Blood lactate concentrations have frequently been used to assess exercise intensity during tennis match play (1,2,4,15,19,20). The observation that tennis players have mean blood lactate values of 1.5-3.0 mmol·L−1 indicates that the rate of glycolytic energy turnover during a match is moderate (3). However, during long and intense rallies, the circulating blood lactate concentrations can increase up to 8 mmol·L−1, which suggests temporal increases in glycolytic processes to fuel on-court tennis movements (15). Because of the unpredictable nature of tennis competition, periods of high-intensity activity may be required on several occasions throughout the match (15,20). It is likely that during these periods of high-intensity activity, important points can be won or lost. The ability to perform high-intensity intermittent exercise during a tennis match will depend on the player's ability to resist the onset of fatigue (7). It is, therefore, important to implement training strategies to offset the development of fatigue and performance impairments associated with tennis match play. This suggests a need for accurate methods to monitor training intensity during tennis practice and on-court training.
Monitoring blood lactate during daily exercise training routines and competitions might be problematic due to the impracticability associated with collection of multiple blood samples. Alternatively, previous research has demonstrated an association between ratings of perceived exertion (RPE) and blood lactate concentrations during different exercise modes (12,21). However, the relationship between RPE and blood lactate concentration during intermittent exercise has been reported to be disrupted (8,18). Thus, although RPE might be a reasonable marker of blood lactate concentration for many activities, this relationship might not be causal. Considering the convenience of using RPE to quantify physiological responses during sport games (5,13), the purpose of the present study was (a) to investigate the relationship between blood lactate concentration and RPE responses and (b) to examine the associations of these metabolic and perceptual responses with variables describing the characteristics of singles tennis matches.
Experimental Approach to the Problem
In the present study, a nonexperimental, descriptive, correlational design was used to examine the relationship between the RPE response and blood lactate concentration and the associations of these responses with the players' activity patterns. Professional tennis players were assessed for blood lactate and perceived exertion responses at selected changeovers during competitive singles matches. Each player was individually filmed for the entire duration of the match and several variables describing the characteristics of the matches were determined.
Eight well-trained, professional, male tennis players (age, 27.0 ± 4.4 years; height, 182.9 ± 5.2 cm; body mass, 80.6 ± 7.5 kg) participated in this study. Seven of them had an Association of Tennis Professionals (ATP) (men's professional tennis association) singles ranking (3 players in the top 100 and 4 players between positions 200 and 500 of the ATP rankings) at the time of the study. Among the subjects, there were 4 ATP tournament winners and 1 Davis Cup winner. All the subjects had reached at least 1 final in a national or international tournament within the last 2 years (e.g., ATP, Future, or Challenger tournaments). The players involved in the present study had a tennis training background (i.e., training experience) of 16 ± 2.8 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 resistance training. All subjects were fully informed of the experimental procedures prior to providing written informed consent to participate in the study, which was approved by the Institutional Research Ethics Committee. The study was also previously approved by the contest organizing committee.
All measurements were taken during a 3-day invitational professional singles tennis tournament held at a local tennis club. Players were invited by the contest organizing committee. The acceptance/rejection to participate in the tournament was based on player-dictated schedule conflicts and financial claims. Every player received a fixed payment, after accepting to participate, plus a variable amount of prize money based on their final position at the end of the tournament.
The tournament was a knock-out format. That is, the 8 players contested in 4 quarterfinals, with the winners progressing to the semi-finals, and then the final. Therefore, the total number of matches was 7 (4 quarterfinals, 2 semifinals, and the final). All the matches (best of 3 sets) were conducted on an outdoor clay court surface (i.e., category 1 court surface, natural clay court) (10). The mean (±SD) climatic conditions during the matches were air temperature 20 ± 1°C, humidity 76 ± 6%, and wind speed 1.4 ± 0.3 m·s−1. International Tennis Federation rules were used to govern the scoring and time characteristics for the matches. A set of 4 new balls (Dunlop Fort, Carlsbad, CA) was used in the first 7 games and then every ninth game for the rest of the match. Fluids (water and sports drinks) were available ad libitum throughout the matches.
Blood Lactate Concentration
Blood lactate concentration was determined from 25 μL of capillarized blood samples drawn from the ear lobe and collected into heparinized tubes. All the blood samples were taken while the players were seated during selected changeover breaks in play, which occurred at the end of the first, third, and every subsequent odd game of each set, and were latter analyzed in duplicate by means of an electroenzymatic method (Analox Micro Stat GM7; Analox Instruments Ltd., London, UK). The number of blood samples was, therefore, variable, depending on the duration of the match (i.e., 2 or 3 sets). In same cases, players prevented us from blood sampling. In most cases, players were “upset” by the current score (normally they had lost an important game), and they preferred to sit and concentrate on the next game, without any external disruption.
Ratings of Perceived Exertion
Ratings of perceived exertion were obtained using the Borg category (6-20) RPE scale (17). The scale was explained before the exercise. The subjects were asked, “how hard do you feel the exercise was?” during selected changeovers while they were sitting. Subjects had to give ratings corresponding to their sensations during the last game.
The game analysis of tennis singles was determined by filming each match. Video recordings were collected using 2 cameras (Sony Handycam, DCR-HC24E, Japan) positioned 2 m away from the side of the court, at the level of the service line, and approximately 2 m above the court. Each player was individually tracked for the entire duration of the match. The videotapes were later replayed on a monitor for computerized recording of their activity patterns. The analyses of all 7 matches were performed by the same experienced researcher.
A modified match protocol developed by Smekal et al. (19) was used to monitor and record the duration of each game and each rally, the duration of the rest intervals between games, the number of shots per rally, and the total duration of the matches. From these data, the following variables were calculated for the 264 games analyzed: (a) the duration of rallies (DR in seconds), (b) the rest times between rallies (in seconds), (c) effective playing time (expressed in percent of the total time of play in a game), and (d) strokes per rally (SR). Duration of rallies was recorded from the time the service player hit the ball in the first serve to the moment when one of the players won the point. Changes between changeovers were excluded from the resting time (15). Effective playing time was determined by dividing the entire playing time of a game (from the beginning of the first rally until the end of the last rally) by the real playing time (sum of the single DR) performed in specific game. SR was quantified as the number of balls hit by the players from the first service to the end of the point.
Means and SDs were calculated for each of the variables analyzed. The Kolmogorov-Smirnov test (with Lilliefors' correction) was used to test data for normality. Independent sample t tests were used to calculate differences between serve-and-return games. The relationship between variables describing the characteristics of the match (i.e., SR and DR) and the physiological-perceptual responses (i.e., blood lactate and RPE) obtained at the end of each game examined was determined using Pearson's product moment correlation analysis. Ninety-five percent confidence limits were also calculated. The level of significance was set at p ≤ 0.05.
The variables describing the characteristics of the matches are shown in Table 1. Mean RPE was significantly higher (p < 0.05) following service games (13.5 ± 1.9; n = 24) than following receiving games (12.2 ± 2.0; n = 22). Mean blood lactate concentrations were significantly higher (p < 0.05) following service games (4.4 ± 2.4 mmol·L−1; n = 24) than following receiving games (3.0 ± 1.3 mmol·L−1; n = 22). Figure 1 provides individual RPE and blood lactate values during the matches. Significant (p < 0.01) correlations were found for RPE and blood lactate responses during the games. These data are presented in Table 2.
Ratings of perceived exertion values were significantly correlated with the variables describing the characteristics of the match (i.e., SR and DR) (r = 0.80-0.47; p < 0.05) in both service and return games (Figure 2). Blood lactate values were significantly correlated with SR and DR (r = 0.80; p <0.05) only when players were serving (Figure 3). Conversely, in receiving games, the relationship between blood lactate concentrations and both SR and DR was found to be low and nonsignificant (r = 0.26-0.28; p > 0.1) (Figure 3).
The intent of the present study was to investigate whether the processes that mediate the RPE response during tennis match play are associated with the duration (i.e., DR) and/or number (i.e., SR) of efforts, and/or the blood lactate concentration. The results indicate that there were increases in blood lactate concentrations and RPE in response to increases in DR or SR, with better correlations in service than in receiving games. Although correlation does not indicate causality, these data appear to provide support for a functional link among the 3 main effort continua (physiological, perceptual, and physical demands) during tennis match play.
The results of this study demonstrated a positive correlation between blood lactate concentration and RPE. Previous investigations have reported similar findings during resistance exercise (22), graded exercise testing (10), and steady state exercise (21), suggesting that blood lactate concentration is correlated with RPE during physical exercise. To our knowledge, this is the first study reporting such a relationship during tennis match play, where activity patterns are characterized by a random fluctuation of periods of maximal or near maximal work and longer periods of moderate- and low-intensity activity. However, although significant, the correlations obtained between RPE and blood lactate concentration in the present study were moderate (Table 1), suggesting that blood lactate concentration might only play an auxiliary role in influencing RPE during tennis match play.
Similar correlations and lactate have been reported in previous studies using intermittent exercise models (8,18). For example, Green et al. (8) reported significant moderate to weak correlations (r = 0.43-0.22) between RPE and blood lactate concentrations during interval cycling consisting of five 2-minute intervals separated by 3 minutes of recovery. This limited influence of blood lactate concentration on RPE is also consistent with the notion that not one single physiological variable is responsible for RPE (9,17). Rather, perceptual estimations reflect a complex array of physiological and psychological internal and external stimuli (17). It is reasonable to assume that competitive tennis match play is associated with a higher number of psychological stressors (e.g., current score, public, etc) than experiments conducted in controlled laboratory conditions (11). Thus, it is plausible that, in addition to the physiological load, RPE obtained during tennis play would concomitantly reflect those unmeasured psychological stressors (11). Moreover, Girard et al. (6) reported a dissociation between cardiovascular stress (i.e., HR) and RPE during a 3-hour tennis match, suggesting that other factors, such as mechanical factors related to the eccentric loading associated with tennis match play, might help to determine the level of perceive exertion in this type of exercise. Thus, although RPE can be used as an estimate of exercise intensity during on-court tennis practice, more work is needed to fully understand its utility during this type of exercise as well as the correspondence between perceptual and physiological variables.
Both RPE and blood lactate concentration increased as the duration of the rallies increased and the number of strokes was higher. These results are consistent with previous investigations that reported significantly higher physiological responses (higher oxygen uptake, heart rate, and blood lactate concentrations) in matches with longer rallies (15,16,20). We also found higher RPE scores and blood lactate concentrations in service than in return games. This higher physiological-perceptual load recorded in service players can be attributed to the more demanding role of servers in dictating the game (14,15,16). In this regard, servers are typically subjected to a higher physical strain as reflected by the high number of short rallies (∼50% of rallies recorded in the present study had 1-2 strokes), with these rallies consisting of either aces or service winners (return faults) (15). Consistent with these results, Johnson and McHugh (14) found that the serve is the predominant stroke in professional tennis, accounting for 45-60% of strokes during service games. Thus, it is clear that the intensity and duration of the server's activity is typically higher than that of the return player. One practical application that can be derived from the observed increases in markers of exercise intensity (i.e., blood lactate and RPE) in rallies with a long duration and a high number of strokes when players are serving is that all these situations can be conveniently manipulated to provide an adequate physiological stimulus during on-court tennis practice. Thus, prescribing on-court tennis-specific endurance drills having players serving and/or playing long-duration rallies would result in more physiological stress than having players receiving and/or playing shorter-duration rallies.
In conclusion, the present study investigated the relationship between metabolic (i.e., blood lactate concentration) and perceptual (i.e., RPE) responses and their association with variables describing the characteristics of the matches. Within the limitations of this study (e.g., small number of players, no other markers of physiological load such as heart rate and oxygen uptake), it can be concluded that blood lactate concentration, a physiological marker of exercise intensity, exhibits analogous responses with perceptual responses as tennis match play intensity increases. Further research is needed to investigate the validity of RPE responses to quantify the physiological load during tennis match play and training.
Tennis coaches often prescribe on-court game-specific exercise drills to concurrently develop technical, tactical, and physiological factors. As on-court drills appear to be relatively difficult to quantify, a practical method of monitoring global training intensity is needed. The results of this study have shown that both blood lactate concentration and Borg 20-point RPE scale appear to be valid measures of exercise intensity during tennis practice. Based on the results of this study, long-duration rallies with players serving could be used to impose an increased physiological load under game-specific conditions in tennis players. RPE might be used as an adjunct to monitor exercise intensities during those training drills.
The authors would like to thank all the players involved in the tournament, especially Esteban Carril, for giving their time and effort to participate in this study, as well as the Dionisio Nespral Tournament organizing committee.
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