participants performed two famili- arization sessions (done on consecutive days) at the same ball speed and lateral oscillation as applied in the test, and at varying ball shot frequencies. Each familiarization session lasted approximately 10 minutes. Experimental procedures Critical frequency test (critf): All the athletes performed three or four trials (separated by at least 2 hours and no more than 2 exercises per day) on a table tennis table. Exercise frequencies (intensities) corresponded to ap- proximately 48, 56, 65, and 72 (balls.min -1 ) and were performed until technical or voluntary exhaustion (techni- cal exhaustion occurred when four consecutive errors occurred in the offensive strokes developed

three or four trials (separated by at least 2 hours and no more than 2 exercises per day) on a table tennis table. Exercise frequencies (intensities) corresponded to ap- proximately 48, 56, 65, and 72 (balls.min -1 ) and were performed until technical or voluntary exhaustion (techni- cal exhaustion occurred when four consecutive errors occurred in the offensive strokes developed with aid of coach). Exhaustion time (Tlim) was recorded. The critf was obtained by linear regression techniques between ball frequency (f) and the inverse of the Tlim (Tlim -1 ), corre- sponding to linear coefficient (y-intercept) (Figure 2). Figure 2. Represents the linear relationship between ball shot frequencies versus inverse of exercise time (1/Tlim) used to determine the critical frequency (critf). The critf corresponded to linear coefficient between frequency and the inverse of time. Lactate steady state test: A sport-specific continuous test was applied after critf to verify the blood lactate behavior in intensities below critf (90% of critf), at critf (100% of critf) and upper critf (106% of critf). The test lasted 20 minutes at constant workload intensity. Capillary blood samples were taken from the ear lobe (25 µl) every four minutes of constant load to determine the blood lactate

Zagatto et al. 463 Figure 3. Determination of frequency corresponding of anaerobic threshold determined in specific protocol through visual inspection of abrupt increase of [Lac] using bi-segmented linear regression model (AnT BI) (Dash lines) and the OBLA 3.5 determined by fixed blood lactate concentration corresponding to 3.5 mmol·L -1 (short dot lines). concentration during the test. The lactate steady state was defined as highest work rate that could be maintained without an increase of blood lactate by more than 1.0 mmol.L -1 between the 8 th and the 20 th min of constant load of the lactate steady state test (Gobatto et al., 2001). Incremental test: The sport-specific incremental test consisted of an initial frequency of 34 balls⋅min -1 and increments of 5 balls⋅min -1 every 3 minutes until volun- tary exhaustion. After each exercise stage blood samples (25 µl) were collect from the ear lobe to determine the lactatemia. Blood samples were also taken at 1, 3, 5 and 7 minutes after exercise. Determination of anaerobic threshold (AnT) and Onset of blood lactate accumulation (OBLA) intensities: Anaerobic threshold (AnT BI) was determined by visual inspection of abrupt increase of the lactate concentration response using bi-segmented linear regression model (determined by three specialists in physiology of exercise) and the onset of blood lactate accumulation was corre- sponded to 3.5 mmol.L -1 fixed blood lactate concentration (OBLA 3.5) (Figure 3). Blood sample analysis: Blood samples (25 µL) were collected from a participant’s ear lobe and trans- ferred to 1.5 mL Eppendorf tubes containing 50 µL NaF (1% sodium fluoride). The homogenate was injected (25 µL) into an electrochemical lactate analyzer (Yellow Springs Instruments model 1500 Sport, Ohio, USA). The electrochemical lactate analyzer was calibrated after every five blood samples analysis using a standard-5.0 mmol.L -1 lactate solution. Blood lactate concentrations are ex- pressed in millimoles per liter (mmol.L -1 ). Statistical analysis Data are expressed as mean ± SD. Significant differences for critf, AnT BI and OBLA3.5 were tested by one-way ANOVA. Newman-Keuls post hoc test was performed if statistical significance was obtained to identify which variables differed. Relationships between variables

a standard-5.0 mmol.L -1 lactate solution. Blood lactate concentrations are ex- pressed in millimoles per liter (mmol.L -1 ). Statistical analysis Data are expressed as mean ± SD. Significant differences for critf, AnT BI and OBLA3.5 were tested by one-way ANOVA. Newman-Keuls post hoc test was performed if statistical significance was obtained to identify which variables differed. Relationships between variables were examined by using a Product Moment Linear Correlation Analysis. The intensity of lactate steady state was deter- mined by variation lower than 1.0 mmol·L -1 between the 8th and 20th minutes of constant exercise. The program STATISTIC for Windows 6.0 (Statsoft, Inc. 2001) was used for statistical analysis. In all cases, the statistical significance was set at p < 0.05. Results The Tlim obtained in the exercise frequencies (48, 56, 65 and 72 balls . min -1 ) corresponded to 578.57 ± 203.95 s, 342.67 ± 109.70 s, 259.60 ± 38.90 s, and 188.83 ± 60.47 s, respectively. The critf was determined by linear regres- sion between intensity of exercise and Tlim -1 and corre- sponded to 39.87 ± 3.31 balls⋅min -1 . The coefficient of determination (R 2 ) of regression was 0.88 ± 0.11. A dy- namic equilibrium of lactate was found at frequencies of 90% of critf (lactate mean value correspondents at 2.88 ± 1.19 mmol·L -1 and variation of lactatemia equivalent to 0.27 mmol . L -1 ) and 100% of critf (lactate mean value correspondents at 3.51 ± 0.34 mmol·L -1 and variation of lactatemia equivalent to 0.75 mmol . L -1 ). However, there was no lactate equilibrium at the frequency of 106 % of critf (lactate mean value correspondent at 3.80 ± 1.80 mmol·L -1 and variation of lactatemia equivalent at 1.46 mmol . L -1 ). The relationship between lactate concentration and exercise time verified by constant workloads at given frequencies of critf are showed in Figure 4. The AnT BI was determined by three specialists in exercise physiology through a visual inspection following bi-segmented linear regression and were obtained, among them, a results variation of 2.86 ± 2.59% for [Lac]

mmol . L -1 ). The relationship between lactate concentration and exercise time verified by constant workloads at given frequencies of critf are showed in Figure 4. The AnT BI was determined by three specialists in exercise physiology through a visual inspection following bi-segmented linear regression and were obtained, among them, a results variation of 2.86 ± 2.59% for [Lac] and 0.79 ± 0.36% for intensity (shot frequency). The AnT BI occurred at frequency of 48.11 ± 7.36 balls·min -1 and the [Lac] at AnT BI was 3.09 ± 1.65 mmol·L -1 . The OBLA3.5 determined by fixed blood lactate concentration occurred at frequency of 49.36 ± 12.04 balls . min -1 . The maximal frequency obtained in the incremental test was 58.81 ± 12.76 balls . min -1 . The critf, AnT BI and OBLA3.5 were not signifi- cantly different [F(1,6) = 3.03; p = 0.72]. However,

Critical frequency test for table tennis 464 Continuum Test 2 3 4 5 6 7 8 048121620 Time (min) 90 % 100 % 106 % Lactate (mmol.L-1) Figure 4. The relationship between blood lactate concentration and time of exercise in the lactate steady state test. The vertical bars indicate SEM. despite the lack of significant differences between these variables, the AnT BI and the OBLA3.5 were 20.7% and 23.8% higher than critf, respectively. The critf was significantly correlated with the AnT BI (r = 0.78; p = 0.03) and also with the frequency at exhaustion (r = 0.79, p = 0.02), but not with OBLA 3.5 (r = 0.42; p = 0.34). The AnT BI was also correlated with frequency at exhaustion (r = 0.94; p = 0.002). Discussion The critical frequency test was shown to be a good method to aerobic endurance evaluation in a table tennis sport-specific test, and of lactate concentration was found to stabilize at 100% of critf intensity and significantly correlate with critf and AnT BI. The critical power model made some adaptations to the original model described by Monod and Scherrer (1965), for application to swimming (Wakayoshi et al., 1993), cycle ergometer (Bishop et al., 1998; Jenkins and Quigley, 1990; 1992; Pringle and Jones, 2002), running (Bosquet et al., 2006; Smith and Jones, 2001), and kayaking (Clingeleffer et al., 1994), with valid and reliable results. The critical power model has been validated and correlated with the aerobic endur- ance determined by ventilatory threshold (Moritani et al., 1981), fatigue threshold (DeVries et al., 1982), individual anaerobic threshold (McLellan and Cheung, 1992), onset of blood lactate accumulation (OBLA) (Papoti et al., 2005; Wakayoshi et al., 1993) and maximal oxygen up- take (Jenkins and Quigley, 1992), showing it to be a good tool for assessing the aerobic parameter. Wakayoshi et al. (1993) adapted the critP concept for swimming and called it critical swimming. Wakayoshi et al. (1993) found high correlation between critical swimming and anaerobic threshold and showed that in exercise 100% intensity of critical swimming a dynamic equilibrium occurred be- tween the production and the disposal

it to be a good tool for assessing the aerobic parameter. Wakayoshi et al. (1993) adapted the critP concept for swimming and called it critical swimming. Wakayoshi et al. (1993) found high correlation between critical swimming and anaerobic threshold and showed that in exercise 100% intensity of critical swimming a dynamic equilibrium occurred be- tween the production and the disposal of blood lactate. However, this dynamic equilibrium did not occur when the intensity of exercise was increased by only 2%. Simi- lar result was found by Jenkins and Quigley (1990) on the cycle ergometer. In the present study we adapted the critical power model for table tennis using a mechanical ball thrower (robot) to control the exercise intensity (fre- quency). This adaptation for table tennis was initially reported by Zagatto and Gobatto (2002; 2007), but these researchers did not validate this test. The values of critf (39.87 ± 3.31 shots⋅min -1 ) found here were similar to the ones previously obtained by Zagatto and Gobatto (2002) (39.9 ± 1.3 shots⋅min -1 ), but in this investigation higher values of AWC (99.46 ± 29.11 balls and 50.9 ± 6.9 balls, respectively) and linear coefficient (R 2 = 0.88 ± 0.11, and R 2 = 0.77 ± 0.06, respectively) were obtained. Table ten- nis requires a larger contribution of the ATP-CP system in effort periods (Faccini et al., 1989; Zagatto et al., 2008) and the difference found in AWC in these studies could be due a better ability of the athletes in this study. Many investigations use approximately four trials in the critical power test, but other authors have used only two trials (Housh et al., 1990; Wakayoshi et al., 1993). Housh et al. (1990) investigated the number of workloads necessary to accurately determine the critical power. These authors found that critical power could be meas- ured using only two trials. However, it should be noted that a possible mistake in Tlim in one or two of the work- loads applied could have negative effects in the determi- nation of the critical power and AWC results. In the pre- sent investigation, three

accurately determine the critical power. These authors found that critical power could be meas- ured using only two trials. However, it should be noted that a possible mistake in Tlim in one or two of the work- loads applied could have negative effects in the determi- nation of the critical power and AWC results. In the pre- sent investigation, three or four trials were used for criti- cal frequency determination. Nevertheless, the number of workloads used did not influence the results. The duration of the time trial could also influence the results of the critical power model (Bishop et al., 1998; Poole, 1986). Poole (1986) reported that the ideal duration of trials that result in Tlim between 2 and 10 minutes. Workloads that generate a Tlim higher than 10 minutes can overestimate the AWC, and effort that generate short Tlim can overes- timate the critP. The Tlim used in the present study re- spected the relation described by Poole (1986) with Tlim variation between 3 and 9 minutes (188.33 ± 60.47 s to 578.57 ± 203.95 s). The blood lactate concentration analyzed during the lactate steady state test showed a dynamic equilibrium in production and disappearance of blood lactate at fre-

Zagatto et al. 465 quencies of 90 and 100 % of critf. Nevertheless, with an increase of only 6% in the frequency (106% of critf), this dynamic equilibrium was not verified. Similar results were also found by Wakayoshi et al. (1993) in swimming and Jenkins and Quigley (1990) on cycle ergometer. The frequency of 106% of critf applied in the lactate steady state test was chose, because of the difficulty in adjusting lower values of the equipment. The MLSS is usually applied in 30 minutes exercises, analyzing the lactate steady state in the last 20 minutes. But, the table tennis match and training consist in intermittent exercise, and the application of 30 minutes exercise would be very difficult for table tennis players perform. Moreover, even with the lactate steady state test lasting 20 minutes, it was very hard for the athletes to continuously perform the exercise for a long duration. Moritani et al. (1981) found a high correlation between anaerobic threshold, determined for ventilatory threshold, and critical power on the cycle ergometer (r = 0.92), also McLellan and Cheung (1992) found a correlation between the critP and the individual anaerobic threshold (IAT) (r = 0.98) on the same ergome- ter. In the present investigation the critf also significantly correlated with the AnT BI (r = 0.78), but not with OBLA 3.5. The OBLA3.5 was determined using a fixed [Lac] corresponding to 3.5 mmol.L -1 , as proposed by Heck et al. (1985) who used this concentration when the exercise stage lasted three minutes. However, this proto- col determined the anaerobic threshold for fixed [Lac] though mean values and not through individual values, and this could cause more variability in results. Heck el at. (1985) found a range of 2.40 to 4.35 mmol.L -1 for [Lac] for this duration of exercise. The lack of correlation between critf and OBLA 3.5 can be explained by a possible mistake in the utilization of a fixed lactate concentration protocol. The mean lactate concentration in AnT BI was 3.09 ± 1.65 mmol·L -1 which is lower than 3.5 mmol·L -1 used in OBLA 3.5. This