Caffeine is widely ingested by athletes and nonathletes to facilitate improved performance during intermittent (30) and endurance (2) exercise. Furthermore, although evidence thus far seems equivocal (3) with some studies reporting no enhancements in anaerobic exercise performance, several authors have reported enhanced resistance exercise performance after caffeine ingestion (16,17). Despite these reports, the exact mechanisms for these improvements remain to be fully elucidated. Tarnopolsky (35) has suggested that caffeine increases contractility of skeletal muscle and force production by enhanced neuromuscular transmission and therefore maximal muscle activation. Furthermore, Behrens et al. (7) reported that caffeine ingestion increased voluntary activation of the quadriceps and neural drive. In addition, caffeine-induced improvements in exercise performance have also been proposed to originate from an increase in catecholamine secretion (19) and enhanced calcium release from sarcoplasmic reticulum (35). However, in vivo, the dose required to have this effect would be toxic to the body, meaning it is improbable that enhancements in exercise performance occur through increased calcium release (29). Caffeine has also been implicated in enhanced sodium-potassium pump activity that may enhance excitation contraction coupling (13). Caffeine also facilitates central effects by antagonizing adenosine receptors, thereby inhibiting the negative effects of adenosine on neurotransmission, arousal, and pain perception, and it is considered the leading hypothesis as to how caffeine could have an ergogenic effect during anaerobic exercise (13). However, Astorino and Roberson (3) concluded that the mechanism by which caffeine provides an ergogenic effect in short-term high-intensity exercise is likely to be multifactorial, with central factors such as adenosine antagonism being the most probable mechanism with alterations in perceived exertion, reaction time, cognition, and mood also having an influence on performance.
One potential issue with the use of anhydrous caffeine is the possibility of an overdose, and so if the same performance benefits observed in some studies could be procured from the caffeine found in coffee, this risk could be minimized while still providing ergogenic benefits. Caffeine in large doses can cause conditions such as sinus tachycardia, metabolic acidosis, hyperglycemia, and ketosis, with a fatal dose of caffeine being reported to be approximately 14 g, or approximately 150–200 mg·kg−1 (12). It has previously been reported that caffeine is most commonly supplemented by athletes in the form of coffee (15), but more recently, caffeinated energy drinks seem to have become more popular (40). To date, studies that have examined coffee as a potential ergogenic aid have revealed equivocal results (e.g., Refs. (20,21)). The inconsistency in results of coffee ingestion between studies may be the result of the brands of coffee used, significant variances in caffeine content within the coffee, chlorogenic acid content, exercise modality, and participant training status. It is also important to consider that coffee is suggested to contain more than 1,000 other compounds (18), which have undergone very little investigation into their role in influencing exercise performance either alone or in synergy with caffeine.
To date, only 1 other study has investigated the use of coffee and caffeine in a resistance exercise protocol. Trexler (37) reported that coffee improved leg press 1 repetition maximum (1RM) performance to a greater extent than caffeine, whereas both coffee and caffeine attenuated the reduction in total work performed compared with placebo. Therefore, as a result of the equivocal data on performance regarding coffee ingestion, and a number of studies displaying improvements in anaerobic exercise with caffeine supplementation (16), it would be beneficial to investigate the effect of caffeine and coffee on performance during a resistance exercise protocol.
Experimental Approach to the Problem
The aim of the present study was to determine if performance benefits exist during a resistance exercise protocol, when ingesting anhydrous caffeine or a dose-matched amount of caffeine contained within coffee, or decaffeinated coffee plus anhydrous caffeine. A randomized, Latin square, crossover, placebo-controlled design was used during this study. Each subject attended the strength and conditioning suite on 6 occasions, the first being a familiarization session to establish 1RM for the squat and bench press. The following 5 sessions were to complete the experimental protocols under each condition. Trials were performed within half an hour on separate days to ensure an accurate comparison of strength and to limit strength fluctuations because of the effects of circadian variation (16). Trials were separated by at least 2 days to allow recovery and to ensure complete caffeine washout. As caffeine has a highly variable half-life in healthy individuals of between 1.5 and 9 hours (34), subjects were instructed to avoid caffeine ingestion for a minimum of 12 hours and strenuous exercise for 24 hours before each trial to avoid fatigue (6). The main considerations were the response to coffee (hypothesis 1) and caffeine ingestion (hypothesis 2).
Nine resistance-trained men with an age range of 22 to 30 years (mean ± SD: age, 24 ± 2 years; weight, 84 ± 8 kg; height, 180 ± 8 cm; 1RM for the squat, 135 ± 28 kg; and bench press, 102 ± 20 kg) participated in the present study. All subjects were free from injury and were required to have been including bench press and squat pattern exercises in their training for at least 1 year before the study and also performing resistance training 3–4 times a week, to be considered intermediately resistance trained (5). Habitual caffeine consumption was assessed using an adapted version of the Landrum et al. (27) caffeine consumption questionnaire, with subjects having a habitual caffeine intake of 241 ± 122 mg·day−1. The study was approved by the Coventry University Ethics Committee, and subjects were made fully aware of the exact procedures, including any risks and benefits of participation in the study before providing written informed consent and undertaken in accordance with the Declaration of Helsinki.
During the familiarization session, subjects practiced the exercise protocol following technical teaching points by Baechle and Earle (5) for both the squat and bench press, while form was observed by a United Kingdom Strength and Conditioning Association–accredited coach. Subjects then established their 1RM weight for both exercises using a protocol outlined by Baechle and Earle (5). Subjects followed this procedure to establish 1RM, so 60% of 1RM could be used in both the squat (81 ± 17 kg) and bench press (61 ± 12 kg) protocols.
For each trial, subjects followed the protocol outlined in Figure 1. Subjects completed a warm-up that involved 5 minutes of submaximal cycling on a cycle ergometer (16) and then performed no more than 2 sets of 12 repetitions at a self-selected, light intensity for both squat and bench press. The warm-up weights selected during the initial trial were recorded and repeated before subsequent trials. Each subject then started the 60% to failure squat protocol. The squat protocol was performed before the bench press protocol to replicate a real-world training plan because in most instances, the most complex exercises using the largest muscle groups are performed first (25). A 5-minute rest period was allowed before subjects began the bench press protocol. The number of repetitions was recorded to calculate total weight lifted under each condition (repetitions performed multiplied by weight on the bar). Subjects then completed the 60% to failure protocol for the bench press in the same way. For both the exercises, a metronome (30 b·min−1) was used to provide a cadence of 2 seconds for both the eccentric and concentric phases of movement (16). Subjects were given encouragement under all experimental conditions. The felt arousal scale (33) was used to monitor arousal throughout the trials, and the rating of perceived exertion (RPE) (8) was recorded immediately after each of the protocols. Heart rate (HR) was measured using HR telemetry (Polar Electro Oy, Kempele, Finland) and was recorded before and after caffeine consumption, after the warm-up, after the squat protocol, and before and after the bench press protocol.
Each subject performed the exercise protocol after the ingestion of 0.15 g·kg−1 caffeinated coffee (COF), 0.15 g·kg−1 decaffeinated coffee (DEC), 0.15 g·kg−1 decaffeinated coffee plus 5 mg·kg−1 anhydrous caffeine decaffeinated coffee (D + C), 5 mg·kg−1 anhydrous caffeine (CAF), or a placebo (PLA; 5 mg·kg−1 maltodextrin; MyProtein, Manchester, United Kingdom). Nescafé original coffee was used because Hodgson et al. (21) reported that it contains 3.4 g of caffeine per 100 g of coffee, meaning that each subject consumed 0.15 g·kg−1 of coffee to achieve the 5 mg·kg−1 of caffeine required. Nescafé decaffeinated coffee was used, and it provides approximately 0.17 mg·kg−1 (21) and was prepared in identical fashion and concentration to the coffee trial. For the D + C trial, anhydrous caffeine (MyProtein, Manchester, UK) was measured out at 5 mg·kg−1 taking into consideration the 0.17 mg·kg−1 of caffeine from the decaffeinated coffee and added to the same amount of decaffeinated coffee as described above. All coffee trials were dissolved in 600 ml of hot water (68.9 ± 2.5° C) and served in mugs (21). The anhydrous caffeine and placebo trials were provided in capsule form (2 opaque gelatin capsules) and ingested with 600 ml of water (16.4 ± 3.8° C). At the start of each trial, subjects had a maximum of 15 minutes to fully consume either the treatment beverage or capsules and water, and they were then required to rest for the remainder of the hour, after this time the warm-up began. The absolute amount of caffeine ingested per trial was as follows: COF, 433 ± 40 mg; DEC, 2 ± 0 mg; D + C, 425 ± 39 mg; CAF, 425 ± 39 mg. A 24-hour dietary recall was completed by each subject during the familiarization session, and it was photocopied and handed back to the subjects in order for the same diet to be followed before subsequent trials (4).
Data are reported as the mean ± SD. All variables, except for felt arousal and HR, were assessed using a 1-way analysis of variance (ANOVA) with repeated measures. Felt arousal and HR were analyzed with a 2-way ANOVA with repeated measures. Sphericity was analyzed by Mauchly's test of sphericity followed by the Greenhouse-Geisser adjustment where required. Where any differences were identified, pairwise comparisons with Bonferroni's correction were used to show where they lay. All statistical procedures were conducted using IBM SPSS Statistics for Windows (version 22.0; IBM Corp., Armonk, NY, USA). An a priori power calculation based on previously published data with similar subject characteristics (11) suggested that a sample size of 8 subjects would be necessary to detect a statistical difference given an estimated effect size of 0.5, a 1-β error probability of 0.8, and a p value significance level less than 0.05. Furthermore, effect sizes using partial eta squared () were calculated, which were defined as trivial, small, moderate, or large (22).
There were significant differences in the number of repetitions performed for the squat between conditions (COF, 17 ± 5; DEC, 14 ± 5; D + C, 18 ± 5; CAF, 15 ± 5; PLA, 13 ± 4; F[4,32] = 9.869; p < 0.01; = 0.55). Pairwise comparisons revealed that the number of repetitions performed under the D + C condition were significantly greater than those performed under DEC (p < 0.01; 95% confidence interval [CI], 1.8–5.0), CAF (p ≤ 0.05; 95% CI, 0.1–5.2), and PLA (p ≤ 0.05; 95% CI, 0.3–8.4) conditions. Significantly more repetitions were also performed under the COF condition compared with the DEC (p ≤ 0.05; 95% CI, 0.4–5.6) and PLA (p ≤ 0.05; 95% CI, 0.5–7.2) conditions. Consequently, there were significant differences in the total weight lifted for the squat between conditions (F[4,32] = 9.200; p < 0.01; = 0.54; Figure 2). Pairwise comparisons revealed that the total weight lifted under the D + C condition was significantly greater than that lifted under DEC (p < 0.01; 95% CI, 129–445 kg), CAF (p ≤ 0.05; 95% CI, 17–430 kg), and PLA (p ≤ 0.05; 95% CI, 1–673 kg) conditions. Total weight lifted under the COF condition was significantly greater than that lifted under PLA (p < 0.01; 95% CI, 81–510 kg) but not significantly greater than that lifted under the DEC condition (p = 0.082; 95% CI, −24 to 515 kg). Furthermore, no significant order effect was observed (F[4,32] = 0.198; p = 0.937; = 0.02).
There were no significant differences in the number of repetitions performed (COF, 13 ± 2; DEC, 12 ± 2; D + C, 12 ± 3; CAF, 13 ± 2; PLA, 12 ± 2; F[4,32] = 1.871; p = 0.140; = 0.19) or the total weight lifted in the bench press protocol between conditions (F[4,32] = 1.651; p = 0.186; = 0.17; Figure 3). Furthermore, no significant order effect observed (F[4,32] = 0.528; p = 0.716; = 0.06). In addition, Figure 4 shows the individual responses to the conditions for total weight combined from both protocols and it reveals that during the D + C condition, 5 of 9 subjects lifted their greatest total weight and the remaining 4 lifted their greatest total weight in their COF trial.
There were no significant differences in RPE for squats between conditions (F[4,32] = 0.564; p = 0.690; = 0.07; Figure 5), and no significant differences in RPE for bench press between conditions (F[4,32] = 1.744; p = 0.165; = 0.18; Figure 5). There were no significant differences in arousal between conditions during the exercise protocol (F [4,32] = 2.582; p = 0.056; = 0.24; Figure 6), but there were significant differences between time points (F[2,16] = 16.855; p < 0.01; = 0.68). Pairwise comparisons revealed that arousal before treatment was significantly lower than both after treatment (p < 0.01; 95% CI, −1.5 to −0.6) and before bench press (p = 0.01; 95% CI, −1.9 to −0.3) across all conditions. But posttreatment was not significantly different from before bench press (p = 1.000; 95% CI, −0.7 to 0.6) during any condition.
There were significant differences in HR between conditions during the exercise protocols (F[4,32] = 5.100; p < 0.01; = 0.39; Figure 7) with significant differences between time points (F[5,40] = 99.601; p < 0.01; = 0.93). There was also a significant interaction between condition and time points (F[20,160] = 1.768; p ≤ 0.05; = 0.18). Pairwise comparisons revealed that HR during the D + C condition was not significantly different compared with the DEC condition (p = 0.051; 95% CI, 1–15 b·min−1). Heart rate during the CAF condition was significantly higher than both DEC (p ≤ 0.05; 95% CI, 2–19 b·min−1) and PLA (p ≤ 0.05; 95% CI, 3, 14 b·min−1). During the COF condition, HR was significantly higher than during the DEC condition (p ≤ 0.05; 95% CI, 2–14 b·min−1) and was not significantly different from PLA (p = 0.055; 95% CI, 0, 11 b·min−1). Changes in HR between pre and post treatment were not significantly different (p = 0.483; 95% CI, −14 to 7 b·min−1) during any trial, but HR for pre-squat, post-squat, pre-bench, and post-bench were all significantly different from each other (p < 0.01).
The major finding of the present study is that both COF and D + C can significantly improve performance in a resistance exercise protocol. Furthermore, total weight lifted in the squat protocol was significantly greater during the D + C condition than during the DEC, CAF, and PLA conditions, and the total weight lifted under the COF condition was significantly greater than that lifted under the PLA condition. These improvements were evident despite no significant differences in RPE and arousal. However, RPE during CAF was 7% lower than during D + C and 5% lower than during DEC when performing the bench press protocol and was 2–3% lower than all other conditions. For the CAF condition, RPE during the squat protocol was also 2–3% lower than all other conditions except for PLA, which was 1% greater. There has been only 1 other previous investigation in the use of coffee and caffeine in a resistance exercise protocol. The findings of Trexler (37) support the present study in that coffee is able to improve resistance exercise performance and also implies that coffee could potentially impact maximal strength during lower limb exercise to a greater extent than anhydrous caffeine.
The effects of COF and D + C were less pronounced during the bench press protocol and are in contrast with others that have examined caffeine supplementation and bench press performance (16,17). However, the practical significance of the findings may need further investigation because the total weight lifted under the COF condition was 10% greater than that lifted under DEC and 6% greater than that under PLA. In addition, during CAF, the weight lifted was 9% greater than that lifted under DEC and 5% greater than that under PLA, and during the D + C trial, the weight lifted was 6% greater than that lifted under DEC and 3% greater than that lifted under PLA. Timmins and Saunders (36) suggested that the effect of caffeine may be dependent on the muscle group size and consequently, the number of adenosine receptors, which may partially explain the contrasting results between squat and bench press. Therefore, further investigation is required to ascertain whether these treatments are able to significantly improve performance over multiple bouts of resistance exercise or are muscle mass specific.
Previously, Graham et al. (20) observed that 4.45 mg·kg−1 of anhydrous caffeine significantly improved exhaustion running time at 85% of maximal oxygen consumption, and both dose-matched COF and D + C did not. However, more recently Hodgson et al. (21) reported that time trial performance improved by similar magnitudes when supplementing with CAF or COF. These inconsistencies may be the result of the different exercise protocols used. Graham et al. (20) used a time to exhaustion test that has proven to be highly variable from day to day (approximately 27%) (23), whereas Hodgson et al. (21) used a time trial that has been shown to be much more reproducible (approximately 3% variance) (23) and would be more likely to allow subtle differences in performance to be detected.
Graham et al. (20) concluded that because there were no differences in the bioavailability of caffeine between trials, there must be other components in coffee (i.e., chlorogenic acids) that blunted the ergogenic effects of caffeine. Furthermore, de Paulis et al. (14) demonstrated that coffee caused a blunted response of adrenaline, which was attributed to chlorogenic acids antagonizing the binding of caffeine to adenosine receptors. In addition, Hodgson et al. (21) reported that the significant increase in glucose, fatty acids, and glycerol found when supplementing with caffeine had an attenuated response in coffee and was significantly blunted in the decaffeinated trial compared with placebo. This again was attributed to other compounds in coffee that induce subtle effects on the antagonism of A1 and A2A adenosine receptors (21). Despite these findings, performance improvements observed by Hodgson et al. (21) were similar for both caffeine and coffee, leading the authors to conclude that it was unclear on why compounds in coffee modulated metabolite responses but not the ergogenic effects on performance.
Although the potential mechanisms that explain the findings of the present study are beyond the scope of this article, it may be possible that other components of coffee have a beneficial effect on performance in synergy with caffeine. It could also be possible that the antioxidant properties of polyphenols present in coffee could influence performance. Although Graham et al. (20) reported no difference in the bioavailability of caffeine between trials, in the present study, it may be possible that different water temperatures and delivery vehicles affected the bioavailability of caffeine. Furthermore, it may have been possible that absorption of caffeine via the buccal cavity may have taken place in the coffee-containing trials. Liguori et al. (28) reported that peak saliva caffeine levels of coffee consumption appeared to be greater and occurred faster than those of caffeine capsules (42 and 67 minutes, respectively). This may mean that an hour was not sufficient to allow the same plasma concentrations to be achieved from the caffeine capsules in the present study. In support of this, Vanderveen et al. (38) suggested that the bioavailability of caffeine from capsules and tablets may be less than when caffeine is consumed in oral liquids. Additionally, the caffeine in the CAF trial was contained in capsules, but the coffee trials were consumed with 600 ml of hot water, meaning there may have been some absorption of caffeine through the buccal cavity because this is the suggested mechanism of how caffeine from chewing gum is absorbed faster than orally ingested caffeine capsules (24). Moreover, ingestion of a hot beverage can produce mild stimulation of autonomic nervous system function, as well as positive mood changes, without increasing salivary cortisol or state anxiety levels (31), which may be able to positively influence exercise performance. Finally, there may also be adenosine receptors present in the mouth, as they have been found in the cheek pouch of a fellow mammal (32), which may mean caffeine could exert central effects via the buccal cavity; however, caffeine mouth rinsing has not previously had an ergogenic effect on resistance exercise performance (11).
Coffee is rich in polyphenols, which are classified into flavonoids, phenolic acids, lignans, and stilbenes, and it has been shown to have antioxidant potential (39). Although the effects of antioxidants and reactive oxygen species on sporting performance are currently not well understood (9), a cocktail of antioxidants administered before a single bout of lower limb resistance exercise has been shown to improve muscle contractile performance (1). Also, polyphenols have been linked with a number of functions that could enhance exercise performance, such as reducing levels of exercise-induced reactive oxygen species and improved nitric oxide synthesis, which improves blood flow (9). Indeed, Lafay et al. (26) reported that polyphenols from grape extract increased physical performance in handball players and concluded that the enhancement in performance may be at least partly because of the protective action of grape extract during physical exercise.
The present study is not without limitations, as plasma caffeine concentrations were not measured. However, Graham et al. (20) reported no differences in caffeine or methylxanthine plasma concentrations 1 hour after the intake or at the end of exercise for both caffeine and coffee. Furthermore, the temperature at which caffeine is consumed is suggested to determine its bioavailability (10), and therefore, there may have been differences in caffeine concentrations between COF, CAF, and D + C. Finally, this experiment cannot be considered double blind because some of the trials were capsules and some were fluid, which left participants with some idea of the trial they were consuming.
In conclusion, COF and D + C improved resistance exercise performance, and therefore, coffee could be used as an inexpensive alternative to caffeine ingestion before resistance exercise. Further research should be conducted to establish the mechanisms behind the enhanced performance, as well as optimal timings, dosage, and what other compounds in coffee, if any, may be enhancing performance.
The practical implications of the present study are that the ingestion of caffeinated coffee, providing 5 mg·kg−1 of caffeine (approximately 2 large cups, i.e., 300 ml per cup) before resistance exercise, can enhance performance. However, the improvements may only relate to squat performance and not the bench press. The fact that COF and D + C conditions improved squat performance but not the subsequent bench press performance suggests that there may need to be consideration of when to use these treatments because they may not be effective over multiple bouts of resistance exercise, although this possibility requires further investigation.
In the present study, the amount of caffeine from coffee used was 5 mg·kg−1, which when consumed with hot water equated to 2 cups of very strong tasting coffee, which may not appealing or practical to many people. Furthermore, the temperature and volume of the hot water means that it takes an individual in the region of 10–15 minutes to fully consume, which may not be practical or time efficient for athletes. Further research should be conducted to observe if enhancements in performance can be procured from a lower dose of caffeine from coffee, for example, 3 mg·kg−1, which would be a more practical amount of coffee to drink (approximately 1 large cup, i.e., 300 ml).
1. Ackerman J, Clifford T, McNaughton LR, Bentley DJ. The effect of an acute antioxidant supplementation compared with placebo on performance and hormonal response during a high volume resistance training session. J Int Soc Sports Nutr 11: 10, 2014.
2. Ahrens JN, Crixell SH, Lloyd LK, Walker JL. The physiological effects of caffeine
in women during treadmill walking. J Strength Cond Res 21: 164–168, 2007.
3. Astorino TA, Roberson DW. Efficacy of acute caffeine
ingestion for short-term high-intensity exercise performance: A systematic review. J Strength Cond Res 24: 257–265, 2010.
4. Astorino TA, Rohmann RL, Firth K. Effect of caffeine
ingestion on one-repetition maximum muscular strength. Eur J Appl Physiol 102: 127–132, 2008.
5. Baechle TR, Earle RW. Essentials of Strength Training and Conditioning. Leeds: Human Kinetics, 2008.
6. Beaven CM, Maulder P, Pooley A, Kilduff L, Cook C. Effects of caffeine
and carbohydrate mouth rinses on repeated sprint performance. Appl Physiol Nutr Metab 38: 633–637, 2013.
7. Behrens M, Mau-Moeller A, Weippert M, Fuhrmann J, Wegner K, Skripitz R, Bader R, Bruhn S. Caffeine
-induced increase in voluntary activation and strength of the quadriceps muscle during isometric, concentric and eccentric contractions. Sci Rep 5, 2015.
8. Borg GA. Psychophysical bases of perceived exertion. Med Sci Sports Exercise 14: 377–381, 1982.
9. Braakhuis AJ, Hopkins WG. Impact of dietary antioxidants on sport performance: A review. Sports Med 45: 939–955, 2015.
10. Cendrowska-Pinkosz M, Łuszczewska-Sierakowska I, Dworzański W, Tomaszewski M, Krauze M, Grzebalska AM, Madej-Czerwonka B, Burdan F. Influence of caffeine
used at various temperature ranges on the concentrations of glucose and total serum protein as well as body weight gain in pregnant rats. Bull Vet Inst Pulawy 58: 311–314, 2014.
11. Clarke ND, Kornilios E, Richardson DL. Carbohydrate and caffeine
mouth rinses do not affect maximum strength and muscular endurance performance. J Strength Cond Res, 2015.
12. Clauson KA, Shields KM, McQueen CE, Persad N. Safety issues associated with commercially available energy drinks. J Am Pharm Assoc (2003) 48: e55–63. quiz e64–57, 2008.
13. Davis JK, Green JM. Caffeine
and anaerobic performance: Ergogenic value and mechanisms of action. Sports Med 39: 813–832, 2009.
14. de Paulis T, Schmidt DE, Bruchey AK, Kirby MT, McDonald MP, Commers P, Lovinger DM, Martin PR. Dicinnamoylquinides in roasted coffee
inhibit the human adenosine transporter. Eur J Pharmacol 442: 215–223, 2002.
15. Desbrow B, Leveritt M. Awareness and use of caffeine
by athletes competing at the 2005 Ironman Triathlon World Championships. Int J Sport Nutr Exercise Metab 16: 545–558, 2006.
16. Duncan MJ, Oxford SW. The effect of caffeine
ingestion on mood state and bench press performance to failure. J Strength Cond Res 25: 178–185, 2011.
17. Duncan MJ, Oxford SW. Acute caffeine
ingestion enhances performance and dampens muscle pain following resistance exercise to failure. J Sports Med Phys Fitness 52: 280–285, 2012.
18. Gonzalez de Mejia E, Ramirez-Mares MV. Impact of caffeine
on our health. Trends Endocrinol Metab 25: 489–492, 2014.
19. Graham TE. Caffeine
and exercise: Metabolism, endurance and performance. Sports Med 31: 785–807, 2001.
20. Graham TE, Hibbert E, Sathasivam P. Metabolic and exercise endurance effects of coffee
ingestion. J Appl Physiol 85: 883–889, 1998.
21. Hodgson AB, Randell RK, Jeukendrup AE. The metabolic and performance effects of caffeine
compared to coffee
during endurance exercise. PLoS One 8: e59561, 2013.
22. Hopkins WG, Marshall SW, Batterham AM, Hanin J. Progressive statistics for studies in sports medicine and exercise science. Med Sci Sports Exercise 41: 3–13, 2009.
23. Jeukendrup A, Saris WH, Brouns F, Kester AD. A new validated endurance performance test. Med Sci Sports Exercise 28: 266–270, 1996.
24. Kamimori GH, Karyekar CS, Otterstetter R, Cox DS, Balkin TJ, Belenky GL, Eddington ND. The rate of absorption and relative bioavailability of caffeine
administered in chewing gum versus capsules to normal healthy volunteers. Int J Pharm 234: 159–167, 2002.
25. Kraemer WJ, Ratamess NA. Fundamentals of resistance training: Progression and exercise prescription. Med Sci Sports Exercise 36: 674–688, 2004.
26. Lafay S, Jan C, Nardon K, Lemaire B, Ibarra A, Roller M, Houvenaeghel M, Juhel C, Cara L. Grape extract improves antioxidant status and physical performance
in elite male athletes. J Sports Sci Med 8: 468–480, 2009.
27. Landrum RE, Meliska CJ, Loke WH. Effects of caffeine
and task experience on task performance. Psychologia, 1988.
28. Liguori A, Hughes JR, Grass JA. Absorption and subjective effects of caffeine
, cola and capsules. Pharmacol Biochem Behav 58: 721–726, 1997.
29. Magkos F, Kavouras SA. Caffeine
use in sports, pharmacokinetics in man, and cellular mechanisms of action. Crit Rev Food Sci Nutr 45: 535–562, 2005.
30. Mohr M, Nielsen JJ, Bangsbo J. Caffeine
intake improves intense intermittent exercise performance and reduces muscle interstitial potassium accumulation. J Appl Physiol 111: 1372–1379, 2011.
31. Quinlan P, Lane J, Aspinall L. Effects of hot tea, coffee
and water ingestion on physiological responses and mood: The role of caffeine
, water and beverage type. Psychopharmacology (Berl) 134: 164–173, 1997.
32. Rubinstein I, Chandilawa R, Dagar S, Hong D, Gao X-P. Adenosine A1 receptors mediate plasma exudation from the oral mucosa. J Appl Physiol 91: 552–560, 2001.
33. Svebak S, Murgatroyd S. Metamotivational dominance: A multimethod validation of reversal theory constructs. J Pers Soc Psychol 48: 107, 1985.
34. Syed SA, Kamimori GH, Kelly W, Eddington ND. Multiple dose pharmacokinetics of caffeine
administered in chewing gum to normal healthy volunteers. Biopharm Drug Dispos 26: 403–409, 2005.
35. Tarnopolsky MA. Effect of caffeine
on the neuromuscular system–potential as an ergogenic aid. Appl Physiol Nutr Metab 33: 1284–1289, 2008.
36. Timmins TD, Saunders DH. Effect of caffeine
ingestion on maximal voluntary contraction strength in upper- and lower-body muscle groups. J Strength Cond Res 28: 3239–3244, 2014.
37. Trexler ET. Effects of creatine, coffee
, and caffeine
anhydrous on strength and sprint performance: The University of North Carolina at Chapel Hill, 2015.
38. Vanderveen J, Armstrong L, Butterfield G, Chenoweth W, Dwyer J, Fernstrom J, Kanarek R, Levander O, Sternberg E. Caffeine
for the Sustainment of Mental Task Performance: Formulations for Military Operations: National Academy. Washington, DC, 2001.
39. Wang Y, Ho CT. Polyphenolic chemistry of tea and coffee
: A century of progress. J Agric Food Chem 57: 8109–8114, 2009.
40. Wiens K, Erdman KA, Stadnyk M, Parnell JA. Dietary supplement usage, motivation, and education in young, Canadian athletes. Int J Sport Nutr Exercise Metab 24: 613–622, 2014.