Efficacy of caffeine as an ergogenic aid in multiple cycling time trials

INTRODUCTION

Evidence that caffeine supplementation is effective to improve performance in cycling time trials has been obtained in single rather than multiple time trials. We investigated whether 5 mg.kg-1 of caffeine enhanced performance in multiple 4 km cycling time trials (TT4km) conducted within the same day and across different days.

METHODS

After selection of eligible cyclists and familiarization with the TT4km, thirteen well-trained cyclists participated in a balanced placebo-controlled designed with two caffeine sessions (CAF-1 and CAF-2) and a placebo session. In each session, cyclists performed a baseline TT4km before caffeine, and two supplemented TT4km (50 min and 80 min after supplementation). Relative and absolute reliability were obtained through intraclass coefficient correlation and standard error of the measurement (SEM), respectively. The cyclists' performance responses to caffeine were classified as beneficial, unchanged, and adverse by calculating the change between caffeine and placebo relative to SEM.

RESULTS

Caffeine enhanced performance in the first supplemented TT4km in CAF-1 and CAF-2 (0.5% and 1.8%, respectively), although only CAF-2 was significantly different from placebo (p < 0.001). Analysis with grouped data showed good absolute and relative reliability of caffeine effects within and across days. However, analysis of individual data showed that 38% and 31% of the cyclists changed their classification of responsiveness to caffeine between the supplemented trials across days.

CONCLUSIONS

Despite the good reliability of caffeine to enhance performance in a single TT4km performed within and across days, individual analysis challenged the use of caffeine supplementation protocols based on grouped data.

Effect of caffeine ingestion on time trial performance in cyclists: a systematic review and meta-analysis

BACKGROUND

Caffeine, widely recognized as an ergogenic aid, has undergone extensive research, demonstrating its effectiveness to enhance endurance performance. However, there remains a significant gap in systematically evaluating its effects on time trial (TT) performance in cyclists.

PURPOSE

This meta-analysis aimed to determine the efficacy of caffeine ingestion to increase cycling TT performance in cyclists and to evaluate the optimal dosage range for maximum effect.

METHODS

A search of four databases was completed on 1 December 2023. The selected studies comprised crossover, placebo-controlled investigations into the effects of caffeine ingestion on cycling TT performance. Completion time (Time) and mean power output (MPO) were used as performance measures for TT. Meta-analyses were performed using a random-effects model to assess the standardized mean differences (SMD) in individual studies.

RESULTS

Fifteen studies met the inclusion criteria for the meta-analyses. Subgroup analysis showed that moderate doses of caffeine intake (4-6 mg/kg) significantly improved cycling performance (SMD Time = -0.55, 95% confidence interval (CI) = -0.84 ~ -0.26, p < 0.01, I2 = 35%; SMD MPO = 0.44, 95% CI = 0.09 ~ 0.79, p < 0.05, I2 = 39%), while the effects of low doses (1-3 mg/kg) of caffeine were not significant (SMD Time = -0.34, 95% CI = -0.84 ~ 0.17, p = 0.19, I2 = 0%; SMD MPO = 0.31, 95% CI = -0.02 ~ 0.65, p = 0.07, I2 = 0%).

CONCLUSION

A moderate dosage (4-6 mg/kg) of caffeine, identified as the optimal dose range, can significantly improve the time trial performance of cyclists, while a low dose (1-3 mg/kg) does not yield improvement. In addition, the improvements in completion time and mean power output resulting from a moderate dose of caffeine are essentially the same in cycling time trails.

The effect of caffeine chewing gum on muscle performance and fatigue after severe-intensity exercise: isometric vs. dynamic assessments in trained cyclists

This study investigated the effect of caffeinated chewing gum (GUMCAF) on muscle fatigue (isometric vs. dynamic) after severe-intensity cycling bouts. Fifteen trained male cyclists participated in four visits. Each visit involved two severe-intensity cycling bouts (Δ1 and Δ2) lasting 6 min, separated by a 5-min recovery period. Muscle fatigue was assessed by isometric maximal voluntary knee extension contraction (IMVC) with twitch interpolation technique and dynamically by 7 s all-out cycling sprints. Assessments were performed before GUMCAF (Pre-GUM) and after the cycling bouts (Post-Exercise). GUMCAF and placebo gum (GUMPLA) were administered in a randomized double-blind procedure with participants receiving each gum type (GUMCAF and GUMPLA) during two separate visits. The results showed no significant interaction between gum types and time for the isometric and dynamic measurements (p > 0.05). The percentage change in performance from Pre-GUM to Post-Exercise showed no significant difference between GUMCAF and GUMPLA for either the dynamic-derived TMAX (~ -17.8% and -15.1%, respectively; p = 0.551) or isometric IMVC (~ -12.3% and -17.7%, respectively; p = 0.091) measurements. Moderate to large correlations (r = 0.31-0.51) were found between changes in sprint maximal torque and maximal power output measurements and isometric force, for both gum conditions. GUMCAF was not effective in attenuating muscle force decline triggered by severe-intensity cycling exercises, as measured by both isometric and dynamic methods. The correlations between IMVC and cycling maximal torque and power output suggest caution when interpreting isometric force as a direct measure of fatigue during dynamic cycling exercises.

Caffeine Increases Endurance Performance via Changes in Neural and Muscular Determinants of Performance Fatigability

PURPOSE

In the present study, we tested the hypothesis that caffeine would increase endurance performance via attenuation of neural and muscular determinants of performance fatigability during high-intensity, whole-body exercise.

METHODS

Ten healthy males cycled until exhaustion (89% ± 2% of V̇O2max) after the ingestion of caffeine or placebo. During another four visits, the same exercise was performed after either caffeine or placebo ingestion but with exercise discontinued after completing either 50% or 75% of the duration of placebo trial. An additional trial with caffeine ingestion was also performed with interruption at the placebo time to exhaustion (isotime). Performance fatigability was measured via changes in maximal voluntary contraction, whereas neural and muscular determinants of performance fatigability were quantified via preexercise to postexercise decrease in quadriceps voluntary activation (VA) and potentiated twitch force, respectively.

RESULTS

Compared with the placebo, caffeine increased time to exhaustion (+14.4 ± 1.6%, P = 0.017, 314.4 ± 47.9 vs 354.9 ± 40.8 s). Caffeine did not change the rate of decline in maximal voluntary contraction (P = 0.209), but caffeine reduced the twitch force decline at isotime when stimulating at single twitch (-58.6 ± 22.4 vs -45.7 ± 21.9%, P = 0.014) and paired 10 Hz electrical stimuli (-37.3 ± 13.2 vs -28.2 ± 12.9%, P = 0.025), and reduced the amplitude of electromyography signal during cycling at isotime (P = 0.034). The decline in VA throughout the trial was lower (P = 0.004) with caffeine (-0.5 ± 4.2%) than with placebo (-5.8 ± 8.5%). Caffeine also maintained peripheral oxygen saturation at higher levels (95.0 ± 1.9%) than placebo (92.0 ± 6.2%, P = 0.016).

CONCLUSIONS

Caffeine ingestion improves performance during high-intensity, whole-body exercise via attenuation of exercise-induced reduction in VA and contractile function.

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.

Effects of caffeine ingestion on physiological indexes of human neuromuscular fatigue: A systematic review and meta-analysis

BACKGROUND

Caffeine is often used as a stimulant during fatigue, but the standard of characteristic physiological indicators of the effect of caffeine on neuromuscular fatigue has not been unified. The purpose of this systematic review and meta-analysis is to summarize current experimental findings on the effects of caffeine on physiological indexes before and after neuromuscular fatigue, identify some characteristic neuromuscular physiological indexes to assess the potential effects of caffeine.

METHODS

The Preferred Reporting Items for Systematic Reviews and Meta-analyses are followed. We systematically searched PubMed, Google academic, and Web of Science for randomized controlled trials. We searched for studies on caffeine's (i) effects on neuromuscular fatigue and (ii) the influence of physiological indexes changes. Meta-analysis was performed for standardized mean differences (SMD) between caffeine and placebo trials in individual studies.

RESULTS

The meta-analysis indicated that caffeine significantly improves voluntary activation (VA) (SMD = 1.46;95%CI:0.13, 2.79; p < .00001), PTw (SMD = 1.11, 95%CI: -1.61, 3.84; p < .00001), and M-wave (SMD = 1.10, 95%CI: -0.21, 2.41; p < .00001), and a significant difference (p = .003) on measures of Peak Power (PP), and insignificant difference on measures of heart rate (HR) (I²  = 0.0, p = .84) and Maximal oxygen uptake (VO₂ ) (I²  = 0.0, p = .76).

CONCLUSION

The analysis showed that caffeine intake had a relatively large effect on VA, potentiated twitch (PTw), M-wave, which can be used as characteristic indexes of caffeine's impact on neuromuscular fatigue. This conclusion tends to indicate the effects of caffeine on neuromuscular fatigue during endurance running or jumping or muscle bending and stretching. The caffeine intake had a big effect on the electromyogram (EMG) and peak power (PP), and its effect role needs to be further verified, this conclusion tends to indicate the effect of caffeine on neuromuscular fatigue during jumping or elbow bending moment movements. HR, VO₂ , maximal voluntary contraction (MVC) cannot be used as the characteristic indexes of caffeine on neuromuscular fatigue. This conclusion tends to indicate the effect of caffeine on neuromuscular fatigue during endurance exercise. However, the results of meta-analysis are based on limited evidence and research scale, as well as individual differences of participants and different physical tasks, so it is necessary to interpret the results of meta-analysis cautiously. Therefore, future research needs to explore other physiological indicators and their indicative effects in order to determine effective and accurate characteristic indicators of caffeine on neuromuscular fatigue.

Effects of caffeine on central and peripheral fatigue following closed- and open-loop cycling exercises

We examined whether endurance performance and neuromuscular fatigue would be affected by caffeine ingestion during closed- and open-loop exercises. Nine cyclists performed a closed-loop (4,000-m cycling time trial) and an open-loop exercise (work rate fixed at mean power of the closed-loop trial) 60 min after ingesting caffeine (CAF, 5 mg/kg) or placebo (PLA, cellulose). Central and peripheral fatigue was quantified via pre- to post-exercise decrease in quadriceps voluntary activation and potentiated twitch force, respectively. Test sensitivity for detecting caffeine-induced improvements in exercise performance was calculated as the mean change in time divided by the error of measurement. Caffeine ingestion reduced the time of the closed-loop trial (PLA: 375.1±14.5 s vs CAF: 368.2±14.9 s, P=0.024) and increased exercise tolerance during the open-loop trial (PLA: 418.2±99.5 s vs CAF: 552.5±106.5 s, P=0.001), with similar calculated sensitivity indices (1.5, 90%CI: 0.7-2.9 vs 2.8, 90%CI: 1.9-5.1). The reduction in voluntary activation was more pronounced (P=0.019) in open- (-6.8±8.3%) than in closed-loop exercises (-1.9±4.4%), but there was no difference between open- and closed-loop exercises for the potentiated twitch force reduction (-25.6±12.8 vs -26.6±12.0%, P>0.05). Caffeine had no effect on central and peripheral fatigue development in either mode of exercise. In conclusion, caffeine improved endurance performance in both modes of exercise without influence on post-exercise central and peripheral fatigue, with the open-loop exercise imposing a greater challenge to central fatigue tolerance.

No additive effect of acetaminophen when co-ingested with caffeine on cycling performance in well-trained young men

We investigated the effect of caffeine and acetaminophen on power output during a 6-min performance test, peripheral fatigue, and muscle protein kinase A (PKA) substrate phosphorylation. Fourteen men [age (means ± SD): 26 ± 6 yr; V̇o_2max: 63.9 ± 5.0 mL·min^-1·kg^-1] completed four randomized trials with acetaminophen (1,500 mg), caffeine (5 mg·kg body wt^-1), combined caffeine and acetaminophen (caffeine + acetaminophen), or placebo. Mean power output during the 6-min performance test (placebo mean: 312 ± 41 W) was higher with caffeine (+5 W; 95% CI: 1 to 9; P = 0.017) and caffeine + acetaminophen (+6 W; 95% CI: 0 to 12; P = 0.049) than placebo, but not with acetaminophen (+1 W; 95% CI: -4 to 7; P = 0.529). Decline in quadriceps maximal isometric voluntary torque immediately after the performance test was lower (treatment × time; P = 0.035) with acetaminophen (-40 N·m; 95% CI: -53 to -30; P < 0.001) and caffeine + acetaminophen (-44 N·m; 95% CI: -58 to -30; P < 0.001) than placebo (-53 N·m; 95% CI: -71 to -39; P < 0.001) but was similar with caffeine (-54 N·m; 95% CI: -69 to -38; P < 0.001). Muscle phosphocreatine content decreased more during the performance test (treatment × time; P = 0.036) with caffeine + acetaminophen (-55 mmol·kg dry wt^-1; 95% CI: -65 to -46; P < 0.001) than placebo (-40 mmol·kg dry wt^-1; 95% CI: -52 to -24; P < 0.001). Muscle net lactate accumulation was not different from placebo (+85 mmol·kg dry wt^-1; 95% CI: 60 to 110; P < 0.001) for any treatment (treatment × time; P = 0.066), being +75 mmol·kg dry wt^-1 (95% CI: 51 to 99; P < 0.001) with caffeine, +76 mmol·kg dry wt^-1 (95% CI: 58 to 96; P < 0.001) with acetaminophen, and +103 mmol·kg dry wt^-1 (95% CI: 89 to 115; P < 0.001) with caffeine + acetaminophen. Decline in muscle ATP and glycogen content and increase in PKA substrate phosphorylation was not different between treatments (treatment × time; P > 0.1). Thus, acetaminophen provides no additive performance enhancing effect to caffeine during 6-min maximal cycling. In addition, change in PKA activity is likely not a major mechanism of performance improvement with caffeine.NEW & NOTEWORTHY Here, we show that acetaminophen does not provide additive performance improvement to caffeine during a 6-min cycling ergometer performance test, and that acetaminophen does not improve performance on its own. Neither substance affects peripheral fatigue, muscle glycolytic energy production, or phosphorylation of muscle proteins of importance for ion handling. In contrast to previous suggestions, increased epinephrine action on muscle cells does not appear to be a major contributor to the performance enhancement with caffeine.