Many athletes use caffeine as an ergogenic aid to enhance exercise performance (13,17,29-30). A variety of foods and beverages, including chocolate, tea, coffee, and cola drinks, contain caffeine. Energy drinks are a relatively new category of beverages that contain caffeine in amounts similar to the concentrations found in coffee (33).
Caffeine is currently a monitored or restricted drug for competitive athletes. The World Anti-Doping Agency (WADA) monitors caffeine use during competition to detect patterns of misuse in sport (42). The National Collegiate Athletic Association (NCAA) considers caffeine a prohibited substance when the concentration in the urine exceeds 15 μg/mL (44). An athlete would need to consume approximately 13 mg mg/kg body weight (BW) of caffeine (approximately 700 mg caffeine or 8 cups of brewed coffee) to reach the maximum allowable urinary concentrations (17,21).
Evidence from the research literature suggests that 150 to 250 mg of ingested caffeine improves exercise performance (2,9). Caffeine may enhance performance during endurance exercise, particularly prolonged and exhaustive exercises (1,5,7,10,18,20,22). Caffeine may also reduce fatigue, improve concentration, and enhance mental alertness (31,41). However, the evidence for caffeine as an ergogenic aid for short-term, high-intensity athletic performance is more mixed. For example, some studies report an improvement in anaerobic exercise performance with caffeine (14,16,25,42). Other studies have reported no improvement in anaerobic exercise performance with caffeine (11,23,26).
Mixed results could be due to the different exercise protocols, doses of caffeine, and the fitness level of the research participants. Caffeine may affect trained and nontrained participants differently; trained individuals may benefit more from the ergogenic effect of caffeine. Elite athletes generally have more muscle mass than recreational athletes. Researchers have speculated that caffeine acts directly on muscle fibers, and, thus, the effects of caffeine would be greater in individuals with a larger muscle mass.
Historic use of caffeine could also impact study results. No improvement in 15-second cycling sprints was observed in untrained males that were low caffeine users (<70 mg/d) after ingesting 7 mg/kg BW of caffeine (43). A higher dose of caffeine (7 mg/kg BW) given to low caffeine users may produce a shaky or sick feeling and possibly decrease performance.
Recently, the impact of caffeine during high-intensity team sports activity has been examined. Players of high-intensity team sports engage in many short-term, high-intensity exercise bouts throughout the game. Stuart et al. (40) examined the effects of caffeine (6 mg/kg BW) on high-intensity team-sport performance. The participants were amateur male rugby players and were all regular caffeine consumers. The exercise tests included 14 circuit stations of activities examining sprinting abilities (straight-line and agility sprints), peak-power generation in 2 consecutive drives, and accuracy. Performance improved in the caffeine trial compared with the placebo trial for all exercise tests except peak-power during the second drive. This study suggests that caffeine (6 mg/kg BW) may be effective at improving physical and skill performance in trained male rugby players who are regular users of caffeine.
Only limited research has examined the impact of caffeine on anaerobic-type exercise during field-type exercise. Furthermore, studies do not always report the historic use of caffeine of the research participants. Thus, the purpose of this study was to examine the effect of caffeine on short-term, high-intensity exercise in highly trained, caffeine naïve football athletes using anaerobic field-type exercises such as those used in the National Football League (NFL) Combine.
Experimental Approach to the Problem
The primary purpose of this study was to examine the effects of a moderate dose of caffeine during field-type exercises in highly trained collegiate male football players. A randomized, double-blind crossover study examined the impact of a moderate dose of caffeine (5 mg/kg BW) on anaerobic performance in male football players, 18 to 23 years of age. Our primary null hypothesis was that the ingestion of a beverage with a 5.0 mg/kg BW dose of caffeine would not significantly improve exercise performance in collegiate male football players using the 185 or 225 lb bench press, a 40-yard dash, and a 20-yard shuttle. Our secondary null hypothesis was that the ingestion of a beverage with 5.0 mg/kg BW dose of caffeine would not alter heart rate (HR), blood pressure (BP), or Borg's rating of perceived exertion (RPE).
Recruitment began in November of 2004 and involved both oral communication as well as a recruitment flyer distributed to local community colleges and athletic facilities. Athletes were also recruited from Athletes' Performance, a state-of-the-art facility that trains elite and professional athletes. Participants were eligible if they fit the following inclusion criteria: played collegiate football during the past season. This study was approved by the Human Subjects Institutional Review Board at Arizona State University, and the procedures were conducted in accordance to these guidelines.
Seventeen male collegiate football players (age, 20 ± 2 yr; wt, 100 ± 15 kg; ht, 185 ± 5 cm; body mass index [BMI], 29.4 ± 3.6 kg/m2) completed this study (Table 1). Height was measured (without shoes) to the nearest 0.1 cm using a stadiometer. Weight was measured to the nearest 0.1 kg using a digital platform scale (Avadia model 758, Webb City, MO).
The athletes played collegiate football during the previous season (community college to NCAA Division I). Some of the research participants were currently training for the NFL Combine; other participants were completing off-season training to prepare them for the next football season. To further control the testing conditions, participants completed identical training programs during the 2 weeks of testing.
To assess usual intake of caffeine, participants completed the Arizona Food Frequency Questionnaire (AFFQ) (University of Arizona, Tucson, AZ) (34). Participants were given written and oral instructions on how to complete the questionnaire. The AFFQs were analyzed by the NCS OpScan 5 (Al-Bassam International Company, Al-Khobar, Saudi Arabia). Typical dietary caffeine intake was 16 ± 20 (range, 0-81) mg/day (Table 1). Sixteen participants consumed less than 50 mg/day, and 1 participant consumed greater than 50 mg/day (Table 1).
Participants reported to Athletes' Performance twice, separated by 1 week, after an 8- to 12-hour fast and abstaining from caffeinated products for 48 hours. HR and BP were measured upon arrival to the facility. A Polar heart rate monitor (Polar T1 model CEO 537, Lake Success, NY) was used to monitor HR, and a professional stethoscope (Adscope series 601, Miami, FL) and sphygmomanometer (Moore Medical, New Britain, CT) were used to monitor BP. The participants then consumed a beverage with caffeine (caffeine, 5.0 mg/kg BW; carbohydrate (CHO), 0.125 g/kg BW) or without caffeine (placebo; CHO, 0.125 g/kg BW) and ate a meal 15 minutes later (Table 1). Each participant consumed the same meal during each of the test days. Sixty minutes after consuming the beverage, participants performed 3 exercise tests: a 40-yard dash test, 20-yard shuttle, and a bench press (Olympic bar) with repetitions to fatigue using 185 or 225 lb. The lower weight (185 lb) was used for the participants who could not press 225 lb.
The 40-yard dash assesses speed, power, and explosion. During this exercise test, the research participants ran at maximal effort for 40 yards on an AstroTurf track. The 20-yard shuttle measures lateral speed and coordination. It was also completed on the AstroTurf. The AstroTurf was premarked at 0, 5, and 10 yards. The participants started at the center of the premeasured 10 yards and ran at maximum effort for 5 yards one direction, touching the yard line. They then ran the full 10 yards back the other direction. After touching the yard line, the participants finished the test after running across the center line. The researchers recorded the time it took to complete the 40-yard dash and the 20-yard shuttle. Participants completed the bench press, which tests upper body strength and conditioning, on a flat bench using an Olympic bar. While in a supine position on the bench, participants lifted the Olympic bar (185 or 225 lb) off the rack then lowered it to the chest. The bar was then pressed back up to the starting position and repeated to fatigue. Researchers recorded the number of repetitions for each participant. The research participants estimated perceived effort immediately after each exercise test using Borg's RPE (scale 6-20). Postexercise HR and BP were measured immediately after the final exercise test.
Although we controlled for caffeine and diet, we were not able to ensure compliance in the study. Factors such as rest, prior exercise, and over-the-counter medications may have affected the participants' response or metabolism of the caffeine. We did not ask the participants which treatment they thought they received or how they felt after each treatment. In addition, plasma or urinary caffeine concentrations were not measured.
Descriptive statistics (age, height, weight, BMI) were expressed as mean ± SD. Normality of data was assessed using histograms and the Kolmogorov-Smirnov test statistic. Tests for normality were met for the 40-yard dash, 20-yard shuttle, RPE, and HR. Before analysis, the bench press data were transformed using the logarithm. Mean and SD values in the tables are the untransformed values. Paired t-tests using the Statistical Package for Social Sciences (SPSS, version 14.0, 2005, Chicago, IL) determined the independent effects of caffeine vs. no caffeine on exercise performance during the bench press, 40-yard dash, and 20-yard shuttle. A 2-way repeated-measures analysis of variation + Bonferroni post hoc test was used to examine significant differences in HR and systolic (S)BP for time, treatment, and their interaction. A Wilcoxon signed rank test was used to test for differences for the diastolic (D)BP data.
This study examined the effects of a moderate dose of caffeine (5 mg/kg BW) on anaerobic exercise performance (40-yard dash, 20-yard shuttle, and bench press), RPE, HR, and BP in collegiate male football players.
No significant differences in performance were seen for the 40-yard dash, 20-yard shuttle, or bench press between treatments (40-yard dash: caffeine, 5.01 ± 0.25 vs. placebo, 5.03 ± 0.26 s, p = 0.43; 20-yard shuttle: caffeine, 4.63 ± 0.19 vs. placebo, 4.66 ± 0.24 s, p = 0.51; bench press: caffeine, 17 ± 8 vs. placebo, 17 ± 8 rep, p = 0.51) (Figures 1-3) (Table 2). However, 59% of the participants decreased time during the 40-yard dash, 59% decreased time during the 20-yard shuttle, and 47% increased repetitions during the bench press with the caffeine.
Borg's Rating of Perceived Exertion
Because the RPE data were not normally distributed, the Wilcoxon signed rank test was used for the analysis. No differences were observed for RPE between treatments in any of the exercises (40-yard dash: caffeine, 13 ± 3 vs. placebo, 13 ± 2, p = 1.00; 20-yard shuttle: caffeine, 12 ± 2 vs. placebo, 12 ± 3, p = 0.25; bench press: caffeine, 16 ± 2 vs. placebo, 15 ± 3, p = 0.31) (Table 3).
Heart Rate and Blood Pressure
Postexercise values for HR were significantly higher than pre-exercise values within each treatment (precaffeine, 68 ± 10 vs. postcaffeine, 118 ± 21; preplacebo, 67 ± 6 vs. postplacebo, 110 ± 19 [beats/min]; p < 0.01) (Table 4). Postexercise values were significantly higher for SBP compared with resting values within each treatment (precaffeine, 114 ± 10 vs. postcaffeine, 136 ± 14; preplacebo, 116 ± 12 vs. postplacebo, 135 ± 14 [mm Hg]; p < 0.01) (Table 4). Postexercise values were significantly lower for DBP compared with pre-exercise values within each treatment (precaffeine, 72 ± 8 vs. postcaffeine, 63 ± 6; preplacebo, 76 ± 6 vs. postplacebo, 63 ± 9 [mm Hg]; p < 0.01) (Table 4). No differences in HR and BP were observed between treatments at the pre-exercise or postexercise assessments.
This study examined the effects of a moderate dose of caffeine (5 mg/kg BW) on exercise performance, RPE, HR, and BP in highly trained male collegiate football athletes. In our study, all of the participants consumed less than the U.S. population's average intake of caffeine of 193 mg/d (19). Thus, the participants in our study were caffeine naïve or very low caffeine consumers. To date, only limited studies have examined the impact of caffeine on field-based anaerobic exercise performance (32,37,39,40).
We did not find a statistically significant difference between treatments for the bench press. However, 59% of the participants in our study increased the number of repetitions on the bench press with caffeine.
Jacobs et al. (26) also examined the impact of caffeine on exercise performance during the leg press and bench press. Similar to our study, no significant differences were observed with the caffeine trial compared with the placebo trial. Jacobs et.al. (26) included recreational athletes instead of the highly trained collegiate football players used in our study. Jacobs et al. (26) administered a lower dose (4 mg/kg BW) of caffeine 90 minutes before testing instead of the 5 mg/kg BW dose 60 minutes before testing in our study. However, their study did not provide any information on the historic use of caffeine by the research participants.
In an older study, Jacobson et al. (27) found an improvement in muscle strength and power using knee extension and flexion exercises. Similar to our study, the research participants were elite male football athletes and low users of caffeine. However, a higher dose of caffeine (7 mg/kg BW) was used in that study compared with our study (5 mg/kg BW). These results suggest that a higher dose of caffeine (7 mg/kg BW) may be needed to get an ergogenic effect for short-term, high-intensity exercise. However, several of our participants reported feeling shaky or jittery during the exercise testing. Thus, a higher dose of caffeine might have negatively impacted their performance. Furthermore, the potential to improve may be small and difficult to measure in short-term, high-intensity exercise performance.
Only limited research has examined the impact of caffeine during high-intensity, team-sport field performance exercises (32,37,39,40). In the present study, no significant performance improvement was observed with caffeine ingestion during the 40-yard dash and 20-yard shuttle. For these field-tests, a performance improvement was seen with the caffeine for 59% of the participants during the 40-yard dash but only for 47% during the 20-yard shuttle. Our results are similar to Paton et al. (37), who also did not find significant improvement with caffeine using team-sport athletes in 20-meter repeated sprints. In their study, they used a slightly higher dose of caffeine (6 mg/kg BW). Historic use of caffeine was not reported in this study. Lorino et al. (32) also found that a 6 mg/kg dose of caffeine did not improve agility as measured by the pro-agility run test or power output as measure by the Wingate test. The participants in this study were recreationally active young males who were not habituated to caffeine.
However, Stuart et al. (40) found significant improvements in exercise performance in team-sport athletes in repeated bouts of sprint, power, and accuracy tests lasting 3 to14 seconds. In their study, they also used 6 mg/kg BW of caffeine, and all the participants stated that they were regular consumers of caffeine in their diet. Schneiker et al. (39) also found a positive impact of caffeine on exercise performance in competitive male team-sport athletes. The research participants consumed either caffeine (6 mg/kg) or a placebo and then completed prolonged intermittent-sprint exercise tests on a cycle ergometer. No information was provided on the historic use of caffeine of the research participants.
In the present study, no significant differences were observed between treatments for RPE during any of the exercise tests. These results are similar to the results of several other short-term, high-intensity exercise protocols (4,12). One published study using a short-term, high-intensity exercise protocol reported a significantly lower RPE after caffeine ingestion (16). Furthermore, a recent meta-analysis found that caffeine reduces RPE during exercise, and the study stated that this may explain the ergogenic effect of caffeine on exercise performance (15). However, during short-term, high-intensity exercise protocols, RPE may not be a sensitive dependent variable.
Variable cardiac responses have been reported with acute caffeine ingestion. Most researchers found no impact of acute caffeine ingestion on resting HR (3,6,8,12,36). However, one study reported an increase in resting HR after acute caffeine ingestion (38). Most research also reports that caffeine may increase postexercise HR but not significantly when compared with a placebo trial (4,6,12,28,36). Our data are in agreement with the findings of the majority of the research literature. We found that resting HR was not significantly different between trials. HR was significantly increased after exercise in both trials, but no significant differences were seen between treatments.
Fewer studies have examined the impact of caffeine on BP response to exercise. We found no significant differences in BP between trials. Other research has reported no significant differences in resting (3,24,26) or postexercise (36) BP readings after caffeine ingestion.
Caffeine ingestion did not statistically improve exercise performance during the bench press, 40-yard dash, and 20-yard shuttle. During the caffeine treatment, 59% of the individual participants improved in the bench press, 59% improved in the 40-yard dash, and 47% improved in the 20-yard shuttle. The potential to improve short-term, high-intensity exercise performance may be small and difficult to measure. Our participants were low users of caffeine and may have experienced more negative side effects to the caffeine.
More research is needed examining the effect of caffeine during field tests specific to the athletes' competition or training regimen. Different doses of caffeine should be examined. In addition, the participants in this study were either caffeine naïve or very low caffeine consumers, which may have contributed to the study results. Thus, research is needed that compares the effect of caffeine in caffeine naïve vs. habituated caffeine consumers in anaerobic exercise performance.
This study found that caffeine (5 mg/kg BW) did not statistically improve exercise performance in caffeine naïve football players completing field-based anaerobic exercise tests (40-yard dash, 20-yard shuttle, and a bench press) used during the NFL Combine. Also, no differences were found between treatments for RPE, HR, and BP. However, 59% of the individual participants improved performance with the caffeine during the bench press and the 40-yard dash. Because caffeine did not statistically improve exercise performance, it would not be recommended for a coach or personal trainer to suggest a moderate dose of caffeine in caffeine naïve football players as a method to improve performance.
Grant Support: Experimental and Applied Science, Inc.
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