Toxic doses of caffeine are needed to increase skeletal muscle contractility

Acute effects of caffeine or quercetin ingestion on motor unit firing pattern before and after resistance exercise

The aim of the present study was to investigate the acute effect of caffeine or quercetin ingestion on motor unit firing patterns and muscle contractile properties before and after resistance exercise. High-density surface electromyography (HDs-EMG) during submaximal contractions and electrically elicited torque in knee extensor muscles were measured before (PRE) and 60 min after (POST1) ingestion of caffeine, quercetin glycosides, or placebo, and after resistance exercise (POST2) in ten young males. The Convolution Kernel Compensation technique was used to identify individual motor units of the vastus lateralis muscle for the recorded HDs-EMG. Ingestion of caffeine or quercetin induced significantly greater decreases in recruitment thresholds (RTs) from PRE to POST1 compared with placebo (placebo: 94.8 ± 9.7%, caffeine: 84.5 ± 16.2%, quercetin: 91.9 ± 36.7%), and there were significant negative correlations between the change in RTs (POST1-PRE) and RT at PRE for caffeine (rs = - 0.448, p < 0.001) and quercetin (rs =  - 0.415, p = 0.003), but not placebo (rs =  - 0.109, p = 0.440). Significant positive correlations between the change in firing rates (POST2-POST1) and RT at PRE were noted with placebo (rs = 0.380, p = 0.005) and quercetin (rs = 0.382, p = 0.007), but not caffeine (rs = 0.069, p = 0.606). No significant differences were observed in electrically elicited torque among the three conditions. These results suggest that caffeine or quercetin ingestion alters motor unit firing patterns after resistance exercise in different threshold-dependent manners in males.

Effects of 7-day quercetin intervention on motor unit activity and muscle contractile properties before and after resistance exercise in young adults randomized controlled trials

We investigated whether the alteration of the motor unit recruitment threshold (MURT) caused by quercetin ingestion intervention for 7 days modifies motor unit activation patterns before and after a single session of resistance exercise. Twenty young male and female adults were divided into two groups: ingestion of placebo (PLA) or quercetin glycosides at 200 mg/day (QUE). High-density surface electromyography during submaximal contractions was measured to assess the motor unit firing rate (MUFR) and MURT of the vastus lateralis muscle before (PRE) and after (POST) resistance exercise (DAY1). The same measurements were repeated after 7 days of placebo or quercetin glycoside ingestion (DAY8). In QUE, MURT decreased more from DAY1-PRE to DAY8-PRE (29.1 ± 9.1 to 27.1 ± 9.5% MVC, p < 0.001) but not in PLA (29.8 ± 10.4 to 28.9 ± 9.7% MVC, p < 0.167). For percentage change in MUFR following resistance exercise, there was a significant interaction (day × group, p < 0.001). The degree of changes in MURT from DAY1-PRE to DAY8-PRE was significantly correlated with the percentage change of MUFR from DAY8-PRE to DAY8-POST in QUE (p = 0.014, r = -0.363) but not in PLA (p = 0.518). The study suggests that 7-day quercetin ingestion alters the motor unit recruitment pattern, and this may induce changes in motor unit firing patterns during a single session of resistance training (Trial registration: UMIN000052255, R000059650).

Caffeine intake enhances peak oxygen uptake and performance during high-intensity cycling exercise in moderate hypoxia

PURPOSE

We investigated whether caffeine consumption can enhance peak oxygen uptake ([Formula: see text]) by increasing peak ventilation during an incremental cycling test, and subsequently enhance time to exhaustion (TTE) during high-intensity cycling exercise in moderate normobaric hypoxia.

METHODS

We conducted a double-blind, placebo cross-over design study. Sixteen recreational male endurance athletes (age: 20 ± 2 years, [Formula: see text]: 55.6 ± 3.6 ml/kg/min, peak power output: 318 ± 40 W) underwent an incremental cycling test and a TTE test at 80% [Formula: see text] (derived from the placebo trial) in moderate normobaric hypoxia (fraction of inspired O2: 15.3 ± 0.2% corresponding to a simulated altitude of ~ 2500 m) after consuming either a moderate dose of caffeine (6 mg/kg) or a placebo.

RESULTS

Caffeine consumption resulted in a higher peak ventilation [159 ± 21 vs. 150 ± 26 L/min; P < 0.05; effect size (ES) = 0.31]. [Formula: see text] (3.58 ± 0.44 vs. 3.47 ± 0.47 L/min; P < 0.01; ES = 0.44) and peak power output (308 ± 44 vs. 302 ± 44 W; P = 0.02, ES = 0.14) were higher following caffeine consumption than during the placebo trial. During the TTE test, caffeine consumption enhanced minute ventilation (P = 0.02; ES = 0.28) and extended the TTE (426 ± 74 vs. 358 ± 75 s; P < 0.01, ES = 0.91) compared to the placebo trial. There was a positive correlation between the percent increase of [Formula: see text] following caffeine consumption and the percent increase in TTE (r = 0.49, P < 0.05).

CONCLUSION

Moderate caffeine consumption stimulates breathing and aerobic metabolism, resulting in improved performance during incremental and high-intensity endurance exercises in moderate normobaric hypoxia.

Highly contractile 3D tissue engineered skeletal muscles from human iPSCs reveal similarities with primary myoblast-derived tissues

Skeletal muscle research is transitioning toward 3D tissue engineered in vitro models reproducing muscle's native architecture and supporting measurement of functionality. Human induced pluripotent stem cells (hiPSCs) offer high yields of cells for differentiation. It has been difficult to differentiate high-quality, pure 3D muscle tissues from hiPSCs that show contractile properties comparable to primary myoblast-derived tissues. Here, we present a transgene-free method for the generation of purified, expandable myogenic progenitors (MPs) from hiPSCs grown under feeder-free conditions. We defined a protocol with optimal hydrogel and medium conditions that allowed production of highly contractile 3D tissue engineered skeletal muscles with forces similar to primary myoblast-derived tissues. Gene expression and proteomic analysis between hiPSC-derived and primary myoblast-derived 3D tissues revealed a similar expression profile of proteins involved in myogenic differentiation and sarcomere function. The protocol should be generally applicable for the study of personalized human skeletal muscle tissue in health and disease.

Effect of isolated and combined ingestion of caffeine and citrulline malate on resistance exercise and jumping performance: a randomized double-blind placebo-controlled crossover study

PURPOSE

The aim of this study was to explore the isolated and combined effects of caffeine and citrulline malate (CitMal) on jumping performance, muscular strength, muscular endurance, and pain perception in resistance-trained participants.

METHODS

Using a randomized and double-blind study design, 35 resistance-trained males (n = 18) and females (n = 17) completed four testing sessions following the ingestion of isolated caffeine (5 mg/kg), isolated CitMal (12 g), combined doses of caffeine and CitMal, and placebo. Supplements were ingested 60 min before performing a countermovement jump (CMJ) test (outcomes included jump height, rate of force development, peak force, and peak power), one-repetition maximum (1RM) squat and bench press, and repetitions to muscular failure in the squat and bench press with 60% of 1RM. Pain perception was evaluated following the repetitions to failure tests. The study was registered at ISRCTN (registration number: ISRCTN11694009).

RESULTS

Compared to the placebo condition, isolated caffeine ingestion and co-ingestion of caffeine and CitMal significantly enhanced strength in 1RM bench press (Cohen's d: 0.05-0.06; 2.5-2.7%), muscular endurance in the squat (d: 0.46-0.58; 18.6-18.7%) and bench press (d: 0.48-0.64; 9.3-9.5%). However, there was no significant difference between isolated caffeine ingestion and caffeine co-ingested with CitMal, and isolated CitMal supplementation did not have an ergogenic effect in any outcome. No main effect of condition was found in the analysis for CMJ-derived variables, 1RM squat and pain perception.

CONCLUSION

Caffeine ingestion appears to be ergogenic for muscular strength and muscular endurance, while adding CitMal does not seem to further enhance these effects.

Postsynaptic Calcium Extrusion at the Mouse Neuromuscular Junction Alkalinizes the Synaptic Cleft

Neurotransmission is shaped by extracellular pH. Alkalization enhances pH-sensitive transmitter release and receptor activation, whereas acidification inhibits these processes and can activate acid-sensitive conductances in the synaptic cleft. Previous work has shown that the synaptic cleft can either acidify because of synaptic vesicular release and/or alkalize because of Ca2+ extrusion by the plasma membrane ATPase (PMCA). The direction of change differs across synapse types. At the mammalian neuromuscular junction (NMJ), the direction and magnitude of pH transients in the synaptic cleft during transmission remain ambiguous. We set out to elucidate the extracellular pH transients that occur at this cholinergic synapse under near-physiological conditions and identify their sources. We monitored pH-dependent changes in the synaptic cleft of the mouse levator auris longus using viral expression of the pseudoratiometric probe pHusion-Ex in the muscle. Using mice from both sexes, a significant and prolonged alkalization occurred when stimulating the connected nerve for 5 s at 50 Hz, which was dependent on postsynaptic intracellular Ca2+ release. Sustained stimulation for a longer duration (20 s at 50 Hz) caused additional prolonged net acidification at the cleft. To investigate the mechanism underlying cleft alkalization, we used muscle-expressed GCaMP3 to monitor the contribution of postsynaptic Ca2+ Activity-induced liberation of intracellular Ca2+ in muscle positively correlated with alkalization of the synaptic cleft, whereas inhibiting PMCA significantly decreased the extent of cleft alkalization. Thus, cholinergic synapses of the mouse NMJ typically alkalize because of cytosolic Ca2+ liberated in muscle during activity, unless under highly strenuous conditions where acidification predominates.SIGNIFICANCE STATEMENT Changes in synaptic cleft pH alter neurotransmission, acting on receptors and channels on both sides of the synapse. Synaptic acidification has been associated with a myriad of diseases in the central and peripheral nervous system. Here, we report that in near-physiological recording conditions the cholinergic neuromuscular junction shows use-dependent bidirectional changes in synaptic cleft pH-immediate alkalinization and a long-lasting acidification under prolonged stimulation. These results provide further insight into physiologically relevant changes at cholinergic synapses that have not been defined previously. Understanding and identifying synaptic pH transients during and after neuronal activity provides insight into short-term synaptic plasticity synapses and may identify therapeutic targets for diseases.

The Long Way to Establish the Ergogenic Effect of Caffeine on Strength Performance: An Overview Review

This overview review aimed to describe the evolution of the characteristics of the research on caffeine effects on strength. A total of 189 experimental studies with 3459 participants were included. The median sample size was 15 participants, with an over-representation of men vs. women (79.4 vs. 20.6%). Studies on young participants and elders were scarce (4.2%). Most studies tested a single dose of caffeine (87.3%), while 72.0% used doses adjusted to body mass. Single-dose studies ranged from 1.7 to 7 mg/kg (4.8 ± 1.4 mg/kg), while dose-response studies ranged from 1 to 12 mg/kg. Caffeine was mixed with other substances in 27.0% of studies, although only 10.1% of studies analyzed the caffeine interaction with these substances. Capsules (51.9%) and beverages (41.3%) were the most common forms of caffeine administration. Similar proportions of studies focused on upper (24.9%) or lower body strength 37.6% (37.6% both). Participants' daily intake of caffeine was reported in 68.3% of studies. Overall, the pattern in the study of caffeine's effects on strength performance has been carried out with experiments including 11-15 adults, using a single and moderate dose of caffeine adjusted to participants' body mass in the form of a capsule.

Caffeine ingestion increases endurance performance of trained male cyclists when riding against a virtual opponent without altering muscle fatigue

PURPOSE

Caffeine improves cycling time trial (TT) performance; however, it is unknown whether caffeine is ergogenic when competing against other riders. The aim of this study was to investigate whether caffeine improves performance during a 4-km cycling TT when riding against a virtual opponent, and whether it is associated with increased muscle activation and at the expense of greater end-exercise central and peripheral fatigue.

METHODS

Using a randomized, crossover, and double-blind design, eleven well-trained cyclists completed a 4-km cycling TT alone without supplementation (CON), or against a virtual opponent after ingestion of placebo (OP-PLA) or caffeine (5 mg^.kg^-1, OP-CAF). Central and peripheral fatigue were quantified via the pre- to post-exercise decrease in voluntary activation and potentiated twitch force, respectively. Muscle activation was continually measured during the trial via electromyography activity.

RESULTS

Compared to CON, OP-PLA improved 4-km cycling TT performance (P = 0.018), and OP-CAF further improved performance when compared to OP-PLA (P = 0.050). Muscle activation was higher in OP-PLA and OP-CAF than in CON throughout the trial (P = 0.003). The pre- to post-exercise reductions in voluntary activation and potentiated twitch force were, however, similar between experimental conditions (P > 0.05). Compared to CON, OP-PLA increased the rating of perceived exertion during the first 2 km, but caffeine blunted this increase with no difference between the OP-CAF and CON conditions.

CONCLUSIONS

Caffeine is ergogenic when riding against a virtual opponent, but this is not due to greater muscle activation or at the expense of greater end-exercise central or peripheral fatigue.

Influence of caffeine on the maximal isometric and concentric force produced by skinned fibers

Caffeine is one of the most famous and widely used ergogenic drugs, especially by athletes to improve sports performance. Caffeine is known to enhance muscle contraction by facilitating Ca²⁺ release from the sarcoplasmic reticulum. While the effect of caffeine on the cross-bridge dynamics has also investigated, the results is controversial. Therefore, the purpose of this study was to examine the influence of caffeine on cross-bridge dynamics using skinned fiber preparations from rabbit soleus (N = 19 in total). We performed isometric contractions at an average sarcomere length of 2.4 μm; thereafter, skinned fibers were shortened by 20% of the fiber length at a velocity of 0.1 mm/s (slow shortening) or 0.5 mm/s (fast shortening). The contractions were performed under both normal and caffeine-containing activating solution conditions to compare the isometric, slow concentric, and fast concentric forces between conditions. The isometric force did not differ between normal and caffeine-containing activating solution conditions. Similarly, the concentric forces obtained during the slow and fast shortening trials did not differ between conditions. We also measured the stiffness and the rate of force redevelopment (kTR) during the isometric contraction phase and found that these values were not different between normal and caffeine conditions. Based on these results, we conclude that the influence of caffeine on cross-bridge dynamics is negligible, and the ergogenic effect of caffeine, from the view of muscle contractility, is by facilitating Ca²⁺ release, as suggested in previous studies, and not by modulating the cross-bridge dynamics.

Matrigel 3D bioprinting of contractile human skeletal muscle models recapitulating exercise and pharmacological responses

A key to enhance the low translatability of preclinical drug discovery are in vitro human three-dimensional (3D) microphysiological systems (MPS). Here, we show a new method for automated engineering of 3D human skeletal muscle models in microplates and functional compound screening to address the lack of muscle wasting disease medication. To this end, we adapted our recently described 24-well plate 3D bioprinting platform with a printhead cooling system to allow microvalve-based drop-on-demand printing of cell-laden Matrigel containing primary human muscle precursor cells. Mini skeletal muscle models develop within a week exhibiting contractile, striated myofibers aligned between two attachment posts. As an in vitro exercise model, repeated high impact stimulation of contractions for 3 h by a custom-made electrical pulse stimulation (EPS) system for 24-well plates induced interleukin-6 myokine expression and Akt hypertrophy pathway activation. Furthermore, the known muscle stimulators caffeine and Tirasemtiv acutely increase EPS-induced contractile force of the models. This validated new human muscle MPS will benefit development of drugs against muscle wasting diseases. Moreover, our Matrigel 3D bioprinting platform will allow engineering of non-self-organizing complex human 3D MPS.

Alave-Furrer et al adapted their recently-developed 3D bioprinting platform to allow microvalve-based drop-on-demand printing of cell-laden Matrigel containing primary human muscle precursor cells. Their bioprinting platform recapitulated aspects of exercise and pharmacological responses and thus could aid the engineering of more complex 3D microphysiological systems.

Acute Caffeine Intake Reduces Perceived Exertion But Not Muscle Pain during Moderate Intensity Cycling Exercise in Women with Fibromyalgia

OBJECTIVE

Exacerbated perceived exertion and muscle pain responses during exercise might limit physical activity practice in fibromyalgia patients. Thus, nutritional strategies that can reduce perceived exertion and muscle pain during exercise in fibromyalgia patients would be useful. The purpose of this study was to investigate the effects of acute caffeine intake on the perceptions of exertion and muscle pain during a moderate intensity exercise in women with fibromyalgia.Method: Using a randomized, double-blinded, placebo-controlled and crossover experimental design, eleven sedentary women diagnosed with fibromyalgia (age: 44.6 ± 10.5 years; body mass index: 28.5 ± 4.5 kg.m^-2) ingested a capsule containing either caffeine (5 mg per kg of body mass) or cellulose (placebo), 60 minutes before performing a 30-minute constant-load cycling exercise, with work rate fixed at 50% of their individual peak workload attained in an incremental exercise test. Ratings of perceived leg muscle pain and perceived exertion were assessed every 5 minutes of exercise.Results: The perceived leg muscle pain was similar (F_(1,10) = 1.18, p = 0.30, ŋ² = 0.11) between caffeine (2.1 ± 1.2 arbitrary units) and placebo conditions (2.2 ± 0.9 arbitrary units). The perceived exertion, however, was on average 8 ± 6% lower (F_(1,10) = 12.13; p = 0.006; ŋ² = 0.55) during exercise in the caffeine condition (12.4 ± 1.3 arbitrary units) than in the placebo condition (13.1 ± 1.1 arbitrary units).Conclusions: These findings indicate that acute caffeine intake could be an attractive strategy to attenuate the exacerbated perceived exertion of fibromyalgia patients during moderate intensity exercise.

Effects of Caffeine on Resistance Exercise: A Review of Recent Research

In the last few years, a plethora of studies have explored the effects of caffeine on resistance exercise, demonstrating that this field of research is growing fast. This review evaluates and summarizes the most recent findings. Given that toxic doses of caffeine are needed to increase skeletal muscle contractility, the binding of caffeine to adenosine receptors is likely the primary mechanism for caffeine's ergogenic effects on resistance exercise. There is convincing evidence that caffeine ingestion is ergogenic for (i) one-repetition maximum, isometric, and isokinetic strength; and (ii) muscular endurance, velocity, and power in different resistance exercises, loads, and set protocols. Furthermore, there is some evidence that caffeine supplementation also may enhance adaptations to resistance training, such as gains in strength and power. Caffeine ingestion is ergogenic for resistance exercise performance in females, and the magnitude of these effects seems to be similar to that observed in men. Habitual caffeine intake and polymorphisms within CYP1A2 and ADORA2A do not seem to modulate caffeine's ergogenic effects on resistance exercise. Consuming lower doses of caffeine (e.g., 2-3 mg/kg) appears to be comparably ergogenic to consuming high doses of caffeine (e.g., 6 mg/kg). Minimal effective doses of caffeine seem to be around 1.5 mg/kg. Alternate caffeine sources such as caffeinated chewing gum, gel, and coffee are also ergogenic for resistance exercise performance. With caffeine capsules, the optimal timing of ingestion seems to be 30-60 min before exercise. Caffeinated chewing gums and gels may enhance resistance exercise performance even when consumed 10 min before exercise. It appears that caffeine improves performance in resistance exercise primarily due to its physiological effects. Nevertheless, a small portion of the ergogenic effect of caffeine seems to be placebo-driven.

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.

In vivo cooling-induced intracellular Ca2+ elevation and tension in rat skeletal muscle

It is an open question as to whether cooling‐induced muscle contraction occurs in the in vivo environment. In this investigation, we tested the hypotheses that a rise in intracellular Ca²⁺ concentration ([Ca²⁺]i) and concomitant muscle contraction could be evoked in vivo by reducing muscle temperature and that this phenomenon would be facilitated or inhibited by specific pharmacological interventions designed to impact Ca²⁺‐induced Ca²⁺‐release (CICR). Progressive temperature reductions were imposed on the spinotrapezius muscle of Wistar rats under isoflurane anesthesia by means of cold fluid immersion. The magnitude, location, and temporal profile of [Ca²⁺]i were estimated using fura‐2 loading. Caffeine (1.25–5.0 mM) and procaine (1.6–25.6 mM) loading were applied in separatum to evaluate response plasticity by promoting or inhibiting CICR, respectively. Lowering the temperature of the muscle surface to ~5°C produced active tension and discrete sites with elevated [Ca²⁺]i. This [Ca²⁺]i elevation differed in magnitude from fiber to fiber and also from site to site within a fiber. Caffeine at 1.25 and 5.0 mM reduced the magnitude of cooling necessary to elevate [Ca²⁺]i (i.e., from ~5°C to ~8 and ~16°C, respectively, both p < 0.05) and tension. Conversely, 25.6 mM procaine lowered the temperature at which [Ca²⁺]i elevation and tension were detected to ~2°C (p < 0.05). Herein we demonstrate the spatial and temporal relationship between cooling‐induced [Ca²⁺]i elevation and muscle contractile force in vivo and the plasticity of these responses with CICR promotion and inhibition.

Caffeine increases peripheral fatigue in low- but not in high-performing cyclists

The influence of cyclists’ performance levels on caffeine-induced increases in neuromuscular fatigue after a 4-km cycling time trial (TT) was investigated. Nineteen cyclists performed a 4-km cycling TT 1 h after ingesting caffeine (5 mg·kg−1) or placebo (cellulose). Changes from baseline to after exercise in voluntary activation (VA) and potentiated 1 Hz force twitch (Qtw,pot) were used as markers of central and peripheral fatigue, respectively. Participants were classified as “high performing” (HP, n = 8) or “low performing” (LP, n = 8) in accordance with their performance in a placebo trial. Compared with placebo, caffeine increased the power, anaerobic mechanical power, and anaerobic work, reducing the time to complete the trial in both groups (p < 0.05). There was a group versus supplement and a group versus supplement versus trial interaction for Qtw,pot, in which the postexercise reduction was greater after caffeine compared with placebo in the LP group (Qtw,pot = −34% ± 17% vs. −21% ± 11%, p = 0.02) but not in the HP group (Qtw,pot = −22% ± 8% vs. −23% ± 10%, p = 0.64). There was no effect of caffeine on VA, but there was a group versus trial interaction with lower postexercise values in the LP group than in the HP group (p = 0.03). Caffeine-induced improvement in 4-km cycling TT performance seems to come at the expense of greater locomotor muscle fatigue in LP but not in HP cyclists.

Novelty Caffeine improves exercise performance at the expense of a greater end-exercise peripheral fatigue in low-performing athletes. Caffeine-induced improvement in exercise performance does not affect end-exercise peripheral fatigue in high-performing athletes. High-performing athletes seem to have augmented tolerance to central fatigue during a high-intensity time trial.

Neuronal adenosine A2A receptors signal ergogenic effects of caffeine

Caffeine is one of the most used ergogenic aid for physical exercise and sports. However, its mechanism of action is still controversial. The adenosinergic hypothesis is promising due to the pharmacology of caffeine, a nonselective antagonist of adenosine A1 and A2A receptors. We now investigated A2AR as a possible ergogenic mechanism through pharmacological and genetic inactivation. Forty-two adult females (20.0 ± 0.2 g) and 40 male mice (23.9 ± 0.4 g) from a global and forebrain A2AR knockout (KO) colony ran an incremental exercise test with indirect calorimetry (V̇O2 and RER). We administered caffeine (15 mg/kg, i.p., nonselective) and SCH 58261 (1 mg/kg, i.p., selective A2AR antagonist) 15 min before the open field and exercise tests. We also evaluated the estrous cycle and infrared temperature immediately at the end of the exercise test. Caffeine and SCH 58621 were psychostimulant. Moreover, Caffeine and SCH 58621 were ergogenic, that is, they increased V̇O2max, running power, and critical power, showing that A2AR antagonism is ergogenic. Furthermore, the ergogenic effects of caffeine were abrogated in global and forebrain A2AR KO mice, showing that the antagonism of A2AR in forebrain neurons is responsible for the ergogenic action of caffeine. Furthermore, caffeine modified the exercising metabolism in an A2AR-dependent manner, and A2AR was paramount for exercise thermoregulation.

Caffeine supplementation is ergogenic in soccer players independent of cardiorespiratory or neuromuscular fitness levels

Background

Equivocal findings examining the influence of caffeine on performance and biological responses to exercise may be due to inter-individual variability in cardiorespiratory or neuromuscular fitness. This study examined whether the effects of caffeine ingestion on exercise performance and biological responses to prolonged intermittent exercise to exhaustion depend on cardiorespiratory or neuromuscular fitness.

Methods

Twenty male soccer players, separated according to either cardiorespiratory fitness (high vs medium) or neuromuscular fitness (high vs medium) underwent two trials simulating the cardiovascular demands of a soccer game to exhaustion on treadmill after ingesting either caffeine (6 mg∙kg^− 1) or placebo. Physical performance, cardiorespiratory and metabolic parameters and blood metabolites were evaluated.

Results

Time to exhaustion (719 ± 288 vs 469 ± 228 s), jump height (42.7 ± 4.2 vs 38.6 ± 4.4 cm), heart rate (163 ± 12 vs 157 ± 13 b∙min^− 1), mean arterial blood pressure (98 ± 8 vs 92 ± 10 mmHg), plasma glucose (5.6 ± 0.7 vs 5.3 ± 0.6 mmol∙l^− 1) and lactate (3.3 ± 1.2 vs 2.9 ± 1.2 mmol∙l^− 1) were higher, while rating of perceived exertion (12.6 ± 1.7 vs 13.3 ± 1.6) was lower with caffeine vs placebo (p < 0.01), independent of cardiorespiratory or neuromuscular fitness level. Reaction time; plasma glycerol, non-esterified fatty acids and epinephrine; carbohydrate and fat oxidation rates; and energy expenditure were not affected by caffeine (p > 0.05).

Conclusions

Caffeine was effective in improving endurance and neuromuscular performance in athletes with either high or medium cardiorespiratory and neuromuscular fitness. Cardiorespiratory and neuromuscular fitness do not appear to modulate the ergogenic effects of caffeine supplementation in well-trained athletes.

Effects of caffeine on neuromuscular function in a non-fatigued state and during fatiguing exercise

NEW FINDINGS

What is the central question of the study? What are the effects of caffeine on neuromuscular function in a non-fatigued state and during fatiguing exercise? What is the main finding and its importance? In a non-fatigued state, caffeine decreased the duration of the silent period evoked by transcranial magnetic stimulation. Caffeine-induced reduction of inhibitory mechanisms in the central nervous system before exercise was associated with an increased performance. Individuals who benefit from caffeine ingestion may experience lower perception of effort during exercise and an accelerated recovery of M-wave amplitude postfatigue. This study elucidates the mechanisms of action of caffeine and demonstrates that inter-individual variability of its effects on neuromuscular function is a fruitful area for further work.

ABSTRACT

Caffeine enhances exercise performance, but its mechanisms of action remain unclear. In this study, we investigated its effects on neuromuscular function in a non-fatigued state and during fatiguing exercise. Eighteen men participated in this randomized, double-blind, placebo-controlled crossover trial. Baseline measures included plantarflexion force, drop jump, squat jump, voluntary activation of triceps surae muscle, soleus muscle contractile properties, M-wave, α-motoneuron excitability (H-reflex), corticospinal excitability, short-interval intracortical inhibition, intracortical facilitation, silent period evoked by transcranial magnetic stimulation (SP) and plasma potassium and caffeine concentrations. Immediately after baseline testing, participants ingested caffeine (6 mg·kg^-1 ) or placebo. After a 1-h rest, baseline measures were repeated, followed by a fatiguing stretch-shortening cycle exercise (sets of 40 bilateral rebound jumps on a sledge apparatus) until task failure. Neuromuscular testing was carried out throughout the fatigue protocol and afterwards. Caffeine enhanced drop jump height (by 4.2%) and decreased the SP (by 12.6%) in a non-fatigued state. A caffeine-related decrease in SP and short-interval intracortical inhibition before the fatiguing activity was associated with an increased time to task failure. The participants who benefitted from an improved performance on the caffeine day reported a significantly lower sense of effort during exercise and had an accelerated postexercise recovery of M-wave amplitude. Caffeine modulates inhibitory mechanisms of the CNS, recovery of M-wave amplitude and perception of effort. This study lays the groundwork for future examinations of differences in caffeine-induced neuromuscular changes between those who are deemed to benefit from caffeine ingestion and those who are not.

Effects of Caffeine Supplementation on Power Performance in a Flywheel Device: A Randomised, Double-Blind Cross-Over Study

Despite the demonstrated evidence of the importance of eccentric contractions in sports performance, no research has evaluated the ergogenic effects of caffeine on this type of contraction means during flywheel exercises. Therefore, the aims of the present study were to compare the power outcomes, using different inertial loads, between caffeine and placebo conditions. Twenty-four young, healthy, and active men (age: 22.5 ± 4.8 years) took part in the study. A crossed, randomised double-blind design was used to analyse the effects of caffeine on lower limb power outcomes during a flywheel half-squat exercise. Participants completed four sets of eight all-out repetitions with a fixed three-minutes rest interval, and each set was performed using different inertial loads (i.e., 0.025, 0.050, 0.075 and 0.100 kg·m⁻²). Both the mean power (MP) and peak power (PP) in concentric (CON) and eccentric (ECC) movement phases at each inertial load were recorded after participants were administered either a caffeine supplement (6 mg·kg⁻¹) or placebo (sucrose). Participants receiving a caffeine supplementation demonstrated improvements versus the placebo in total MP (MP_total), as well as MP in CON phase (MP_con) and in ECC phase (MP_ecc) at each inertial load (22.68 to 26.53%; p < 0.01, effect size (ES) = 0.89–1.40). In addition, greater improvements with caffeine ingestion were obtained with respect to the placebo condition (18.79 to 24.98%; p < 0.01, ES = 1.03–1.40) in total PP (PP_total), as well as PP in CON phase (PP_con) and in ECC phase (PP_ecc) at each inertial load. Thus, the supplementation of 6 mg·kg⁻¹ caffeine may be considered to maximise on-field physical performance in those sports characterised by high demands of resistance.