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APPLIED SCIENCES: Physical Fitness and Performance

Effects of Caffeine on Prolonged Intermittent-Sprint Ability in Team-Sport Athletes

SCHNEIKER, KNUT THOMAS1; BISHOP, DAVID1; DAWSON, BRIAN1; HACKETT, LAURENCE PETER2

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Medicine & Science in Sports & Exercise: March 2006 - Volume 38 - Issue 3 - p 578-585
doi: 10.1249/01.mss.0000188449.18968.62
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Abstract

Several studies have reported improved performance following caffeine ingestion during varying intensities and modalities of exercise (2,7,10,11).The erogenic effects of caffeine have been attributed to a number of possible mechanisms, including adenosine receptor antagonism (9), central nervous system (CNS) facilitation (30), increased Na+/K+ ATPase activity (17), mobilization of intracellular calcium (24), increased plasma catecholamine concentration (20), and via carbohydrate sparing (6). It now seems likely that adenosine receptor antagonism is the primary mechanism of action (10). There is, however, some evidence to support each of these mechanisms and it is probable that all contribute to the wide range of physiological responses to caffeine that make it ergogenic. A more detailed discussion on the possible mechanisms underlying the ergogenic effects of caffeine can be found elsewhere (9,10).

Although caffeine appears to be ergogenic for the performance of prolonged, endurance exercise and single bouts of high-intensity exercise, little evidence supports an ergogenic effect of caffeine on intermittent-sprint performance. Those studies that have examined the effects of caffeine on intermittent-sprint performance typically have dealt with intermittent exercise bouts at least 30 s in duration (5,13,27). However, based on time and motion analyses of various team sports including Australian rules football (8), soccer (19), and field-hockey (25), it is apparent that the performance of intermittent 30-s sprints relates poorly to the performance demands typically placed on athletes during team sports. In contrast, these studies have determined that the ability to repeatedly perform maximal or near-maximal efforts of short duration (1-6 s), with sometimes brief recovery periods, over an extended period of time, is more representative of typical team-sport activity.

To the authors' knowledge, only a single study has examined the effect of caffeine ingestion on performance of repeated sprints of a duration similar to those performed by team-sport athletes (22). The test consisted of 10 × 20-m sprints (mean time ≈ 3.6 s per sprint), each performed within 10 s, followed by a rest for the remainder of each 10-s period. It was found that the effect of caffeine ingestion on mean sprint performance and fatigue over 10 sprints was negligible and the authors suggested that, pending confirmatory research, team-sport athletes should not expect caffeine to enhance repeated-sprint performance. Whereas this study appears to be a more valid test of the effect of caffeine ingestion on intermittent-sprint performance than earlier studies, the exercise protocol does not truly reflect the demands placed on team-sport athletes over the course of an entire game. A single bout of 10 repeated sprints would not simulate the metabolic requirements of a full game requiring intermittent high-intensity efforts as well as intense bouts of repeated-sprint activity over 60 min or more. Furthermore, the recovery time that was allowed between sprints is less than what is typically experienced during team sports (8,19,25). The purpose of this study, therefore, was to determine the effects of acute ingestion of a moderate dose of caffeine (6 mg·kg−1) on a prolonged-duration, intermittent-sprint test, specifically designed to simulate the physiological demands of athletes during team sports.

METHODS

Participants.

Ten male, moderately trained, team-sport athletes (age 20 ± 3 yr, height 179 ± 5 cm, mass 77.7 ± 13.9 kg, peak oxygen uptake (V̇O2peak) 56.5 ± 8.0 mL·kg−1·min−1 (mean ± SD)) were recruited for this study from competitive Australian rules football, soccer, and hockey teams. All participants were informed of the study requirements, benefits, and risks before giving written informed consent. Approval for the study was granted by the institutional research ethics committee.

Experimental design.

On day 1, participants performed a familiarization session for both the intermittent-sprint exercise and the graded exercise tests. The graded exercise test was performed 48 h later to determine V̇O2peak. At least 48 h following the graded exercise test, participants performed the first of two intermittent-sprint tests, after ingestion of either caffeine or placebo. One week later, the second intermittent-sprint test was performed, in the same manner and at the same time of day as for the first. The order was randomized and administered double-blind.

On testing days, participants were asked to report to the laboratory 2.5 h before testing and, on arrival, were given 2 g·kg−1 of 100% glucose powder (Glucodin, Boots Healthcare, NSW, Australia) to consume with 1 L of water within 30 min. This was done to standardize food intake before testing and to ensure adequate preexercise carbohydrate and water consumption. One hour before testing, participants ingested opaque gelatin capsules containing either caffeine (6 mg·kg−1) or dextrose, along with 200 mL of water. Participants then rested for 50 min before commencing the pretest warm-up. To avoid the effects of dehydration and depletion of glycogen stores, participants were alternately given 100 mL of a commercial sports drink (Gatorade, 6% CHO) or water, following every third sprint of the intermittent-sprint test (approximately every 6 min; total of 600 mL of sports drink and 600 mL of water).

To standardize diet and exercise, participants were asked to prepare for each testing session as they would for an important match and in an identical manner. To assist in this, participants were asked to keep a food and activity diary for the 48-h period before each test and were asked to avoid alcohol and vigorous exercise during this time. Participants were given a list of caffeine-containing foods and beverages and were asked to abstain from consuming these products from 48 h preceding the initial testing session, until completion of the experimental trials. On testing days, participants consumed no food or beverages before the test (other than what was provided by the experimenters) following arrival at the laboratory.

Graded exercise test.

The graded exercise test was performed on an air-braked, track-cycle ergometer (Evolution Pty. Ltd., Adelaide, Australia) and consisted of graded exercise steps (3-min stages), using an intermittent protocol (1-min break between stages). The test commenced at between 70 and 100 W (depending on the participant's performance during familiarization) and thereafter, intensity was increased by 30 W every 4 min until volitional exhaustion. Participants were required to maintain the set power output, which was displayed on a computer screen in front of them. The test was stopped when the participant could no longer maintain the required power output for two consecutive 15-s epochs or at volitional exhaustion. Strong verbal encouragement was provided to all participants as they approached the end of the test.

Gas analysis (graded exercise test).

During the graded exercise test, expired air was continuously monitored for the analysis of O2 and CO2 concentrations (Ametek gas analyzers SOV S-3A11 and COV CD-3A, respectively, Pittsburgh, PA). Ventilation was recorded every 15 s using a turbine ventilometer (Morgan, Model 096, Kent, England). The gas analyzers were calibrated immediately before and verified after each test using three (alpha-verified) beta-grade gas mixtures [(O2: 18.7 ± 0.11%, CO2: 5.61 ± 0.2%), (O2: 14.5 ± 0.2%, CO2: 2.52 ± 0.05%), (O2: 16.2 ± 0.2%, CO2: 4.14 ± 0.08%), (BOC Gases, Chatswood, Australia)]. The ventilometer was calibrated preexercise and verified after exercise using a 1-L syringe in accordance with the manufacturer's instructions. The ventilometer and gas analyzers were connected to an IBM PC that measured and displayed data every 15 s. The sum of the four highest consecutive 15-s values was recorded as the participant's V̇O2peak.

Intermittent-sprint test.

The intermittent-sprint test was performed on a front-access, air-braked cycle ergometer (Model Ex-10, Repco, Australia). Participants performed a pretest warm-up consisting of 4 min at 50% V̇O2peak followed by two 1-min blocks of 30 s at 70% V̇O2peak and 30 s of rest. This was immediately followed by two, 4-s maximal sprints separated by 2 min of active recovery at 35% V̇O2peak. Participants were then given a 2-min rest before commencing the intermittent-sprint test.

The intermittent-sprint test consisted of two 36-min halves of intermittent-sprint exercise separated by a half-time rest period of 10 min. Each half was divided into 18 × 2-min blocks consisting of a 4-s sprint, 100 s of active recovery, and 20 s of passive recovery (Fig. 1). On two occasions during each half, participants performed a repeated-sprint bout composed of five 2-s sprints, with 18 s of recovery between successive sprints. During active recovery, participants maintained a power output calculated to be 35% of their V̇O2peak. Five seconds before each sprint, participants were asked to stand in a ready position and await the start signal. Strong verbal encouragement was provided during all sprints. During the "half-time" period, participants rested quietly before assuming the ready position 5 s before beginning the second half.

F1-25
FIGURE 1:
Schematic of the first half of the intermittent-sprint exercise test. WUP, warm-up; B, blood sample; RPE, rating of perceived exertion. Measures taken throughout second half were taken at identical times as measures taken throughout first half. U* = posttest urine sample collected within 10 min of completion of second half.

Calculation of test scores (intermittent-sprint test)

The work done (J) and peak power achieved (W) were recorded for each 4-s sprint of the intermittent-sprint test (18 × 4-s sprints per half). From these data, both absolute (total work, mean peak power) and relative (percent decrement over repeated efforts) scores were calculated for each half of the test, and for the two repeated-sprint bouts in each half.

Capillary blood sampling and analysis.

Capillary blood samples were taken immediately before each half of the intermittent-sprint test and 9, 17, 25, and 33 min into each half (Fig. 1). A hyperemic ointment (Finalgon, Boehringer Ingelheim, Germany) was applied to the earlobe 5-7 min before initial blood sampling. Blood was sampled in an aseptic manner and was collected in 35-μL glass capillary tubes (D957G-70-35, Clinitubes, Radiometer Copenhagen). Plasma lactate concentration was determined using a blood-gas analyzer (ABL625, Radiometer Copenhagen), which was regularly calibrated using precision standards, and routinely assessed by external quality controls.

Rating of perceived exertion.

Perception of effort was obtained immediately before blood sampling throughout the intermittent-sprint test (Fig. 1) using the Borg rating of perceived exertion (RPE) scale (4). This is a numerical scale ranging from 6 to 20, with a written description of perceived effort associated with all odd numbers (e.g., 13 is somewhat hard, 15 is hard, and 17 is very hard). Participants were familiarized with this scale during preliminary visits.

Urine sampling and analysis.

Urine samples (~40 mL) were obtained within 10 min of completion of each experimental trial (~2.5 h after ingestion of placebo or caffeine). Urine samples were frozen for later determination of urinary caffeine concentration using high-performance liquid chromatography (WATERS Corporation instrument, Milford, MA).

Pretest questionnaire.

Immediately before each of the intermittent-sprint tests, the participants were questioned regarding which of the two treatments (caffeine or placebo) they thought they had received, and on what basis their opinion (if any) was formed.

Statistical analysis.

The results of each independent variable were analyzed by a two-factor ANOVA (treatment and time) with repeated measures. The alpha level for statistical significance was preset at 0.05 and Fisher's least-squares difference (LSD) was used for post hoc analysis when significance was found. Effect sizes were also calculated for performance data.

RESULTS

Pretest questionnaire.

The pretest questionnaire revealed that, 60 min following placebo ingestion, 7 of 10 participants were unsure which treatment they had received. The remainder believed they had not received the caffeine treatment because they did not feel any effects. Following caffeine ingestion, three participants were unsure, one participant believed he had not been given caffeine because he felt no effects, and six participants correctly believed they had received the caffeine treatment because of feelings of being "amped," hyperactive, restless, or a slight headache.

Work performed.

The average amount of work performed by participants during individual sprints in each half of the intermittent-sprint test is summarized in Figure 2. A significant main effect was seen of treatment (P < 0.05), demonstrating that more work was performed during sprints in each half of the intermittent-sprint test following caffeine ingestion, in comparison with placebo. Also, a significant main effect was noted of time (P < 0.01). Post hoc analysis revealed that work performed during sprints following the repeated-sprint bouts (sprints 9 and 17 of each half) was consistently less than work performed during sprints following the standard 2-min recovery period. Figure 2 also demonstrates a trend for greater work scores during all sprints in each half of the caffeine trial, in comparison with placebo. No significant interaction effect for work scores was found, however.

F2-25
FIGURE 2:
Work (J) performed during individual sprints in the first half (A) and second half (B) of the intermittent-sprint test for the caffeine and placebo conditions. Values are mean ± SEM (N = 10). A significant main effect was found for both treatment and time for both halves (P < 0.05.) ↑ Repeated bout.

As illustrated in Figure 3, the total amount of sprint work done during each half of the intermittent-sprint test was significantly greater for the caffeine trial. The total amount of sprint work performed during the first half of the caffeine trial was 8.5% greater than that performed during the placebo trial (75,165.4 ± 3,902.9 vs 69,265.6 ± 3,719.7 J, P < 0.01), and 7.6% greater during the second half (73,978.7 ± 4,092.6 vs 68,783.2 ± 3,574.4 J, P < 0.05). The effect size (ES) for the difference in total work between the caffeine and placebo trial was moderate for both the first and the second half (ES = 0.50 and 0.46, respectively). No significant difference was seen in the total amount of work performed between halves for either condition.

F3-25
FIGURE 3:
Total amount of work (J) performed during each half of the intermittent-sprint test for the caffeine and placebo conditions. Values are mean ± SEM (N = 10). * Significantly different from placebo (P < 0.05).

Peak power.

The average peak power achieved by participants during individual sprints in each half of the intermittent-sprint test is summarized in Figure 4. A significant main effect of treatment (P < 0.01) was noted, which demonstrated that greater peak power was achieved during sprints in each half of the intermittent-sprint test following caffeine ingestion, in comparison to placebo. Also, a significant main effect of time (P < 0.01) was noted. Post hoc analysis revealed that peak power achieved during the sprints following the repeated-sprint bouts (sprints 9 and 17 of each half) was consistently less than the peak power achieved during sprints following the standard 2-min recovery period. Figure 4 also demonstrates a trend for greater peak power during all sprints in each half of the caffeine trial in comparison with placebo. No significant interaction, however, was found for power scores.

F4-25
FIGURE 4:
Peak power (W) achieved during individual sprints in the first half (A) and second (B) of the intermittent-sprint test for the caffeine and placebo conditions. Values are mean ± SEM (N = 10). A significant main effect was found for both treatment and time for both halves (P < 0.05). ↑ Repeated-sprint bout.

As illustrated in Figure 5, the mean peak power score for each half of the intermittent-sprint test was significantly greater for the caffeine trial. The mean peak power achieved during the first half of the caffeine trial was 7.0% greater than that achieved during the placebo trial (1330.9 ± 68.2 vs 1244.2 ± 60.7 W, P < 0.01), and 6.6% greater during the second half (1314.5 ± 68.4 vs 1233.2 ± 59.9 W, P < 0.01). The effect size for the difference in average peak power achieved between the caffeine and placebo trial was moderate for both the first and the second half (ES = 0.40 and 0.43, respectively). No difference was found in the mean peak power scores between the two halves for either condition.

F5-25
FIGURE 5:
Mean peak power (W) achieved during each half of the intermittent-sprint test for the caffeine and placebo conditions. Values are mean ± SEM (N = 10). * Significantly different from placebo (P < 0.05).

Performance decrements.

No significant differences were found between treatments in percent decrement for total work (7.4 ± 1.8 vs 8.4 ± 3.0% and 8.0 ± 2.0 vs 7.8 ± 1.9%, for the first and second halves, respectively, placebo vs caffeine). Although not significant, a trend was noted toward lower decrements in work done across repeated-sprint bouts (Table 1) during the caffeine trial in comparison with placebo, in both halves of the intermittent-sprint test.

T1-25
TABLE 1:
Performance decrements for repeated-sprint bouts 1 and 2 during each half of the intermittent-sprint test for placebo and caffeine conditions. Values are mean ± SEM (N = 10).

Rating of perceived exertion.

Perceived exertion increased significantly as exercise progressed during each half (P < 0.01); however, no significant difference was found between treatments.

Plasma lactate concentration.

There was a significant main effect of treatment (P < 0.05) and time (P < 0.01) for measures of plasma lactate during both halves of the intermittent-sprint test; however, no significant interaction effect was seen (Fig. 6). Plasma lactate levels were significantly higher in the caffeine trial compared with placebo. During both halves, plasma lactate levels displayed a significant increase from the commencement of exercise until immediately after the first repeated-sprint bout (0-17 min). Following this, plasma lactate levels decreased significantly (25 min) before rising again immediately after the second repeated-sprint bout (33 min).

F6-25
FIGURE 6:
Plasma lactate concentration (mmol·L−1) for the caffeine and placebo conditions during the first half (A) and second half (B) of the intermittent-sprint test. Values are mean ± SEM (N = 10). A significant main effect was seen for treatment (P < 0.05) and significant time effect: a < b < c (P < 0.05).

Postexercise urinary caffeine concentration.

Most (8 of 10) urinary caffeine levels measured following the placebo trial were less than 0.2 μg·mL−1, which is the sensitivity limit for the test used. Following the placebo trial, only two participants tested above this limit (0.3 and 0.8 μg·mL−1). Urinary caffeine levels measured following the caffeine trial ranged from 3.5 to 9.1 μg·mL−1 (6.9 ± 0.6 μg·mL−1).

DISCUSSION

This study was conducted to determine the effects of acute caffeine ingestion on the performance of an intermittent-sprint exercise test designed to simulate the physiological demands specific to athletes participating in team sports. It was found that the total amount of sprint work performed during each half of the intermittent-sprint test, as well as the average peak power attained by participants during sprints in each half, were significantly improved in the caffeine trial in comparison with placebo. In the caffeine trial, total sprint work was 8.5% greater during the first half and 7.6% greater in the second, when compared with placebo. Similarly, average peak power was 7.0% greater in the first half of the caffeine trial and 6.6% greater in the second, when compared with placebo. It is important to note that this study is unique among others that have examined the effect of caffeine on exercise, because the exercise performed was of a prolonged duration (~80 min), but the performance parameters measured throughout were high-intensity in nature (4-s maximal sprint work and power). As a consequence, these results are difficult to compare with previous studies in which the exercise protocols were either prolonged submaximal efforts or sustained high-intensity efforts.

The performance improvements reported in this study fall below the range of performance improvements reported by previous studies that have examined the effect of caffeine on prolonged, submaximal exercise (19-51%) (6,12). The performance parameter typically measured in these studies, however, has been time to fatigue rather than work accomplished in a fixed period of time. Although it is uncertain by which mechanism(s) caffeine affects performance during exercise, it is possible that the effect of caffeine on work done during a fixed amount of time is different than the effect of caffeine on fatigue development during exercise at a fixed workload. In support of this, our results are similar to the 7.4% improvement in total work that was reported in a study by Ivy et al. (14), in which participants performed 120 min of isokinetic cycling following caffeine ingestion.

The performance improvements noted in the present study are also consistent with previous studies that have reported improvements in high-intensity exercise with caffeine. Following caffeine ingestion, Anselme et al. (1) reported an approximately 7% increase in maximal anaerobic power (Wmax) during a single 6-s sprint on a cycle ergometer; Kang et al. (15) noted an approximately 11% increase in work during the 30-s Wingate test, and Collomp et al. (5) reported an increase in the average speed of trained swimmers during a 100-m freestyle sprint (~30 s), corresponding to an increase in power of approximately 4-6% (22). Whereas the caffeine doses used in these studies were variable (2.5-8.8 mg·kg−1), it has been demonstrated that little variation occurs in the ergogenic effect of caffeine doses ranging from approximately 3 to 9 mg·kg−1 (16,21). Therefore, the results of the present investigation support the findings of several previous studies that have found caffeine to be ergogenic for short-duration (≤30 s) supramaximal exercise.

To the authors' knowledge, only Paton et al. (22) have similarly attempted to determine the effect of caffeine ingestion on repeated-sprint performance. In their study, 16 male team-sport athletes ingested either placebo or 6 mg·kg−1 of caffeine, 60 min before performing a repeated-sprint test. The test consisted of 10 × 20-m sprints (mean time ≈ 3.6 s), each performed within 10 s, followed by a rest for the remainder of the 10-s period. As in our study, participants were competitive, male team-sport athletes, who had controlled diets for 48 h preceding each trial and were asked to avoid caffeine-containing foods and beverages for 48 h before the first trial through to the end of the experimental trials. Trials were also conducted in a placebo-controlled, randomized, double-blind fashion. Contrary to our results, a negligible effect was found of caffeine ingestion on mean sprint performance.

The difference in results between the present study and that by Paton et al. (22) may be attributable to differences in the exercise protocols. Whereas a running protocol, as used by Paton et al. (22), more closely replicates the usual form of activity performed by team-sport athletes, the cycle ergometer was used in the present study to allow for a high degree of accuracy in the measurement of work and power output during each 4-s sprint. It is possible that participants in the study by Paton et al. (22) achieved greater peak power while sprinting following caffeine ingestion, but this was not reflected by simply measuring 20-m sprint time.

Another important difference between the two studies was the difference in the length of the recovery periods between repeated sprints. The approximately 6.4-s recovery time that was used in the study by Paton et al. (22) is well below the average length of time between sprints typical of team-sport activity (8,19,25). Hence, it is likely that the exercise protocol used by Paton et al. (22) to test the effect of caffeine on repeated-sprint performance did not reflect the demands typically placed on athletes during team sports. Furthermore, some performance improvements may have been noted had the exercise protocol been longer, thereby allowing for the possibility of delayed fatigue development following caffeine ingestion.

A number of possible mechanisms may have contributed to the increase inwork and power noted in the present study. It is possible that the performance improvement noted was a result of the antagonism of adenosine receptors by caffeine. Caffeine is very similar in structure to adenosine and can bind to its cell membrane receptors, thus blocking their action (9). Because the general neurophysiological actions of adenosine are inhibitory, it is likely that antagonism of adenosine receptors would exert stimulatory actions on the CNS (9). CNS facilitation could enable recruitment of additional motor units or increase the frequency of motor unit activation (30), which theoretically would increase work and power output during sprints. It is also possible that caffeine ingestion had a direct effect on muscle tissue via the mobilization of intramuscular calcium (24). This may have facilitated excitation-contraction coupling and increased muscle contraction efficiency, thereby enabling increases in work and power output. Furthermore, CNS facilitation may also have led to a decreased perception of effort during exercise (6,23), thereby enabling participants to work at a greater intensity for a given period of time. Given that no significant difference in ratings of perceived exertion between treatments was noted, and yet participants were consistently able to perform more work throughout the intermittent-sprint test following caffeine ingestion, the results of the present study support this theory. It should be noted however, that these possibilities are speculative; further research is required to determine the exact mechanism(s) responsible for the ergogenic effects of caffeine on intermittent-sprint performance.

In a study by Greer et al. (13), examining the effects of a 6-mg·kg−1 dose of caffeine on peak and mean power during four 30-s Wingate tests, each separated by 4 min, a nonsignificant trend was noted toward enhanced performance of the first Wingate test and a significantly reduced performance by the fourth test. This led the authors to suggest that, although caffeine may enhance performance during initial efforts, it may be ergolytic as fatigue develops, possibly because of an increase in the by-products of anaerobic metabolism. As in the study by Greer et al. (13), plasma lactate levels in the present study were significantly higher during exercise with caffeine compared with placebo, and this was attributed to an increase in anaerobic work performed during the 4-s sprints. Despite the improvements in work and power, however, no significant difference was found between conditions, in percent decrement for total work performed, during either half of the intermittent-sprint test. Hence, contrary to the results of the study by Greer et al. (13), our data suggest that, although caffeine significantly enhanced performance of intermittent-sprints, resulting in increased plasma lactate concentrations, this did not affect the ability of participants to maintain work efforts in the latter stages of the exercise protocol. That is, no apparent increase in the rate of fatigue development was attributable to initial improvements in work and power achieved during the caffeine trial.

Although no other study has specifically set out to determine decrement in performance of intermittent high-intensity efforts following caffeine consumption (thereby testing the theory that caffeine may be ergolytic as fatigue develops), it has been demonstrated that greater power output during the initial sprints of a repeated-sprint ability test (five 6-s sprints with 24 s of rest between each sprint) is associated with a greater decrement in performance (3). Therefore, the results of the present study suggest that caffeine ingestion before exercise does not increase the rate of fatigue development during intermittent high-intensity exercise and, in fact, may limit the decrement in performance that has otherwise been noted with greater power output achieved during initial sprints. That caffeine may attenuate fatigue development during high-intensity efforts is further supported by the trend (although not statistically significant) toward lower decrements in work done across the repeated-sprint bouts that was noted in this study.

That no significant difference was found in the rate of fatigue development (based on percent decrement scores) may have been caused by a reduction in perceived effort following caffeine ingestion. This is supported by the fact that no significant increase was found in the level of perceived effort with caffeine in comparison with placebo, even though more work was consistently performed throughout the test. It is also possible that caffeine attenuated the effects of fatigue by facilitating Na+/K+ ATPase activity (18). It is believed that K+ loss from the intracellular space of muscle during exercise is causally related to muscle contraction failure and the development of fatigue (17,26). Although K+ levels were not measured in this study, a study by Lindinger et al. (17) has previously reported that, compared with placebo, a 6-mg·kg−1 dose of caffeine significantly enhanced performance and significantly decreased the rise in plasma K+ concentration during exercise. Therefore, caffeine ingestion could theoretically attenuate fatigue development by facilitating Na+/K+ ATPase activity, thereby limiting the rise in plasma K+ that typically occurs during exercise at intensities greater than approximately 30% V̇O2peak (29).

A practical finding of this study is that none of the participants tested more than 12 μg·mL−1 for postexercise urinary caffeine, which, before 2004, was the level set by the International Olympic Committee above which a positive test for caffeine use was recorded. In the present study, the urinary caffeine concentrations, measured approximately 2.5 h following ingestion of 6 mg·kg−1 of caffeine ranged from 3.5 to 9.1 μg·mL−1. This large individual variability in urinary caffeine levels is typical following ingestion of standard doses (21,28) and may be explained by individual variations in the rate at which caffeine is metabolized by the liver.

In conclusion, the results of the present study have shown that acute ingestion of a moderate dose of caffeine can improve the performance of an intermittent-sprint exercise test designed to simulate the physiological demands of team-sport athletes. It was also shown that improvement in sprint performance as a result of caffeine ingestion is not compromised by an increased rate of fatigue development over an extended period of time.

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Keywords:

REPEATED-SPRINT ABILITY; INTERMITTENT HIGH-INTENSITY EXERCISE; ERGOGENIC AIDS; TEAM SPORT

©2006The American College of Sports Medicine