Caffeine ingestion and metabolic responses of tetraplegic humans during electrical cycling

Caffeine during High-Intensity Whole-Body Exercise: An Integrative Approach beyond the Central Nervous System

Caffeine is one of the most consumed ergogenic aids around the world. Many studies support the ergogenic effect of caffeine over a large spectrum of exercise types. While the stimulatory effect of caffeine on the central nervous system is the well-accepted mechanism explaining improvements in exercise performance during high-intensity whole-body exercise, in which other physiological systems such as pulmonary, cardiovascular, and muscular systems are maximally activated, a direct effect of caffeine on such systems cannot be ignored. A better understanding of the effects of caffeine on multiple physiological systems during high-intensity whole-body exercise might help to expand its use in different sporting contexts (e.g., competitions in different environments, such as altitude) or even assist the treatment of some diseases (e.g., chronic obstructive pulmonary disease). In the present narrative review, we explore the potential effects of caffeine on the pulmonary, cardiovascular, and muscular systems, and describe how such alterations may interact and thus contribute to the ergogenic effects of caffeine during high-intensity whole-body exercise. This integrative approach provides insights regarding how caffeine influences endurance performance and may drive further studies exploring its mechanisms of action in a broader perspective.

Effectiveness of caffeine intake on maximal and sub-maximal physiological markers of exercise intensity among wheelchair users compared to able-bodied individuals

The aim of the current study was to assess the effect of caffeine intake on maximal and sub-maximal physiological markers of exercise intensity and whether group (able-bodied and wheelchair users) moderated these findings. Ten able-bodied men (20.3±2.4 years, length 174.3±5.1 cm, 76.4±9.4 kg) and 9 wheelchair users (29.9±7.1 years, length 164±13 cm, 78.6±20.6 kg) participated in the study. Each participant performed 4 exercise tests. Two ramp exercise tests were used to assess the effect of caffeine intake on maximal values of power output (PO), oxygen uptake (VO2), heart rate (HR), lactate and rate of perceived exertion (RPE): one performed with 6 mg/kg body mass of caffeine ingestion as gelatine capsules, and the second one with placebo. Two constant-load exercise tests at 70% POpeak to volitional exhaustion were used to assess the effect caffeine intake on sub-maximal values of VO2, HR, lactate and RPE: one performed with 6 mg/kg body mass of caffeine ingestion as gelatine capsules, and the second one with placebo. Two way ANOVA revealed that caffeine intake does not affect maximal values of VO2, HR, lactate and RPE (P>0.05). Caffeine intake reduced sub-maximal RPE at 5 min (P<0.05) and 10 min of exercise. Sub-maximal HR at 70% POpeak was higher in caffeine than placebo among wheelchair users (P<0.05). Time to exhaustion at 70% POpeak was significantly longer in caffeine than placebo (P<0.05). VO2max and POpeak were significantly higher among able-bodied than wheelchair users (P<0.05). Caffeine has an effect on sub-maximal RPE and time to volitional exhaustion. Closed-loop exercise mode should be employed in future studies. Greater dosage of caffeine could be used but should not exceed the permitted amount of 12 mg/kg body mass. Wheelchair users should exercise and do more physical activity to enhance VO2max and POpeak.

The Influence of Caffeine Supplementation on Resistance Exercise: A Review

This paper aims to critically evaluate and thoroughly discuss the evidence on the topic of caffeine supplementation when performing resistance exercise, as well as provide practical guidelines for the ingestion of caffeine prior to resistance exercise. Based on the current evidence, it seems that caffeine increases both maximal strength and muscular endurance. Furthermore, power appears to be enhanced with caffeine supplementation, although this effect might, to a certain extent, be caffeine dose- and external load-dependent. A reduction in rating of perceived exertion (RPE) might contribute to the performance-enhancing effects of caffeine supplementation as some studies have observed decreases in RPE coupled with increases in performance following caffeine ingestion. However, the same does not seem to be the case for pain perception as there is evidence showing acute increases in resistance exercise performance without any significant effects of caffeine ingestion on pain perception. Some studies have reported that caffeine ingestion did not affect exercise-induced muscle damage, but that it might reduce perceived resistance exercise-induced delayed-onset muscle soreness; however, this needs to be explored further. There is some evidence that caffeine ingestion, compared with a placebo, may lead to greater increases in the production of testosterone and cortisol following resistance exercise. However, given that the acute changes in hormone levels seem to be weakly correlated with hallmark adaptations to resistance exercise, such as hypertrophy and increased muscular strength, these findings are likely of questionable practical significance. Although not without contrasting findings, the available evidence suggests that caffeine ingestion can lead to acute increases in blood pressure (primarily systolic), and thus caution is needed regarding caffeine supplementation among individuals with high blood pressure. In the vast majority of studies, caffeine was administered in capsule or powder forms, and therefore the effects of alternative forms of caffeine, such as chewing gums or mouth rinses, on resistance exercise performance remain unclear. The emerging evidence suggests that coffee might be at least equally ergogenic as caffeine alone when the caffeine dose is matched. Doses in the range of 3-9 mg·kg^-1 seem to be adequate for eliciting an ergogenic effect when administered 60 min pre-exercise. In general, caffeine seems to be safe when taken in the recommended doses. However, at doses as high as 9 mg·kg^-1 or higher, side effects such as insomnia might be more pronounced. It remains unclear whether habituation reduces the ergogenic benefits of caffeine on resistance exercise as no evidence exists for this type of exercise. Caution is needed when extrapolating these conclusions to females as the vast majority of studies involved only male participants.

MUSCLE STRENGTH AND CAFFEINE SUPPLEMENTATION: ARE WE DOING MORE OF THE SAME?

ABSTRACT The purpose of this review was to examine in the current literature the advances made in terms of the effects of caffeine supplementation on maximum strength and its associated mechanisms since the publication of two important papers in 2010. Searches were carried out in the PubMed, Medline, Scielo and Web of Science databases for articles published after 2010. Sixteen studies were included based on inclusion and exclusion criteria. Five studies did not report changes in maximal voluntary strength (31.3%). Four of them used isometric muscle contractions, although this may not be a key factor because five other studies also used isometric contractions and reported ergogenic effects. Furthermore, these four studies evaluated small muscle groups and volunteers were not accustomed to consuming caffeine. Caffeine produced ergogenic effects in eleven of the sixteen studies analyzed (68.8%). None of the doses were clearly related to ergogenic effects; however, a dose of at least 3 mg/kg of caffeine is probably necessary. Caffeine ergogenicity was affected by various factors. There was a lack of standardized protocols and controls for intervening factors (e.g., circadian cycles and nutritional states), which could affect results. An ideal caffeine supplementation protocol that is useful for future research, athletes, and physical activity practitioners, has yet to be defined. A small advance made since 2010 involved a possible lack of gender difference; it would appear that caffeine supplementation affects men and women equally. Level of Evidence I; Systematic Review of Level I Studies.

Toxic doses of caffeine are needed to increase skeletal muscle contractility

Discrepant results have been reported regarding an intramuscular mechanism underlying the ergogenic effect of caffeine on neuromuscular function in humans. Here, we reevaluated the effect of caffeine on muscular force production in humans and combined this with measurements of the caffeine dose-response relationship on force and cytosolic free [Ca²⁺] ([Ca²⁺]_i) in isolated mouse muscle fibers. Twenty-one healthy and physically active men (29 ± 9 yr, 178 ± 6 cm, 73 ± 10 kg, mean ± SD) took part in the present study. Nine participants were involved in two experimental sessions during which supramaximal single and paired electrical stimulations (at 10 and 100 Hz) were applied to the femoral nerve to record evoked forces. Evoked forces were recorded before and 1 h after ingestion of 1) 6 mg caffeine/kg body mass or 2) placebo. Caffeine plasma concentration was measured in 12 participants. In addition, submaximal tetanic force and [Ca²⁺]_i were measured in single mouse flexor digitorum brevis (FDB) muscle fibers exposed to 100 nM up to 5 mM caffeine. Six milligrams of caffeine per kilogram body mass (plasma concentration ~40 µM) did not increase electrically evoked forces in humans. In superfused FDB single fibers, millimolar caffeine concentrations (i.e., 15- to 35-fold above usual concentrations observed in humans) were required to increase tetanic force and [Ca²⁺]_i. Our results suggest that toxic doses of caffeine are required to increase muscle contractility, questioning the purported intramuscular ergogenic effect of caffeine in humans.

Spinal Cord Injury Level Influences Acute Plasma Caffeine Responses

PURPOSE

This study aimed to investigate the absorption curve and acute effects of caffeine at rest in individuals with no spinal cord injury (SCI), paraplegia (PARA), and tetraplegia (TETRA).

METHODS

Twenty-four healthy males (eight able-bodied [AB], eight PARA, and eight TETRA) consumed 3 mg·kg caffeine anhydrous (CAF) in a fasted state. Plasma caffeine [CAF], glucose, lactate, free fatty acid, and catecholamine concentrations were measured during a 150-min rest period.

RESULTS

Peak [CAF] was greater in TETRA (21.5 μM) compared with AB (12.2 μM) and PARA (15.1 μM), and mean peak [CAF] occurred at 70, 80, and 80 min, respectively. Moderate and large effect sizes were revealed for TETRA compared with PARA and AB (-0.55 and -1.14, respectively) for the total area under the [CAF] versus time curve. Large interindividual responses were apparent in SCI groups. The change in plasma catecholamine concentrations after CAF did not reach significance (P > 0.05); however, both adrenaline and noradrenaline concentrations were lowest in TETRA. Significant increases in free fatty acid were seen over time (P < 0.0005), but there was no significant influence of SCI level. Blood lactate concentration reduced over time (P = 0.022), whereas blood glucose concentration decreased modestly (P = 0.695), and no difference between groups was seen (P > 0.05).

CONCLUSION

The level of SCI influenced the caffeine absorption curve, and there was large interindividual variation within and between groups. Individual curves should be considered when using caffeine as an ergogenic aid in athletes with an SCI. The results indicate TETRA should trial low doses in training and PARA may consider consuming caffeine greater than 60 min before exercise performance. The study also supports caffeine's direct effect on adipose tissue, which is not secondary to catecholamine release.

Overexpressed eNOS upregulates SIRT1 expression and protects mouse pancreatic β cells from apoptosis

Loss of sirtuin 1 (SIRT1) activity may be associated with metabolic diseases, including diabetes. The aim of the present study was to investigate the potential effects of overexpressed endothelial nitric oxide synthase (eNOS) on cell proliferation and apoptosis with SIRT1 activation in the Min6 mouse pancreatic β cell line. A pcDNA3.0-eNOS plasmid was constructed and transfected into Min6 cells for 24 h prior to harvesting. eNOS expression was validated and SIRT1 expression was detected following plasmid transfection using reverse transcription-quantitative polymerase chain reaction and western blot analysis, which demonstrated that the expression levels of eNOS and SIRT1 were significantly upregulated. Furthermore, the cell proliferation and cell apoptosis of the Min6 cells were evaluated, using a cell counting kit-8 assay and flow cytometry, respectively. The results suggested that overexpressed eNOS promoted cell proliferation and inhibited cell apoptosis in Min6 cells. The interaction between eNOS and SIRT1 was explored through co-immunoprecipitation, and it found that there was a strong interaction between eNOS and SIRT1. In conclusion, overexpressed eNOS may induce SIRT1 activation, which is implied to play a protective role in Min6 cells, and eNOS may be a new therapeutic target for diseases such as type 2 diabetes.

[MUSCLE FATIGUE: FACTORS OF DEVELOPMENT AND WAYS OF CORRECTION]

The data regarding the analysis of the physiological and biochemical mechanisms of muscle fatigue and ways to prevent it are summarized. The effect of the most common endogenous and exogenous antioxidants in the biochemical processes in muscle fatigue was analyzed. It is shown that biocompatible, non-toxic water-soluble C(60) fullerenes, which possess powerful antioxidative properties, promise great prospects in the correction of skeletal muscle fatigue caused by the destructive action of free radicals.

Effects of Coffee and Caffeine Anhydrous Intake During Creatine Loading

The purpose of this study was to determine the effect of 5 days of creatine (CRE) loading alone or in combination with caffeine anhydrous (CAF) or coffee (COF) on upper-body and lower-body strength and sprint performance. Physically active males (n = 54; mean ± SD; age = 20.1 ± 2.1 years; weight = 78.8 ± 8.8 kg) completed baseline testing, consisting of 1 repetition maximum (1RM) and repetitions to fatigue with 80% 1RM for bench press and leg press, followed by a repeated sprint test of five, 10-second sprints separated by 60-second rest on a cycle ergometer to determine peak power (PP) and total power (TP). At least 72 hours later, subjects were randomly assigned to supplement with CRE (5 g of CRE monohydrate, 4 times per day; n = 14), CRE + CAF (CRE +300 mg·d of CAF; n = 13), CRE + COF (CRE +8.9 g of COF, yielding 303 mg of CAF; n = 13), or placebo (PLA; n = 14) for 5 days. Serum creatinine (CRN) was measured before and after supplementation, and on day 6, participants repeated pretesting procedures. Strength measures were improved in all groups (p ≤ 0.05), with no significant time × treatment interactions. No significant interaction or main effects were observed for PP. For TP, a time × sprint interaction was observed (p ≤ 0.05), with no significant interactions among treatment groups. A time × treatment interaction was observed for serum CRN values (p ≤ 0.05) that showed increases in all groups except PLA. Four subjects reported mild gastrointestinal discomfort with CRE + CAF, with no side effects reported in other groups. These findings suggest that neither CRE alone nor in combination with CAF or COF significantly affected performance compared with PLA.

Effects of caffeine on neuromuscular fatigue and performance during high-intensity cycling exercise in moderate hypoxia

Purpose

To investigate the effects of caffeine on performance, neuromuscular fatigue and perception of effort during high-intensity cycling exercise in moderate hypoxia.

Methods

Seven adult male participants firstly underwent an incremental exercise test on a cycle ergometer in conditions of acute normobaric hypoxia (fraction inspired oxygen = 0.15) to establish peak power output (PPO). In the following two visits, they performed a time to exhaustion test (78 ± 3% PPO) in the same hypoxic conditions after caffeine ingestion (4 mg kg⁻¹) and one after placebo ingestion in a double-blind, randomized, counterbalanced cross-over design.

Results

Caffeine significantly improved time to exhaustion by 12%. A significant decrease in subjective fatigue was found after caffeine consumption. Perception of effort and surface electromyographic signal amplitude of the vastus lateralis were lower and heart rate was higher in the caffeine condition when compared to placebo. However, caffeine did not reduce the peripheral and central fatigue induced by high-intensity cycling exercise in moderate hypoxia.

Conclusion

The caffeine-induced improvement in time to exhaustion during high-intensity cycling exercise in moderate hypoxia seems to be mediated by a reduction in perception of effort, which occurs despite no reduction in neuromuscular fatigue.

Acute Effects of Caffeine on Heart Rate Variability, Blood Pressure and Tidal Volume in Paraplegic and Tetraplegic Compared to Able-Bodied Individuals: A Randomized, Blinded Trial

Caffeine increases sympathetic nerve activity in healthy individuals. Such modulation of nervous system activity can be tracked by assessing the heart rate variability. This study aimed to investigate the influence of caffeine on time- and frequency-domain heart rate variability parameters, blood pressure and tidal volume in paraplegic and tetraplegic compared to able-bodied participants. Heart rate variability was measured in supine and sitting position pre and post ingestion of either placebo or 6 mg caffeine in 12 able-bodied, 9 paraplegic and 7 tetraplegic participants in a placebo-controlled, randomized and double-blind study design. Metronomic breathing was applied (0.25 Hz) and tidal volume was recorded during heart rate variability assessment. Blood pressure, plasma caffeine and epinephrine concentrations were analyzed pre and post ingestion. Most parameters of heart rate variability did not significantly change post caffeine ingestion compared to placebo. Tidal volume significantly increased post caffeine ingestion in able-bodied (p = 0.021) and paraplegic (p = 0.036) but not in tetraplegic participants (p = 0.34). Systolic and diastolic blood pressure increased significantly post caffeine in able-bodied (systolic: p = 0.003; diastolic: p = 0.021) and tetraplegic (systolic: p = 0.043; diastolic: p = 0.042) but not in paraplegic participants (systolic: p = 0.09; diastolic: p = 0.33). Plasma caffeine concentrations were significantly increased post caffeine ingestion in all three groups of participants (p<0.05). Plasma epinephrine concentrations increased significantly in able-bodied (p = 0.002) and paraplegic (p = 0.032) but not in tetraplegic participants (p = 0.63). The influence of caffeine on the autonomic nervous system seems to depend on the level of lesion and the extent of the impairment. Therefore, tetraplegic participants may be less influenced by caffeine ingestion.

Effect of caffeine ingestion on maximal voluntary contraction strength in upper- and lower-body muscle groups

The effect of caffeine on strength-power performance is equivocal, especially with regard to maximal voluntary contraction (MVC) strength. This is partly related to differences in upper- and lower-body musculature. However, there is no evidence to suggest whether this is a product of muscle group location, muscle group size, or both. Consequently, the primary aim of this study was to establish whether the effect of caffeine ingestion on MVC strength in upper- and lower-body muscle groups is significantly different, and if so, to determine whether this is a product of muscle group size. In a randomized, subject-blind crossover manner, 16 resistance-trained men (estimated caffeine intake [mean ± SD] 95.4 ± 80.0 mg·d) received either 6 mg·kg of caffeine (CAF) or a placebo (PLA). Isokinetic peak torque of the knee extensors, ankle plantar flexors, elbow flexors and wrist flexors were measured at an angular velocity of 60°·s. Statistical analyses revealed a significant increase in isokinetic peak torque from PLA to CAF (p = 0.011) and a significant difference in isokinetic peak torque between muscle groups (p < 0.001). However, there was no significant treatment × muscle group interaction (p = 0.056). Nonetheless, the %improvement in isokinetic peak torque with caffeine increased with muscle group size. In conclusion, a moderate dose of caffeine improves MVC strength in resistance-trained men regardless of muscle group location, whereas the influence of muscle group size remains uncertain. This research may be useful for competitive and recreational athletes aiming to increase strength-power performance.

Effects of coffee and caffeine anhydrous on strength and sprint performance

Caffeine and coffee are widely used among active individuals to enhance performance. The purpose of the current study was to compare the effects of acute coffee (COF) and caffeine anhydrous (CAF) intake on strength and sprint performance. Fifty-four resistance-trained males completed strength testing, consisting of one-rep max (1RM) and repetitions to fatigue (RTF) at 80% of 1RM for leg press (LP) and bench press (BP). Participants then completed five, 10-second cycle ergometer sprints separated by one minute of rest. Peak power (PP) and total work (TW) were recorded for each sprint. At least 48 hours later, participants returned and ingested a beverage containing CAF (300 mg flat dose; yielding 3-5 mg/kg bodyweight), COF (8.9 g; 303 mg caffeine), or placebo (PLA; 3.8 g non-caloric flavouring) 30 minutes before testing. LP 1RM was improved more by COF than CAF (p = .04), but not PLA (p = .99). Significant interactions were not observed for BP 1RM, BP RTF, or LP RTF (p > .05). There were no sprint × treatment interactions for PP or TW (p > .05). 95% confidence intervals revealed a significant improvement in sprint 1 TW for CAF, but not COF or PLA. For PLA, significant reductions were observed in sprint 4 PP, sprint 2 TW, sprint 4 TW, and average TW; significant reductions were not observed with CAF or COF. Neither COF nor CAF improved strength outcomes more than PLA, while both groups attenuated sprint power reductions to a similar degree. Coffee and caffeine anhydrous may be considered suitable pre-exercise caffeine sources for high-intensity exercise.

Performance effects and metabolic consequences of caffeine and caffeinated energy drink consumption on glucose disposal

This review documents two opposing effects of caffeine and caffeine-containing energy drinks, i.e., their positive effects on athletic performance and their negative impacts on glucose tolerance in the sedentary state. Analysis of studies examining caffeine administration prior to performance-based exercise showed caffeine improved completion time by 3.6%. Similar analyses following consumption of caffeine-containing energy drinks yielded positive, but more varied, benefits, which were likely due to the diverse nature of the studies performed, the highly variable composition of the beverages consumed, and the range of caffeine doses administered. Conversely, analyses of studies administering caffeine prior to either an oral glucose tolerance test or insulin clamp showed a decline in whole-body glucose disposal of ~30%. The consequences of this resistance are unknown, but there may be implications for the development of a number of chronic diseases. Both caffeine-induced performance enhancement and insulin resistance converge with the primary actions of caffeine on skeletal muscle.

The metabolic and performance effects of caffeine compared to coffee during endurance exercise

There is consistent evidence supporting the ergogenic effects of caffeine for endurance based exercise. However, whether caffeine ingested through coffee has the same effects is still subject to debate. The primary aim of the study was to investigate the performance enhancing effects of caffeine and coffee using a time trial performance test, while also investigating the metabolic effects of caffeine and coffee. In a single-blind, crossover, randomised counter-balanced study design, eight trained male cyclists/triathletes (Mean ± SD: Age 41 ± 7 y, Height 1.80 ± 0.04 m, Weight 78.9 ± 4.1 kg, VO2 max 58 ± 3 ml • kg(-1) • min(-1)) completed 30 min of steady-state (SS) cycling at approximately 55% VO2max followed by a 45 min energy based target time trial (TT). One hour prior to exercise each athlete consumed drinks consisting of caffeine (5 mg CAF/kg BW), instant coffee (5 mg CAF/kg BW), instant decaffeinated coffee or placebo. The set workloads produced similar relative exercise intensities during the SS for all drinks, with no observed difference in carbohydrate or fat oxidation. Performance times during the TT were significantly faster (~5.0%) for both caffeine and coffee when compared to placebo and decaf (38.35 ± 1.53, 38.27 ± 1.80, 40.23 ± 1.98, 40.31 ± 1.22 min respectively, p<0.05). The significantly faster performance times were similar for both caffeine and coffee. Average power for caffeine and coffee during the TT was significantly greater when compared to placebo and decaf (294 ± 21 W, 291 ± 22 W, 277 ± 14 W, 276 ± 23 W respectively, p<0.05). No significant differences were observed between placebo and decaf during the TT. The present study illustrates that both caffeine (5 mg/kg/BW) and coffee (5 mg/kg/BW) consumed 1 h prior to exercise can improve endurance exercise performance.

Caffeine ingestion reverses the circadian rhythm effects on neuromuscular performance in highly resistance-trained men

PURPOSE

To investigate whether caffeine ingestion counteracts the morning reduction in neuromuscular performance associated with the circadian rhythm pattern.

METHODS

Twelve highly resistance-trained men underwent a battery of neuromuscular tests under three different conditions; i) morning (10:00 a.m.) with caffeine ingestion (i.e., 3 mg kg(-1); AM(CAFF) trial); ii) morning (10:00 a.m.) with placebo ingestion (AM(PLAC) trial); and iii) afternoon (18:00 p.m.) with placebo ingestion (PM(PLAC) trial). A randomized, double-blind, crossover, placebo controlled experimental design was used, with all subjects serving as their own controls. The neuromuscular test battery consisted in the measurement of bar displacement velocity during free-weight full-squat (SQ) and bench press (BP) exercises against loads that elicit maximum strength (75% 1RM load) and muscle power adaptations (1 m s(-1) load). Isometric maximum voluntary contraction (MVC(LEG)) and isometric electrically evoked strength of the right knee (EVOK(LEG)) were measured to identify caffeine's action mechanisms. Steroid hormone levels (serum testosterone, cortisol and growth hormone) were evaluated at the beginning of each trial (PRE). In addition, plasma norepinephrine (NE) and epinephrine were measured PRE and at the end of each trial following a standardized intense (85% 1RM) 6 repetitions bout of SQ (POST).

RESULTS

In the PM(PLAC) trial, dynamic muscle strength and power output were significantly enhanced compared with AM(PLAC) treatment (3.0%-7.5%; p≤0.05). During AM(CAFF) trial, muscle strength and power output increased above AM(PLAC) levels (4.6%-5.7%; p≤0.05) except for BP velocity with 1 m s(-1) load (p = 0.06). During AM(CAFF), EVOK(LEG) and NE (a surrogate of maximal muscle sympathetic nerve activation) were increased above AM(PLAC) trial (14.6% and 96.8% respectively; p≤0.05).

CONCLUSIONS

These results indicate that caffeine ingestion reverses the morning neuromuscular declines in highly resistance-trained men, raising performance to the levels of the afternoon trial. Our electrical stimulation data, along with the NE values, suggest that caffeine increases neuromuscular performance having a direct effect in the muscle.

Influence of chronic and acute spinal cord injury on skeletal muscle Na+-K+-ATPase and phospholemman expression in humans

Na(+)-K(+)-ATPase is an integral membrane protein crucial for the maintenance of ion homeostasis and skeletal muscle contractibility. Skeletal muscle Na(+)-K(+)-ATPase content displays remarkable plasticity in response to long-term increase in physiological demand, such as exercise training. However, the adaptations in Na(+)-K(+)-ATPase function in response to a suddenly decreased and/or habitually low level of physical activity, especially after a spinal cord injury (SCI), are incompletely known. We tested the hypothesis that skeletal muscle content of Na(+)-K(+)-ATPase and the associated regulatory proteins from the FXYD family is altered in SCI patients in a manner dependent on the severity of the spinal cord lesion and postinjury level of physical activity. Three different groups were studied: 1) six subjects with chronic complete cervical SCI, 2) seven subjects with acute, complete cervical SCI, and 3) six subjects with acute, incomplete cervical SCI. The individuals in groups 2 and 3 were studied at months 1, 3, and 12 postinjury, whereas individuals with chronic SCI were compared with an able-bodied control group. Chronic complete SCI was associated with a marked decrease in [(3)H]ouabain binding site concentration in skeletal muscle as well as reduced protein content of the α(1)-, α(2)-, and β(1)-subunit of the Na(+)-K(+)-ATPase. In line with this finding, expression of the Na(+)-K(+)-ATPase α(1)- and α(2)-subunits progressively decreased during the first year after complete but not after incomplete SCI. The expression of the regulatory protein phospholemman (PLM or FXYD1) was attenuated after complete, but not incomplete, cervical SCI. In contrast, FXYD5 was substantially upregulated in patients with complete SCI. In conclusion, the severity of the spinal cord lesion and the level of postinjury physical activity in patients with SCI are important factors controlling the expression of Na(+)-K(+)-ATPase and its regulatory proteins PLM and FXYD5.

Methylxanthines and human health: epidemiological and experimental evidence

When considering methylxanthines and human health, it must be recognized that in many countries most caffeine is consumed as coffee. This is further confounded by the fact that coffee contains many bioactive substances in addition to caffeine; it is rich in phenols (quinides, chlorogenic acid, and lactones) and also has diterpenes (fatty acid esters), potassium, niacin, magnesium, and the vitamin B(3) precursor trigonelline. There is a paradox as consumption of either caffeine or caffeinated coffee results in a marked insulin resistance and yet habitual coffee consumption has repeatedly been reported to markedly reduce the risk for type 2 diabetes. There is strong evidence that caffeine reduces insulin sensitivity in skeletal muscle and this may be due to a combination of direct antagonism of A(1) receptors and indirectly β-adrenergic stimulation as a result of increased sympathetic activity. Caffeine may also induce reduced hepatic glucose output. With the exception of bone mineral, there is little evidence that caffeine impacts negatively on other health issues. Coffee does not increase the risk of cardiovascular diseases or cancers and there is some evidence suggesting a positive relationship for the former and for some cancers, particularly hepatic cancer.

Energy drinks: a review of use and safety for athletes

Energy drinks have increased in popularity in adolescents and young adults; however, concerns have been raised regarding the ingredients in energy drinks and their potential negative effects on health. Caffeine, the most physiologically active ingredient in energy drinks, is generally considered safe by the US Food and Drug Administration (FDA), although adverse effects can occur at varying amounts. Guarana, which contains caffeine in addition to small amounts of theobromine, theophylline, and tannins, is also recognized as safe by the FDA, although it may lead to caffeine toxicity when combined with caffeine. The amount of ginseng in energy drinks is typically far below the amount used as a dietary supplement, and is generally considered safe. Taurine, an intracellular amino acid, has been reported to have positive inotropic effects; however, this claim is not supported by research. Most energy drinks also contain sugar in an amount that exceeds the maximum recommended daily amount. Young athletes are increasingly using energy drinks because of the ergogenic effects of caffeine and the other ingredients found in these beverages. Energy drinks combined with alcohol are also gaining popularity in young adults, which poses significant concerns about health risks. Other health concerns related to consumption of energy drinks include case reports of seizures and cardiac arrest following energy drink consumption and dental enamel erosion resulting from the acidity of energy drinks.

The effects of caffeine during exercise in fire protective ensemble

To examine the effects of caffeine during exercise in fire protective ensemble (FPE), 10 healthy males completed 3 x 10 min bouts of treadmill exercise on two separate days. Sixty minutes prior to exercise either 6 mg/kg of caffeine (CAFF) or dextrose placebo (PLA) capsules were ingested (randomly assigned, double blind). End-exercise gastrointestinal temperature (T(gi)) was higher in CAFF compared to PLA (38.80 +/- 0.08 degrees C vs. 38.43 +/- 0.11 degrees C, p < or = 0.01). Ventilation (V(E)) and tidal volume (V(t)) were also significantly higher in CAFF, which resulted in higher consumption of air from the self-contained breathing apparatus. While perceived exertion in the caffeine condition was decreased (p < or = 0.05) compared to placebo, the higher T(gi) values increased calculated physiological strain index in CAFF (p < or = 0.01). Caffeine appears to alter the physiological and psycho-physical responses to exercise in FPE and may influence factors related to work tolerance in firefighting. These findings are relevant to occupations such as firefighting where workers are encapsulated during exposure to heavy physical work and/or environmental heat. The results indicate that workers may be more susceptible to heat-related fatigue, illness or injury with ingestion of significant amounts of caffeine. To the authors' knowledge this is the first study involving humans and exercise to detect an increase in body temperature with caffeine ingestion.

Effect of caffeine on the neuromuscular system--potential as an ergogenic aid

The ergogenic effect of caffeine on endurance exercise performance is multifactorial; however, there is evidence for an effect on both the central nervous system and the excitation-contraction coupling of skeletal muscle. The increase in exercise performance seen following intracerebroventrical caffeine injection in rats provides strong evidence for a central ergogenic effect. The central ergogenic effect is not likely related to the ability of caffeine to promote wakefulness, but could be due to an increase in the pain and effort perception threshold. There is no evidence that caffeine alters peripheral nerve conduction velocity or neuromuscular transmission, and 1 study showed that motor unit synchronization was not altered by caffeine. Studies have also shown that caffeine can have a direct effect on skeletal muscle that could be ergogenic. For example, patients with high cervical spinal cord lesions showed improvements in stimulated contractile force during cycling, in spite of the fact that they have no peripheral pain input and no sympathetic nervous system response. Two studies have found a potentiation of force production during submaximal stimulation intensities, and 1 found that the M-wave amplitude was not altered by caffeine. Together, these studies suggest that caffeine can enhance contractile force during submaximal contractions by potentiating calcium release from the ryanodine receptor, not by altering sarcoplasmic excitability. Furthermore, the potentiation of force during submaximal electrical stimulation is identical in habitual and nonhabitual caffeine consumers. In summary, the ergogenic effects of caffeine during endurance activity are mediated partly by enhanced contractile force and partly by a reduction in perceived exertion, possibly though a blunting of effort and (or) pain.

Does caffeine alter muscle carbohydrate and fat metabolism during exercise?

Caffeine, an adenosine receptor antagonist, has been studied for decades as a putative ergogenic aid. In the past 2 decades, the information has overwhelmingly demonstrated that it indeed is a powerful ergogenic aid, and frequently theories have been proposed that this is due to alterations in fat and carbohydrate metabolism. While caffeine certainly mobilizes fatty acids from adipose tissue, rarely have measures of the respiratory exchange ratio indicated an increase in fat oxidation. However, this is a difficult measure to perform accurately during exercise, and small changes could be physiologically important. The few studies examining human muscle metabolism directly have also supported the fact that there is no change in fat or carbohydrate metabolism, but these usually have had a small sample size. We combined the data from muscle biopsy analyses of several similar studies to generate a sample size of 16-44, depending on the measure. We examined muscle glycogen, citrate, acetyl-CoA, glucose-6-phosphate, and cyclic adenosine monophosphate (cAMP) in resting samples and in those obtained after 10-15 min of exercise at 70%-85% maximal oxygen consumption. Exercise decreased (p < 0.05) glycogen and increased (p < 0.05) citrate, acetyl-CoA, and glucose-6-phosphate. The only effects of caffeine were to increase (p < 0.05) citrate in resting muscle and cAMP in exercise. There is very little evidence to support the hypothesis that caffeine has ergogenic effects as a result of enhanced fat oxidation. Individuals may, however, respond differently to the effects of caffeine, and there is growing evidence that this could be explained by common genetic variations.

Effect of physiological levels of caffeine on Ca2+ handling and fatigue development in Xenopus isolated single myofibers

The purpose of the present study was to determine whether exposure to exogenous physiological concentrations of caffeine influence contractility, Ca(2+) handling, and fatigue development in isolated single Xenopus laevis skeletal muscle fibers. After isolation, two identical contractile periods (separated by 60-min rest) were conducted in each single myofiber (n = 8) at 20 degrees C. During the first contractile period, four fibers were perfused with a noncaffeinated Ringer solution, while the other four fibers were perfused with a caffeinated (70 microM) Ringer solution. The order was reversed for the second contractile period. The single myofibers were stimulated during each contractile period at increasing frequencies (0.16, 0.20, 0.25, 0.33, 0.50, and 1.0 tetanic contractions/s), with each stimulation frequency lasting 2 min until fatigue ensued, defined in this study as a fall in tension development to 66% of maximum. Tension development and free cytosolic [Ca(2+)] (fura-2 fluorescence spectroscopy) were simultaneously measured. There was no significant difference in the peak force generation, time to fatigue, cytosolic Ca(2+) levels, or relaxation times between the noncaffeinated and caffeinated trials. These results demonstrate that physiological levels of caffeine have no significant effect on Xenopus single myofiber contractility, Ca(2+) handling, and fatigue development, and suggest that any ergogenic effects of physiological levels of caffeine on muscle performance during contractions of moderate to high intensity are likely related to factors extraneous to the muscle fiber.

Oxygen consumption during functional electrical stimulation-assisted exercise in persons with spinal cord injury: implications for fitness and health

A lesion in the spinal cord leads in most cases to a significant reduction in active muscle mass, whereby the paralysed muscles cannot contribute to oxygen consumption (VO2) during exercise. Consequently, persons with spinal cord injury (SCI) can only achieve high VO2 values by excessively stressing the upper body musculature, which might increase the risk of musculoskeletal overuse injury. Alternatively, the muscle mass involved may be increased by using functional electrical stimulation (FES). FES-assisted cycling, FES-cycling combined with arm cranking (FES-hybrid exercise) and FES-rowing have all been suggested as candidates for cardiovascular training in SCI. In this article, we review the levels of VO2 (peak [VO2peak] and sub-peak [VO2sub-peak]) that have been reported for SCI subjects using these FES exercise modalities. A systematic literature search in MEDLINE, EMBASE, AMED, CINAHL, SportDiscus and the authors' own files revealed 35 studies that reported on 499 observations of VO2 levels achieved during FES-exercise in SCI. The results show that VO2peak during FES-rowing (1.98 L/min, n = 17; 24.1 mL/kg/min, n = 11) and FES-hybrid exercise (1.78 L/min, n = 67; 26.5 mL/kg/min, n = 35) is considerably higher than during FES-cycling (1.05 L/min, n = 264; 14.3 mL/kg/min, n = 171). VO2sub-peak values during FES-hybrid exercise were higher than during FES-cycling. FES-exercise training can produce large increases in VO2peak; the included studies report average increases of +11% after FES-rowing training, +12% after FES-hybrid exercise training and +28% after FES-cycling training. This review shows that VO2 during FES-rowing or FES-hybrid exercise is considerably higher than during FES-cycling. These observations are confirmed by a limited number of direct comparisons; larger studies to test the differences in effectiveness of the various types of FES-exercise as cardiovascular exercise are needed. The results to date suggest that FES-rowing and FES-hybrid are more suited for high-intensity, high-volume exercise training than FES-cycling. In able-bodied people, such exercise programmes have shown to result in superior health and fitness benefits. Future research should examine whether similar high-intensity and high-volume exercise programmes also give persons with SCI superior fitness and health benefits. This kind of research is very timely given the high incidence of physical inactivity-related health conditions in the aging SCI population.

The Effects of Caffeine Ingestion on Time Trial Cycling Performance

Purpose:

The purpose of this work was to determine the effects of caffeine on high intensity time trial (TT) cycling performance in well-trained subjects.

Subjects:

Six male cyclists with the following physical characteristics (mean ± SD) age 30.7 ± 12, height 179.3 ± 7.5 cm, mass 70.0 ± 7.5 kg, VO2max 65.0 ± 6.3 mL·kg−1·min−1 undertook three 1-h TT performances, control (C), placebo (P) and caffeine (CAF), on a Velotron cycle ergometer conducted in a double-blind, random fashion. Subjects rested for 60 min and were then given CAF or P in a dose of 6 mg·kg−1 body mass and then commenced exercise after another 60 min of rest. Before ingestion, 60 min postingestion, and at the end of the TT, finger-prick blood samples were analyzed for lactate.

Results:

The cyclists rode significantly further in the CAF trial (28.0 ± 1.3 km) than they did in the C (26.3 ± 1.5 km, P < .01) or P (26.4 ± 1.5 km, P < .02) trials. No differences were seen in heart rate data throughout the TT (P > .05). Blood lactate levels were significantly higher at the end of the trials than either at rest or postingestion (P < .0001), but there were no differences between the three trial groups.

Conclusion:

On the basis of the data, we concluded that performance was improved with the use of a caffeine supplement.

Caffeine and other sympathomimetic stimulants: modes of action and effects on sports performance

Stimulants, illegal and legal, continue to be used in competitive sport. The evidence for the ergogenic properties of the most potent stimulants, amphetamines, cocaine and ephedrine, is mostly insubstantial. Low doses of amphetamines may aid performance where effects of fatigue adversely affect higher psychomotor activity. Pseudoephedrine, at high doses, has been suggested to improve high intensity and endurance exercise but phenylpropanolamine has not been proven to be ergogenic. Only caffeine has substantial experimental backing for being ergogenic in exercise. The mode of action of these stimulants centres on their ability to cause persistence of catecholamine neurotransmitters, with the exception of caffeine which is an adenosine receptor antagonist. By these actions, the stimulants are able to influence the activity of neuronal control pathways in the central (and peripheral) nervous system. Rodent models suggest that amphetamines and cocaine interact with different pathways to that affected by caffeine. Caffeine has a variety of pharmacological effects but its affinity for adenosine receptors is comparable with the levels expected to exist in the body after moderate caffeine intake, thus making adenosine receptor blockade the favoured mode of ergogenic action. However, alternative modes of action to account for the ergogenic properties of caffeine have been supported in the literature. Biochemical mechanisms that are consistent with more recent research findings, involving proteins such as DARPP-32 (dopamine and cAMP-regulated phosphoprotein), are helping to rationalize the molecular details of stimulant action in the central nervous system.

Wingate performance and surface EMG frequency variables are not affected by caffeine ingestion

The ergogenic effect of caffeine and its mechanism of action on short-term, high-intensity exercise are controversial. One proposed mechanism is caffeine’s stimulatory effect on the central nervous system and thus, motor-unit excitation. The latter is non-invasively determined from surface electromyographic signal (EMG) frequency measures. The purpose of this study was to determine if power output and surface EMG frequency variables during high-intensity cycling were altered following caffeine ingestion. Eighteen recreationally active college males (mean ± SD age, 21.5 ± 1.8 y; height, 181.8 ± 0.5 cm; body mass, 84.7 ± 11.4 kg) performed the Wingate test (WG) after ingestion of gelatin capsules containing either placebo (PL; dextrose) or caffeine (CAFF; 5 mg/kg body mass). The trials were separated by 1 week and subjects were asked to withdraw from all caffeine-containing products for 48 h before each trial. From the resulting power–time records, peak power (PP; highest power output in 5 s), minimum power (MP; lowest power output in 5 s), and the percent decline in power (Pd) were calculated. Surface EMG records of the right vastus lateralis (VL) and the gastrocnemius (GA) muscles corresponding to the PP and MP periods were collected and used to determine the integrated electromyogram (IEMG), the mean (MNPF), and the median (MDPF) of the signal’s power spectrum. A 2-way repeated measures analysis of variance (ANOVA) (treatment × time) was conducted to determine the effect of caffeine on these variables across levels of time. Caffeine ingestion had no effect on PP (PL, 1049 ± 192 W; CAFF, 1098 ± 198 W), MP (PL, 762 ± 104 W; CAFF, 802 ± 124 W), or the Pd (PL, 47% ± 8.9%; CAFF, 48.2% ± 7.3%) compared with the placebo. For both muscles, MNPF and MDPF diminished significantly (p < 0.001) across time and to a similar degree in both the CAFF and PL trials. Regardless of muscle, CAFF had no effect on the percent change in IEMG from the first 5 s to the last 5 s. For both treatments, the GA displayed a significantly (p < 0.05) greater pre vs. post percent decline in the EMG signal amplitude compared with the VL. These results indicate that caffeine does not impact power output during a 30 s high-intensity cycling bout. Furthermore, these data suggest that caffeine does not impact the neuromuscular drive as indicated by the similar IEMG scores between treatments. Similarly, caffeine does not seem to impact the frequency content of the surface EMG signal and thus the nature of recruited motor units before and after the expression of fatigue. The lack of decline in the IEMG in the VL despite the decline in power output over the course of the WG suggests a peripheral as opposed to a neural mechanism of fatigue in this muscle. The significant difference in the pre vs. post percent decline in the GA IEMG score further supports this notion. The pre vs. post decline in the IEMG noted in the GA may suggest a fatigue-triggered change in pedaling mechanics that may promote dominance of knee extensors with less reliance on plantar flexors.

Metabolic effects of caffeine ingestion and physical work in 75-year old citizens. A randomized, double-blind, placebo-controlled, cross-over study

OBJECTIVE

Whereas caffeine has been demonstrated to impact substantially on the metabolic response to exercise in healthy young subjects, this issue remains to be addressed in healthy elderly subjects.

DESIGN AND PATIENTS

The metabolic response to caffeine ingestion (6 mg/kg) and exercise in healthy elderly citizens at 70 years was examined in a randomized, double-blind, placebo-controlled, cross-over study. We included 30 subjects attending for driver license renewal at their general practitioner. Participants abstained from caffeinated drinks and food for 48 h and were randomized to receive placebo-caffeine or caffeine-placebo with 1 week between sessions.

MEASUREMENTS

A cycling endurance test at 65% of the expected maximal heart rate was performed 1 h after intervention. Blood samples were taken before intervention, before cycling, after 5 min of cycling, and at exhaustion. Analysis was by intention-to-treat and P < 0.05 was regarded as significant.

RESULTS

Caffeine significantly increased the concentration of plasma epinephrine (by 42%, 39%, and 49%), serum-free fatty acids (by 53%, 44%, and 50%), and plasma lactate (by 46%, 36%, and 48%), and insulin resistance (homeostasis model assessment-IR) (by 21%, 26%, and 23%) during rest, after 5 min of cycling, and at exhaustion. At exhaustion, the concentration plasma norepinephrine was elevated by 29%. A decrease was seen with caffeine treatment in blood potassium after 5 min of cycling and at exhaustion (by 3% and 2%, respectively).

CONCLUSIONS

Caffeine treatment increased epinephrine, fatty acids, lactate and norepinephrine at different times during test session and led to insulin-resistance. Hence, caffeine ingestion elicits a similar metabolic response in elderly participants at 70 years old to that seen in younger subjects.

Caffeine increases time to fatigue by maintaining force and not by altering firing rates during submaximal isometric contractions

Caffeine increases time to fatigue [limit of endurance (Tlim)] during submaximal isometric contractions without altering whole muscle activation or neuromuscular junction transmission. We used 10 male volunteers in a randomized, double-blind, repeated-measures experiment to examine single motor unit firing rates during intermittent submaximal contractions and to determine whether administering caffeine increased Tlim by maintaining higher firing rates. On 2 separate days, subjects performed intermittent 50% maximal voluntary contractions of the quadriceps to Tlim, 1 h after ingesting a caffeine (6 mg/kg) or placebo capsule. Average motor unit firing rates recorded with tungsten microelectrodes were constant for the duration of contractions. Caffeine increased average Tlim by 20.5 ± 8.1% ( P < 0.05) compared with placebo conditions. This increase was due to seven subjects, termed responders, who increased Tlim significantly. Two other subjects showed no response, and a third had a shorter Tlim. Neither the increased Tlim nor the responders' performance could be explained by alterations in firing rates or other neuromuscular variables. However, the amplitude of the evoked twitch and its maximal instantaneous rate of relaxation did not decline to the same degree in the caffeine trial of the responders; this resulted in values 20 and 30% higher at the time point matching the end of the placebo trial ( P < 0.05). The amplitude of the evoked twitch and the maximal instantaneous rate of relaxation were linearly correlated (caffeine r = 0.72, placebo r = 0.80, both P < 0.001), suggesting that the increase in Tlim may be partially explained by caffeine's effects on calcium reuptake and twitch force.

Effects of caffeine ingestion on rating of perceived exertion during and after exercise: a meta-analysis

The purpose of this study was to use the meta-analytic approach to examine the effects of caffeine ingestion on ratings of perceived exertion (RPE). Twenty-one studies with 109 effect sizes (ESs) met the inclusion criteria. Coding incorporated RPE scores obtained both during constant load exercise (n=89) and upon termination of exhausting exercise (n=20). In addition, when reported, the exercise performance ES was also computed (n=16). In comparison to placebo, caffeine reduced RPE during exercise by 5.6% (95% CI (confidence interval), -4.5% to -6.7%), with an equivalent RPE ES of -0.47 (95% CI, -0.35 to -0.59). These values were significantly greater (P<0.05) than RPE obtained at the end of exercise (RPE % change, 0.01%; 95% CI, -1.9 to 2.0%; RPE ES, 0.00, 95% CI, -0.17 to 0.17). In addition, caffeine improved exercise performance by 11.2% (95% CI; 4.6-17.8%). Regression analysis revealed that RPE obtained during exercise could account for approximately 29% of the variance in the improvement in exercise performance. The results demonstrate that caffeine reduces RPE during exercise and this may partly explain the subsequent ergogenic effects of caffeine on performance.

Informed decision-making on sympathomimetic use in sport and health

The International Olympic Committee, the World Anti-Doping Agency, and International Sport Federations have banned and restricted the use of many stimulants including prescription and over-the-counter medications and dietary supplements. In addition to elite athletes, people of all ages use stimulants in attempts to improve athletic performance, alter body composition, and increase levels of energy. Here we introduce a seven-stage model designed to facilitate informed decision-making by individuals taking or thinking of taking stimulants for sport, health, and/or appearance reasons. We review for amphetamines, over-the counter sympathomimetics, and caffeine their performance-enhancing and performance-degrading effects, health benefits and mechanisms of action, medical side effects, and legal, ethical, safety, and financial implications.

Caffeine and exercise: metabolism, endurance and performance

Caffeine is a common substance in the diets of most athletes and it is now appearing in many new products, including energy drinks, sport gels, alcoholic beverages and diet aids. It can be a powerful ergogenic aid at levels that are considerably lower than the acceptable limit of the International Olympic Committee and could be beneficial in training and in competition. Caffeine does not improve maximal oxygen capacity directly, but could permit the athlete to train at a greater power output and/or to train longer. It has also been shown to increase speed and/or power output in simulated race conditions. These effects have been found in activities that last as little as 60 seconds or as long as 2 hours. There is less information about the effects of caffeine on strength; however, recent work suggests no effect on maximal ability, but enhanced endurance or resistance to fatigue. There is no evidence that caffeine ingestion before exercise leads to dehydration, ion imbalance, or any other adverse effects. The ingestion of caffeine as coffee appears to be ineffective compared to doping with pure caffeine. Related compounds such as theophylline are also potent ergogenic aids. Caffeine may act synergistically with other drugs including ephedrine and anti-inflammatory agents. It appears that male and female athletes have similar caffeine pharmacokinetics, i.e., for a given dose of caffeine, the time course and absolute plasma concentrations of caffeine and its metabolites are the same. In addition, exercise or dehydration does not affect caffeine pharmacokinetics. The limited information available suggests that caffeine non-users and users respond similarly and that withdrawal from caffeine may not be important. The mechanism(s) by which caffeine elicits its ergogenic effects are unknown, but the popular theory that it enhances fat oxidation and spares muscle glycogen has very little support and is an incomplete explanation at best. Caffeine may work, in part, by creating a more favourable intracellular ionic environment in active muscle. This could facilitate force production by each motor unit.

Opposite actions of caffeine and creatine on muscle relaxation time in humans

The effect of creatine and caffeine supplementation on muscle torque generation and relaxation was investigated in healthy male volunteers. Maximal torque (T(max)), contraction time (CT) from 0.25 to 0.75 of T(max), and relaxation time (RT) from 0.75 to 0.25 of T(max) were measured during an exercise test consisting of 30 intermittent contractions of musculus quadriceps (2 s stimulation, 2 s rest) that were induced by electrical stimulation. According to a double-blind randomized crossover design, subjects (n = 10) performed the exercise test before (pretest) and after (posttest) creatine supplementation (Cr, 4 x 5 g/day, 4 days), short-term caffeine intake (Caf, 5 mg x kg(-1) x day(-1), 3 days), creatine supplementation + short-term caffeine intake (Cr+Caf), acute caffeine intake (ACaf, 5 mg/kg) or placebo. Compared with placebo, Cr shortened RT by approximately 5% (P < 0.05). Conversely, Caf increased RT (+ approximately 10%, P < 0.05), in particular as RT increased because of fatigue. RT was not significantly changed by either Cr+Caf or ACaf. T(max) and CT were similar during all experimental conditions. Initial T(max) was approximately 20% of voluntary maximal isometric contraction force, which was not different between treatments. It is concluded that Caf intake (3 days) prolongs muscle RT and by this action overrides the shortening of RT due to creatine supplementation.

Caffeine, Coffee and Ephedrine: Impact on Exercise Performance and Metabolism

This paper addresses areas where there is controversy regarding caffeine as an ergogenic aid and also identifies topics that have not been adequately addressed. It is clear that caffeine, in moderate amounts, can be used orally as an ergogenic aid in aerobic activity lasting for more than 1 min. It increases endurance and speed, but not maximal [Formula: see text] and related parameters. While there are fewer well-controlled studies for resistance exercise, the literature would suggest similar improvements: increased endurance at submaximal tension and power generated in repeated contractions and no change in maximal ability to produce force. It is likely that theophylline (a related methylxanthine) has similar actions and it has been suggested that the combination of caffeine and sympathomimetics may be a more potent erogenic aid. The voids in our understanding of caffeine include the dose (what amount is optimal, what vehicle is used to deliver the drug as well as method, pattern, and mode of administration), the potential side effects (particularly in competitive settings), health implications (insulin resistance and if combined with ephedrine, cardiovascular risks) and mechanisms of action. It appears unlikely that increased fat oxidation and glycogen sparing is the prime ergogenic mechanism.

Caffeine ingestion decreases glucose disposal during a hyperinsulinemic-euglycemic clamp in sedentary humans

The purpose of this investigation was to examine the effect of caffeine (an adenosine receptor antagonist) on whole-body insulin-mediated glucose disposal in resting humans. We hypothesized that glucose disposal would be lower after the administration of caffeine compared with placebo. Healthy, lean, sedentary (n = 9) men underwent two trial sessions, one after caffeine administration (5 mg/kg body wt) and one after placebo administration (dextrose) in a double-blind randomized design. Glucose disposal was assessed using a hyperinsulinemic-euglycemic clamp. Before the clamp, there were no differences in circulating levels of methylxanthines, catecholamines, or glucose. Euglycemia was maintained throughout the clamp with no difference in plasma glucose concentrations between trials. The insulin concentrations were also similar in the caffeine and placebo trials. After caffeine administration, glucose disposal was 6.38 +/- 0.76 mg/kg body wt compared with 8.42 +/- 0.63 mg/kg body wt after the placebo trial. This represents a significant (P < 0.05) decrease (24%) in glucose disposal after caffeine ingestion. In addition, carbohydrate storage was 35% lower (P < 0.05) in the caffeine trial than in the placebo trial. Furthermore, even when the difference in glucose disposal was normalized between the trials, there was a 23% difference in the amount of carbohydrate stored after caffeine administration compared with placebo administration. Caffeine ingestion also resulted in higher plasma epinephrine levels than placebo ingestion (P < 0.05). These data support our hypothesis that caffeine ingestion decreases glucose disposal and suggests that adenosine plays a role in regulating glucose disposal in resting humans.

Caffeine ingestion does not alter carbohydrate or fat metabolism in human skeletal muscle during exercise

This study examined the effect of ingesting caffeine (6 mg kg-1) on muscle carbohydrate and fat metabolism during steady-state exercise in humans. Young male subjects (n = 10) performed 1 h of exercise (70% maximal oxygen consumption (VO2,max)) on two occasions (after ingestion of placebo and caffeine) and leg metabolism was quantified by the combination of direct Fick measures and muscle biopsies. Following caffeine ingestion serum fatty acid and glycerol concentration increased (P< or =0.05) at rest, suggesting enhanced adipose tissue lipolysis. In addition circulating adrenaline concentration was increased (P< or =0.05) at rest following caffeine ingestion and this, as well as leg noradrenaline spillover, was elevated (P< or =0.05) above placebo values during exercise. Caffeine resulted in a modest increase (P< or =0.05) in leg vascular resistance, but no difference was found in leg blood flow. Arterial lactate and glucose concentrations were increased (P< or =0.05) by caffeine, while the rise in plasma potassium was dampened (P< or =0.05). There were no differences in respiratory exchange ratio or in leg glucose uptake, net muscle glycogenolysis, leg lactate release or muscle lactate, or glucose 6-phosphate concentration. Similarly there were no differences between treatments in leg fatty acid uptake, glycerol release or muscle acetyl CoA concentration. These findings indicate that caffeine ingestion stimulated the sympathetic nervous system but did not alter the carbohydrate or fat metabolism in the monitored leg. Other tissues must have been involved in the changes in circulating potassium, fatty acids, glucose and lactate.

Caffeine potentiates low frequency skeletal muscle force in habitual and nonhabitual caffeine consumers

The mechanism of action underlying the ergogenic effect of caffeine is still unclear. Caffeine increases the force of muscular contraction during low-frequency stimulation by potentiating calcium release from the sarcoplasmic reticulum. Studies have also suggested an enhancement of lipid oxidation and glycogen sparing as potential mechanisms. Given that several studies have found an ergogenic effect of caffeine with no apparent metabolic effects, it is likely that a direct effect upon muscle is important. Twelve healthy male subjects were classified as habitual (n = 6) or nonhabitual (n = 6) caffeine consumers based on a 4-day diet record analysis, with a mean caffeine consumption of 771 and 14 mg/day for each group, respectively. Subjects were randomly allocated to receive caffeine (6 mg/kg) and placebo (citrate) in a double-blind, cross-over fashion approximately 100 min before a 2-min tetanic stimulation of the common peroneal nerve in a custom-made dynamometer (2 trials each of 20 and 40 Hz). Tetanic torque was measured every 30 s during and at 1, 5, and 15 min after the stimulation protocol. Maximal voluntary contraction strength and peak twitch torque were measured before and after the stimulation protocol. Caffeine potentiated the force of contraction during the final minute of the 20-Hz stimulation (P<0.05) with no effect of habituation. There was no effect of caffeine on 40-Hz stimulation strength nor was there an effect on maximal voluntary contraction or peak twitch torque. These data support the hypothesis that some of the ergogenic effect of caffeine in endurance exercise performance occurs directly at the skeletal muscle level.

Comparison of caffeine and theophylline ingestion: exercise metabolism and endurance

This two-part investigation compared the ergogenic and metabolic effects of theophylline and caffeine. Initially (part A), the ergogenic potential of theophylline on endurance exercise was investigated. Eight men cycled at 80% maximum O(2) consumption to exhaustion 90 min after ingesting either placebo (dextrose), caffeine (6 mg/kg; Caff), or theophylline (4.5 mg/kg Theolair; Theo). There was a significant increase in time to exhaustion in both the Caff (41.2+/-4.8 min) and Theo (37.4+/-5.0 min) trials compared with placebo (32.6+/-3.4 min) (P<0.05). In part B, the effects of Theo on muscle metabolism were investigated and compared with Caff. Seven men cycled for 45 min at 70% maximum O(2) consumption (identical treatment protocol as in part A). Neither methylxanthines (MX) affected muscle glycogen utilization (P>0.05). Only Caff increased plasma epinephrine (P<0.05), but both MX increased blood glycerol levels (P<0.05). Muscle cAMP was increased (P<0.05) by both MX at 15 min and remained elevated at 45 min with Theo. This demonstrates that both MX are ergogenic and that this can be independent of muscle glycogen.

Metabolic and exercise endurance effects of coffee and caffeine ingestion

Caffeine (Caf) ingestion increases plasma epinephrine (Epi) and exercise endurance; these results are frequently transferred to coffee (Cof) consumption. We examined the impact of ingestion of the same dose of Caf in Cof or in water. Nine healthy, fit, young adults performed five trials after ingesting (double blind) either a capsule (Caf or placebo) with water or Cof (decaffeinated Cof, decaffeinated with Caf added, or regular Cof). In all three Caf trials, the Caf dose was 4.45 mg/kg body wt and the volume of liquid was 7.15 ml/kg. After 1 h of rest, the subject ran at 85% of maximal O2 consumption until voluntary exhaustion (approximately 32 min in the placebo and decaffeinated Cof tests). In the three Caf trials, the plasma Caf and paraxanthine concentrations were very similar. After 1 h of rest, the plasma Epi was increased (P < 0.05) by Caf ingestion, but the increase was greater (P < 0.05) with Caf capsules than with Cof. During the exercise there were no differences in Epi among the three Caf trials, and the Epi values were all greater (P < 0.05) than in the other tests. Endurance was only increased (P < 0. 05) in the Caf capsule trial; there were no differences among the other four tests. One cannot extrapolate the effects of Caf to Cof; there must be a component(s) of Cof that moderates the actions of Caf.