Development of a tennis-specific fatigue inducing protocol to assess potential ergogenic strategies

Resumen tennis-specific fatigue inducing protocol to assess potential ergogenic strategies

Tennis performance is multifaceted, characterised by an intricate integration of physical attributes, perceptual skill and technical proficiency. The dynamics of the sport and the various styles adopted by players (e.g., baseline or serve and volley), due to the different court surfaces, ensures that there is no predetermined match intensity or duration. Similarly, the variety of environmental conditions players experience, are obvious variables that challenge sustained performance excellence. Success at the elite level is often determined by one’s ability to resist fatigue in a survival of the fittest style showdown. While anecdotal links between fatigue and the impairment of skill proficiency are common, there is a lack of uniform empirical support. The prevailing view of those investigations that have examined the effects of fatigue on tennis skill and performance (Davey, Thorpe, & Williams, 2002, 2003; Dawson, Elliott, Pyke, & Rogers, 1985; Struder, Hollmann, Duperly, & Weber, 1995; Vergauwen, Brouns, & Hespel, 1998; Vergauwen, Spaepen, Lefevre, & Hespel, 1998) is that fatigue manifests in a number of forms, either centrally or through other homeostatic perturbations, largely hypoglycaemia, hyperthermia and dehydration. However, very few, if any of these studies, have adequately identified the specific facets of fatigue that affect tennis performance in situ.

Introduction

Tennis performance is multifaceted, characterised by an intricate integration of physical attributes, perceptual skill and technical proficiency. The dynamics of the sport and the various styles adopted by players (e.g., baseline or serve and volley), due to the different court surfaces, ensures that there is no predetermined match intensity or duration. Similarly, the variety of environmental conditions players experience, are obvious variables that challenge sustained performance excellence. Success at the elite level is often determined by one’s ability to resist fatigue in a survival of the fittest style showdown. While anecdotal links between fatigue and the impairment of skill proficiency are common, there is a lack of uniform empirical support. The prevailing view of those investigations that have examined the effects of fatigue on tennis skill and performance (Davey, Thorpe, & Williams, 2002, 2003; Dawson, Elliott, Pyke, & Rogers, 1985; Struder, Hollmann, Duperly, & Weber, 1995; Vergauwen, Brouns, & Hespel, 1998; Vergauwen, Spaepen, Lefevre, & Hespel, 1998) is that fatigue manifests in a number of forms, either centrally or through other homeostatic perturbations, largely hypoglycaemia, hyperthermia and dehydration. However, very few, if any of these studies, have adequately identified the specific facets of fatigue that affect tennis performance in situ. Caffeine supplementation by elite sports people has seemingly increased in recent years. It is very practical, available without prescription and is used by professional tennis players for its central stimulatory effects. At present many players integrate potentially ergogenic strategies in their ritualistic match preparation or as routine behaviour during breaks in play. Examples of such behaviours adopted with the intended purpose of preventing fatigue and affording a competitive edge include, consuming strong coffee pre-match, drinking Coke® or Redbull®. Unfortunately, little explorative research (specific to tennis) has been conducted to firstly identify the underlying mechanisms of fatigue and secondly to substantiate implementation of these potential strategies to minimise the impact of fatigue. A number of investigators have attempted to offset the development of fatigue and performance impairment, however methodological shortcomings (methods to induce fatigue, sensitivity of performance measures and measuring only performance outcomes) limit the strength of the findings (Burke & Ekblom, 1982; Struder, Ferrauti, Gotzmann, Weber, & Hollmann, 1999; Vergauwen, Brouns et al., 1998). Consequently, tennis players currently employ these strategies based on anecdotal experiences, evidence from other sports or advice from coaches. The purpose of this investigation was to build upon previous research (Davey et al., 2002; Ferrauti & Weber, 1998; Ferrauti, Weber, & Struder, 1997; Struder et al., 1999; Vergauwen, Brouns et al., 1998) and examine the ergogenic potential of caffeine, using a multifaceted measurement approach to tennis performance.

Methodology

Participants Twelve highly trained, male tennis players (mean + SD age 18.3 + 3.0 yr; height 178.8 + 8.5 cm; body mass 73.95 + 12.3 kg; sum of seven skinfolds 62.3 + 20.9 mm) participated in the investigation. Participants trained at least 15 – 20 hr per week and had at least 5 years of competitive tournament experience. Participants (and parents) received explicit details of the experimental protocol before voluntarily providing written informed consent. The study was reviewed and approved by the Australian Institute of Sport Ethics Committee and the University of Ballarat Ethics Committee. Testing Protocol Participants performed a prolonged simulated tennis match, 4 sets (~2 hr 40 min), on four occasions. The four trials (1 x placebo-control and 3 x interventions), were performed in a single-blinded and counterbalanced manner. Figure 1 illustrates the timeline of the experimental design. Matches were conducted indoors on hard courts (Synthetic category 2, Rebound Ace, A.V. Syntec Pty. Ltd. Queensland, Australia) at the National Tennis Centre, Melbourne Park, Australia, home of the Australian Open Grand Slam Tennis Tournament.

Figure 1: A schematic illustration of the experimental design.

Simulated Match The protocol commenced with the ball machine (SAM Millennium II, Maximum Sports, Victoria) projecting 17 tennis balls (Slazenger Hydroguard Ultra VIS, Dunlop Sport Equipment, Regents Park, Australia) in a pre-programmed random sequence over a duration of 40 s. The balls landed approximately 1.5 m to the left and right of the centre mark of the baseline, and at a depth approximating the midpoint between the baseline and service line. The participants’ role was to return the balls to designated areas at the opposite end of the court. Participants were given a different hitting sequence prior to each game (e.g., cross court only, one shot to the left two shots to the right, etc.). On completion of the rally, participants rested at the baseline for 20 s, after which the next 40 s rally immediately commenced. This process continued until ten rallies were completed (10 min). Thereafter, players served six first serves then sat and recovered courtside for 90 – 120 s. During this time participants performed a computer based return of serve test to examine perceptual skill and physiological variables were recorded (core body temperature (TC), heart rate (HR), ratings of perceived exertion (RPE) and thermal sensation (TS)). Blood lactate (BLa) and blood glucose (BGl) was measured during set breaks only (every third break in play). Once all variables were recorded, participants were informed of the next hitting sequence then walked to the baseline and commenced the subsequent 10 min groundstroke assessment. Therefore, one “game” (~ 13 min block) comprised: groundstroke performance assessment (~ 10 min: 40 s rally, 20 s rest); first serve analysis (~ 1 min); perceptual skill test (~ 90 – 120 s, also recovery).

Three completed games constituted one “set” and 4 sets constituted the simulated match. The successive sets replicated the format of the first set, with the only variation being the instructions to players regarding the direction balls were to be returned. Participants were instructed to attempt to maintain an intensity equivalent to that during match play. Performance Assessment There were three facets to performance assessment; stroke quality (serve and groundstroke velocity and accuracy), stroke kinematics (serve only), and perceptual skill (return of serve anticipation accuracy). Serve and groundstroke velocity and accuracy. Two radar guns (Stalker Professional Sports Radar, Radar Sales, Plymouth, MN) were used to measure first serve and forehand groundstroke velocity. The radar recording serve velocity was positioned on the centre of the baseline at the opposite end of the court to the server, aligned with the approximate height of ball contact (~ 2.2 m), and pointing down the centre of the court. The radar recording forehand groundstroke velocity was positioned on the forehand side of the court, behind the participant pointed at net height down the singles sideline. Accuracy scores were determined, using a 3, 2, 1, 0 scoring system, by counting the number of times the ball landed within the designated target area. A total score, expressed as a percentage of the maximum, was recorded for each game. Serve kinematics. First serve service actions were captured using a high-speed (100 frames per second) digital video camera (Phantom, USA), downloaded and converted to video files.

The files were subsequently viewed using a sport analysis tool (Swinger Plus, Webbsoft Solutions) and divided into five distinct temporal phases (phase 1: preparation to ball release, phase 2: ball release to maximum height of the ball toss, phase 3: maximum height of the ball toss to racquet-ball impact, phase 4: racquet-ball impact to follow through, phase 5: entire serve sequence – preparation to follow through). The duration of each phase was determined by counting the number of frames. Perceptual skill. Participants viewed 12 clips, displayed on a laptop computer screen, of a professional player serving. The footage was captured from the perspective of a player attempting to return the serve. The participant was instructed to assume the role of a receiving player and attempt to anticipate the direction of a serve from the footage shown. Two temporal occlusion conditions were presented to manipulate the exposure time available to the participant to predict the direction of the serve. One occlusion condition occluded the vision at the point of racquet-ball contact (T1), while the other condition presented the complete service action, including ball flight (T2). Participants used the computer mouse to click on the side of the service box they believed was the intended service direction. A response accuracy percentage was then generated from the 12 trials presented (six randomly ordered trials of each occlusion condition). The test took approximately 90 s, which is equivalent to the allocated time between games in a tournament match. Intervention During the caffeine trial participants ingested 3 mg.kg-1 of body mass N?-D?z® Awakeners (Key Pharmaceuticals Pty Ltd, Rhodes, Australia) 30 min prior to commencing the simulated match. In the placebo-control trial participants ingested a powder-placebo (Polycose®, Ross Nutrition, Abbott Laboratories, Ohio, USA).

Both supplements were ingested inside a gelatine capsule. Euhydration was maintained during trials by having participants consume (approximately 14 ml.kg-1.hr-1) a flavoured carbohydrate-placebo, Gatorade® Lemon Lime Placebo (Pepsico Australia Holdings Pty Ltd., Sydney, Australia). Materials/Physiological Measures Venous blood samples (10 ml) were extracted from a superficial vein from the Cubital Fossa of the (non-playing) forearm. Samples were later centrifuged and the supernatant was analysed for PRL concentration using an Immulite (Diagnostic Products Corporation, Los Angeles, California, USA). Blood lactate, BGl and CK were analysed from capillary samples using a Lactate Pro (Arkray Factory Inc. Shiga Japan), HemoCue (Angelholm Sweden) and Reflotron (Roche Diagnostics, Indianapolis IN USA), respectively. Core body temperature was measured via short range telemetry, from a single use capsule, ingested at least 3 hr prior to commencing each trial, to a data logger BCTM3 (Fitsense Technology, USA). A Polar S610 (Polar Electro Oy, Finland) was used to monitor HR. Statistical Analyses The experiment used a repeated measures design (with each participant undertaking one trial under each condition), with longitudinal data collected throughout each trial. The data were analysed using linear mixed modelling, with provision for fixed effects of trials and time, random effects for participants, and either constant correlation or autoregressive correlation between the random errors within each trial. Where overall significant differences were detected, subsequent Bonferroni-adjusted post-hoc analyses were conducted to determine the pattern of significance. Results are reported as mean + standard deviation, unless stated otherwise, and significance was identified where p < 0.05. Effect sizes were also calculated for all measured variables and interpreted as previously described (Batterham & Hopkins, 2006).

Results

The purpose of this investigation was to first develop an ecologically valid tennis protocol that induced fatigue equivalent to that experienced during match play and second to identify the ergogenic potential of caffeine to enhance tennis performance. It was envisaged that the performance enhancing efficacy of caffeine would be underscored by the ability to counteract fatigue that manifests during match play through a number of physiological disruptions. The results first present the physiological perturbations induced by the simulated match and an associated performance profile and second the specific effects of the experimental strategy. Protocol effects. Significant effects of time (p < 0.05) were revealed for increasing levels of TC, HR, RPE, TS, CK, PRL and decreasing BGl over the duration of the simulated match. Table 1 illustrates some selected physiological responses and comparisons between conditions. The velocity of first serves (p < 0.05) and groundstrokes (p < 0.05) and groundstroke accuracy (p < 0.05) significantly deteriorated over time. The racquet-arm acceleration phase of the serve slowed significantly (p < 0.05) over the duration of the protocol. The results reflect the efficacy of the devised protocol to induce fatigue and performance impairment in an ecologically valid on-court setting.

Table 1: Comparative physiological responses between conditions. Values presented are mean + SD. Values for core temperature, heart hate, thermal sensation and blood glucose are averages taken over the duration in which they were recorded.

Condition effects. Caffeine supplementation did not elicit any significant physiological or performance enhancement. Trends were revealed for caffeine to mitigate increasing RPE (CAF: 14 + 1 vs. PLA: 14 + 1, p > 0.05, ES = 0.16) and to enhance serve velocity (CAF: 164 + 14 kph vs. PLA 161 + 15 kph, p > 0.05, ES = 0.19) over the duration of the simulated match. These responses were particularly evident in the latter stages of the protocol (see Figure 2) and reflected by small and moderate effect magnitudes and significant interactions between condition and time (serve velocity, p < 0.05). Interaction effects also substantiated trends for caffeine to facilitate the racquet-arm acceleration phase of the serve (phases 3, p < 0.05), revealed by the temporal analysis of serve kinematics. Perceptual skill was not facilitated when averaged over the duration of the protocol, irrespective of the occlusion condition (T1 CAF: 78 + 23% vs. PLA: 78 + 23%, p > 0.05, ES = 0.02; T2 CAF: 81 + 20% vs. PLA: 82 + 19%, p > 0.05, ES = 0.05). Similarly, serve accuracy (CAF: 28 + 15% vs. PLA: 26 + 14%, p > 0.05, ES = 0.15) groundstroke accuracy (CAF: 36 + 4% vs. PLA: 36 + 5%, p > 0.05, ES = 0.03), and groundstroke velocity (CAF: 120 + 8 kph vs. PLA: 120 + 8, p > 0.05, ES = 0.00) were not enhanced through caffeine supplementation.

Figure 2: Ratings of perceived exertion and serve velocity over the duration of the simulated match. Values presented are mean + SE. Comparison of mean differences for RPE revealed *small effects at S3R2 (ES = 0.34), S3R3 (ES = 0.34), S4R3 (ES = 0.58) and #moderate effects for S4R2 (ES = 0.90). Comparison of mean differences for serve velocity revealed $small effects at S1R1 (ES = 0.20), S1R2 (ES = 0.27), S2R3 (ES = 0.25), S4R1 (ES = 0.45), S4R2 (ES = 0.39) and S4R3 (ES = 0.44). There was a significant interaction between condition and time for serve velocity (p < 0.05).

Discussion

The purpose of this investigation was to first identify the effect of fatigue, equivalent to levels experienced during match play, on the proficiency of tennis performance skills. Second, the investigation sought to identify the ergogenic potential of caffeine to counteract fatigue and enhance performance. Previous investigators demonstrated significant skill deterioration when players performed under a state of physiological duress (Davey et al., 2002; Dawson et al., 1985; Vergauwen, Spaepen et al., 1998). The application of these findings is limited by the methods selected to induce fatigue and the levels of fatigue experienced not representing match specific conditions. Further, investigators quantified only outcome performance measures. This investigation built on previous attempts and confirmed that the proficiency of outcome and process performance skills, integral to tennis success, is compromised during prolonged match-specific exercise. The prolonged simulated match elicited physiological and performance responses equivalent to those observed during tennis competition and simulated scenarios. In this instance, only moderate physiological strain was experienced, relative to the duress that has been reported under more challenging playing conditions (Bergeron, 2003; McCarthy, Thorpe, & Williams, 1998; Therminarias, Dansou, Chirpaz, Eterradossi, & Favre – Juvin, 1994). The more commonly reported homeostatic disruptions (dehydration, thermal strain, hypoglycaemia or cardiovascular stress) were not induced in this investigation. Instead, it is suggested that the proliferation of creatine kinase and prolactin underscored performance impairment. These findings implicate both muscle trauma and central fatigue as mechanisms challenging sustained performance proficiency and encourage further exploration of the area. This investigation measured process facets of performance in an attempt to address some of the limitations of previous research and substantiate the facilitative role of caffeine in tennis performance.

No significant physiological or performance effects were revealed to prescribe caffeine supplementation as an ergogenic aid for tennis players. These results are similar to Ferrauti and Weber (1998) who reported no performance benefits (in males) from a similar caffeine dosage administered during match play. Likewise, Vergauwen, Brouns et al. (1998) reported no added effects of ingesting caffeine with carbohydrates over carbohydrate supplementation alone. However, the current study did offer some encouraging trends in respect to fatigue resistance, increased serve velocity and augmented serve kinematics. These responses became more pronounced towards the latter stages of the prolonged simulated match where fatigue-associated performance deficits are most likely to occur. Previous investigations have reported fatigue resistance and enhanced cognition with caffeine supplementation (Bridge & Jones, 2006; Lorist & Snel, 1997). This investigation examined this issue through the application of a computer-based sport-specific perceptual skill test and did not reveal enhanced perceptual skill for the players. A number of issues may have contributed to this finding. The small number of test trials, specifically pre-racquet-ball contact occlusion conditions, able to be completed in the allotted time (change of ends) may have reduced the sensitivity of the test to pick up any perceptual skill changes. Alternatively, the simulated match scenarios may not have elicited equivalent central demands to that experienced in real game situations (Royal et al., 2006). Based on the findings of the current investigation and previous research (Ferrauti & Weber, 1998; Vergauwen, Brouns et al., 1998), the benefits of caffeine supplementation for tennis players remains equivocal. Although trends appeared towards the latter stages of the simulated match, caffeine did not attenuate fatigue or facilitate any quantifiable facet of tennis performance. Difficulties associated with the conduct of field-based testing such as measurement sensitivity, sample size restrictions, and the lack of attentional resource demands associated with simulated match conditions need to be overcome before a true indication of the effects of caffeine on tennis performance can be elucidated. Future researchers are encouraged to persist with the incorporation of both process and outcome measures of tennis performance and the inclusion of capacities such as perceptual skill.

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