Resumen specific incremental test
The intermittent exercise profile in tennis consists of near maximal intensity bouts (5-10 seconds) followed by longer resting periods (10-20 seconds) during 1 to 5 hours (10,16). Technical and tactical skills, psychological preparation, motor skills such as power, strength, agility, speed, and explosiveness but also a highly developed neuromuscular coordinating ability are strongly correlated with tennis tournament performance (21). In addition, a major determinant is the player’s aerobic fitness, which not solely enables to repeatedly generate explosive strokes and rapid on-court movements but also assures a fast recovery and contributes to maintain the concentration and preparation of the next rally during extended matches (12,15). To date, a variety of test procedures are used to evaluate performance ability in tennis players (6,7,9,24,25,27,28). The standard test for assessing aerobic fitness is the direct measurement of the player’s maximal oxygen uptake (V.O2max) whilst running to exhaustion on a treadmill in a laboratory environment. Studies using breath-by-breath gas exchange measurements have identified two specific ventilatory changes corresponding to the ventilatory threshold (VT) and the respiratory compensation point (RCP) (22). These reproducible ventilatory breakpoints are useful markers to characterize training effects, evaluate physical fitness and identify training intensity zones that are distinguished by meaningful differences in sympathetic stress load, motor unit involvement, and duration to fatigue (11). However, during treadmill testing, the mode of exercise (continuous activity) cannot simulate the specific demands of tennis (intermittent activity) and do not reflect the specific muscular involvement of both lower and upper limbs with respect to the stop, start and change of direction movement patterns (9).
The intermittent exercise profile in tennis consists of near maximal intensity bouts (5-10 seconds) followed by longer resting periods (10-20 seconds) during 1 to 5 hours (10,16). Technical and tactical skills, psychological preparation, motor skills such as power, strength, agility, speed, and explosiveness but also a highly developed neuromuscular coordinating ability are strongly correlated with tennis tournament performance (21). In addition, a major determinant is the player’s aerobic fitness, which not solely enables to repeatedly generate explosive strokes and rapid on-court movements but also assures a fast recovery and contributes to maintain the concentration and preparation of the next rally during extended matches (12,15). To date, a variety of test procedures are used to evaluate performance ability in tennis players (6,7,9,24,25,27,28). The standard test for assessing aerobic fitness is the direct measurement of the player’s maximal oxygen uptake (V.O2max) whilst running to exhaustion on a treadmill in a laboratory environment. Studies using breath-by-breath gas exchange measurements have identified two specific ventilatory changes corresponding to the ventilatory threshold (VT) and the respiratory compensation point (RCP) (22). These reproducible ventilatory breakpoints are useful markers to characterize training effects, evaluate physical fitness and identify training intensity zones that are distinguished by meaningful differences in sympathetic stress load, motor unit involvement, and duration to fatigue (11). However, during treadmill testing, the mode of exercise (continuous activity) cannot simulate the specific demands of tennis (intermittent activity) and do not reflect the specific muscular involvement of both lower and upper limbs with respect to the stop, start and change of direction movement patterns (9). Recent tennis field tests have been proposed to determine with an acceptable accuracy the exercise capacity or technical performance of athletes under standardized conditions (6,7,24,25,27,28). However, because these tests require either expensive equipments – that is ball machine, video, radar – (7,25,27,28), only simulate rallies from the baseline (25) or do not reflect precisely time intervals of tennis play (7), they cannot be routinely used to evaluate an individual player’ fitness level in a context close to the game. In addition, although specific fitness tests for badminton (5,29) and squash (13,26) have been validated to assess the metabolic and physiological demands of these sports, there is a scarcity of tennis field procedures to determine the exercise capacity of athletes and the appropriate on-court training intensity. Therefore the aims of this study were to: (a) develop a tennis specific incremental fitness test including some elements of tennis play; (b) compare physiological responses recorded during this field test to those observed during an incremental treadmill test. We hypothesized that the physiological responses would differ because of the differences in movement patterns between the tennis specific (combined use of arms and legs) and treadmill (forward running only) tests.
2.1 Subjects Nine male junior competitive tennis players (mean (SD) age 16.0 (1.6) yrs; height 179.8 (9.4) cm; body mass 65.3 (11.9) kg; training frequency 8.2 (3.1) h/wk) of regional to national level (international tennis ranking ranging from 2 to 4) volunteered to participate in the study. All players were from the Regional under-18 squad. Both the players and their parents provided written informed consent for the study, which was approved by the Ethics Committee of the University of Montpellier, France. 2.2 Study protocol All subjects carried out two incremental protocols to exhaustion in randomized order: a treadmill test (non-specific) and a tennis fitness test (sport-specific). Each test was conducted under standard environmental conditions (temperature ~20 °C, relative humidity ~50%) at the same time of day. 2.3 Experimental procedures 2.3.1 Treadmill testing The treadmill incremental test to exhaustion (TT) was performed on a motorized treadmill (S 2500, Medical development, France) and consisted in an initial 3 minutes continuous workload of 9 km/h followed by increases of 0.5 km/h every minute (0% incline). Each stage was composed of a 45 seconds running period followed by 15 seconds of active recovery during which subjects had to walk at 5 km/h. The test ended with voluntary exhaustion of the subjects.
Figure 1. Set-up of the specific incremental fitness test for tennis players.
The position of forward (black cones), lateral (grey cones) and backward (white cones) targets are indicated. See Methods for further details. 2.3.2 Field testing A tennis specific incremental fitness test (FT) was developed in which subjects repeated displacements replicating the tennis game, at an increasing speed on the court. Each stage consisted of seven shuttle runs, performed from a central basis to one of the six targets located around the court, alternated with 15 seconds of active recovery (Fig. 1). The sets of seven displacements included two forward (offensive), three lateral (neural) and two backward (defensive) courses performed randomly. When the subject arrived at the target, he was instructed to mime a powerful stroke as in official competition before moving back to baseline after each drive. Subjects were asked to use the same running technique as in competition. The duration of the first sequence was 40.5 seconds and then progressively decreased by 0.8 second at each stage. Movement velocities and directions were controlled by visual and sound feed back from a PC. Briefly, specialized software was used in order to simultaneously activate a tune and project a picture of a player moving toward the target. These velocities and sequences of movement were calculated from data collected during official competitions (unpublished data). The test ended when the player failed to reach the target in time (i.e. a 1 m delay was permitted) or was no longer able to fulfill the criteria of the test (i.e. perform strokes with acceptable technique). 2.4 Physiological measurements During the TT (CPX/D; MedGraphics, Saint Paul, Minnesota, USA) and FT (K4b²; Cosmed, Rome, Italy), the following gas exchanges data were measured using breath-by-breath gas analyzers which were calibrated prior each test using the manufacturers’ recommendations: V.O2, carbon dioxide production (V.CO2), respiratory exchange ratio (RER = V.CO2/V.O2), minute ventilation (V.E), breathing frequency, and tidal volume. 5 seconds heart rate (HR) values were recorded by HR monitor with the athletes wearing a chest belt (S810, Polar, Kempele, Finland). The difference between the two analysers has been shown to be non-significant (17) and in our laboratory the differences in V.O2 values between the analysers were less than 2% (20). The Cosmed K4 system weights only 0.7 kg and was carried on the trunk (the main sample unit on the back and a battery pack on the chest). Rating of perceived exertion (RPE) responses were recorded using the Borg 6–20 scale and 25-?L capillary blood samples were taken from the fingertip and analyzed for blood lactate concentrations ([La]) by using the Lactate Pro (LT-1710, Arkray, Japan) portable analyzer at the point of volitional fatigue. In both tests, the gas samples were averaged every 15 seconds, and the highest values for V.O2 and HR over 15 seconds were regarded as V.O2max and heart rate (HRmax). Four criteria were used to determine maximal efforts (18): 1) A plateau or leveling off in V.O2, defined as an increase of less than 1.5 ml/min/kg despite progressive increases in exercise intensity. 2) A final RER of 1.1 or above. 3) A final HR above 95% of the age related maximum. 4) A final [La] above 8 mmol/l. Time to exhaustion (Te, seconds) was recorded in each test. 2.5 Determination of VT and RCP VT was determined using the criteria of an increase in V.E/V.O2 with no increase in V.E/V.CO2 and the departure from linearity of V.E, whereas RCP corresponded to an increase in both V.E/V.O2 and V.E/V.CO2 (8). All assessments of the VT and RCP were made by visual inspection of graphs of time plotted against each relevant respiratory variable measured during testing. The visual inspections were made by two experienced exercise physiologists; the results were compared and then averaged. The difference in the individual determinations of VT and RCP was <3%. Each physiological variable corresponding to VT, RCP, and maximal load was expressed in absolute terms and relative to V.O2max and HRmax. 2.6 Statistical analysis Edita: Alto Rendimiento Mean (SD) was calculated for all variables. Four subjects performed two FTs within four days to assess its reliability. This was done by calculating the relative difference and the coefficient of variation between test and re-test. Data obtained at VT, RCP and maximal load were compared between FT and TT, using paired sample t-tests. V.O2 curves were compared using a two-factorial analysis of variance (factor 1: FT v TT; factor 2: measurement time). The Bonferroni test was used for post-hoc comparisons. Statistical significance was accepted at p<0.05. The statistical analyses were performed using SigmaStat 2.03 software (Jandel Corporation, San Rafael, CA, USA).
3.1 Reproducibility No difference was found in Te (1479 (68) v 1454 (103) seconds; CV = 1.2%) V.O2 (57.4 (6.4) v 58.2 (6.5) ml/min/kg; CV = 1.0%), HR (194.3 (6.7) v 187.3 (1.2) beats/min; CV = 2.6%), [La] (8.0 (2.8) v 7.4 (2.1) mmol/l; CV = 5.2%), RPE (17.3 (1.2) v 16.7 (1.5) points; CV = 2.8%), VT (1479 (68) v 1454 (103) % of V.O2max; CV = 2.7%) and RCP (1479 (68) v 1454 (103) % of HRmax; CV = 3.0%) between the two FTs performed within four days (n = 4). 3.2 Te, [La] and RPE Te (1666 (188) v 1491 (64) seconds; 10.5%) was higher (p<0.05) in TT than in FT. Mean values of [La] (2.2 (0.5) v 2.2 (0.6) and 10.6 (4.3) v 10.7 (3.0) mmol/l) and RPE (9.0 (2.1) v 8.6 (2.1) and 17.7 (1.0) v 18.5 (0.9)) measured before and after exercise did not differ between TT and FT, respectively.
Figure 2. Oxygen uptake in the time course of tennis field (FT) and treadmill (TT) tests in tennis players (n = 9).
3.3 Physiological variables at VT, RCP and maximal load At VT and RCP, V.CO2 and RER values were significantly higher in FT than in TT (table 1). It is of interest to note that %HRmax and %V.O2max at VT and RCP were not different between FT and TT. Again, V.O2, V.CO2 and RER values measured at maximal loads were significantly higher in FT than in TT. As presented in Figure 2, V.O2 (p<0.001) displayed a significant interaction effect between measurement time and testing condition.
Table 1. Physiological values in tennis players corresponding to the ventilatory threshold (VT), respiratory compensation point (RCP) and maximum work load (Max) in tennis field (FT) and treadmill (TT) tests (n = 9).
Determination of aerobic fitness is assumed to be dependent upon the mode of testing in continuous activities, which means that runners are generally tested on a treadmill, rowers on a rowing ergometer and cyclists on a cycle ergometer (1,11). The physiological demands in racquet games such as tennis are highly influenced by the fact that players have to accelerate, decelerate, change direction, move quickly, maintain balance and generate optimum stroke production repeatedly (16). Laboratory testing on treadmill cannot simulate the specific muscular involvement of both lower and upper limbs with respect to the change of pace and direction movement patterns of the tennis game and therefore is inadequate to evaluate the specific demands of this activity (9,24,25). As a consequence, we designed a tennis specific incremental fitness test including some technical characteristics (performed on a tennis court; similar displacement technique to competition; uncertain direction of motion; simulation of ball hitting). Of interest is that the FT had a high reproducibility which suggests that this test is sensitive and valid to provide information on a player’s fitness level and training intensity zones. Since during the FT, subjects wore the Cosmed K4 system that weights 0.7 kg, one may assume that their V.O2max expressed to body mass was slightly underestimated. In the present study, the difference between wearing or not the portable device was of 1.1 ± 0.2% and therefore did not modify the main findings of this study. This is confirmed by previous findings (23) that reported that a 0.1 kg additional charge carried on the trunk (near the center of gravity) induced only a 0.1% increase in V.O2. 4.1 Submaximal intensities Although several questions regarding the cause-effect relationship among ventilatory, lactate, EMG and sympathetic hormone changes during incremental testing (11) are still unanswered; by comparing visual and computerized methods, Santos and Gianella-Neto (22) have recently confirmed that VT and RCP are valid and reliable markers for establishing exercise intensity zones. Surprisingly, only limited data are available regarding VT and RCP values in tennis players since V.O2max has traditionally been considered as the ‘gold standard’. However, there is increasing evidence that the ventilatory breakpoints may be a better predictor for submaximal endurance performance (8). This is especially true in tennis where the performance is multifaceted, involving technical, tactical, psychological and physiological factors (16). The intensity at VT and RCP found in the present study for junior competitive tennis players is higher to that generally reported for physically active subjects (80 v 90% of HRmax and 50 v 80% of V.O2max at VT and RCP, respectively)(11). By comparing metabolic profiles of young tennis players and untrained boys, Mero et al. (19) have reported that tennis players had significantly lower V.O2 at the RCP (38 v 47 ml/min/kg) than the controls but with the same corresponding treadmill speed and the same V.O2max. According to König et al. (15), the high VT and RCP values could reflect the ability to tolerate high exercise intensity during tennis competitions. These values of ventilatory breakpoints are, however, lower than the values (88 v 95% and 85 v 91% of the HRmax and V.O2max for VT and RCP, respectively) reported recently in elite squash players tested similarly than the present test but with time intervals specific to the squash game (shorter resting periods between stages: 10 seconds; longer stage durations: 9 simulations of ball hitting) (13). The discrepancies between these studies are mainly the result of the training status of the subjects. Nevertheless, comparing results between studies appear awkward since subject’s characteristics, equipment, protocols and modes of tests as well as methods used to detect ventilatory breakpoints are often different. An interesting finding of the present study is that the load increments during TT and FT were similar as evidenced by the progressive increases in V.O2 (Fig 2) and by the fact that rest intervals (15 seconds) were identical in the two tests. These data are not in good agreement with previous findings (24,25) reporting lower submaximal HR and V.O2 values during laboratory testing than under sport-specific conditions. It has been shown that the length of the stage per se affects the peak metabolic responses during an incremental test (2). However, even stages of very different duration (3 v 8 min) did not induce different submaximal values (for example, at onset of blood lactate accumulation) (3). In the present study, the difference in stage duration between FT and TT was very small. One may therefore assume that the two different protocols did not result in different submaximal values. The lack of difference in physiological variables (%HRmax and %V.O2max) at VT and RCP between the two tests lead us to suggest that treadmill testing remains the ‘gold method’ to detect ventilatory breakpoints in order to define intensity areas for tennis on-court aerobic exercises. Interestingly, the VT intensity found in our players is similar to the relative HR and V.O2 reported for tennis competitions (70-90% of HRmax and 60-75% of V.O2max) (10,14-16). This is, however, not consistent with the findings of Mero et al. (19) suggesting that tennis is played on average at an intensity slightly below RCP. 4.2 Maximal loads The mean end-exercise V.O2, HR and [La] values showed that during the last stages of both tests players experienced an elevated cardiovascular stress and that the anaerobic energy system was highly taxed for energy furniture. It is interesting to note that HR responses during the last stages of TT and FT were similar to the levels observed during the intense parts of a tennis match play (190-200 beats/min) (15). Also, at maximal loads, [La] and RPE were similar in both tests, which differ from previous findings reporting higher [La] values following treadmill than field testing (24,25). A possible explanation could be the intermittent design of the present treadmill test contrasting with previous protocols with a continuous load profile. Indeed, it is well established that lactate can be oxidized locally or transported from production sites to oxidative muscle fibers for subsequent oxidation during recovery periods (4). V.O2max values measured in the FT or the TT are in similar range or slightly higher than those reported previously (50-60 ml/min/kg) in players of similar standard (10). This confirms that a high aerobic power is a prerequisite in tennis to successfully sustain an elevated level of technical, tactical, physiological and psychological capacity during several hours. Of interest is also that the V.O2, V.CO2 and RER values were significantly higher in the FT than in the TT at maximal loads, suggesting that V.O2max values derived from laboratory testing were not relevant for an accurate estimate of fitness in tennis players. Although the design of the two tests was intermittent in nature, it is noteworthy that during the FT, players were asked to perform repeated specific displacements in all directions with changing pace. These specific patterns included accelerations, decelerations, changeovers as well as upper arm involvement with racquet holding and stroke miming actions. On contrary, running on a treadmill was only characterized by a steady pace and little or no lateral movement. As suggested by Smekal et al. (25), one may therefore assume that greater muscle mass was involved during the FT and that muscles were recruited at a higher rate than during the TT which may have in turn increased V.O2 in the FT.
In conclusion, although physiological variables were not different at submaximal intensities between the two tests, suggesting that treadmill testing gives valid information to detect ventilatory breakpoints in order to establish tennis on-court aerobic exercises, V.O2max values derived from laboratory measurements were significantly lower than those measured under sport-specific conditions. V.O2max values derived from treadmill testing were therefore not relevant to accurately estimate fitness in junior competitive tennis players. Furthermore, the present study showed that the FT had a high reproducibility. Thus, using field testing in addition to treadmill testing provides a more valid assessment of a player’s individual fitness level and may be routinely used to accurately prescribe aerobic exercise training in a context appropriate to the game.
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