Abstract
udy aimed to explore the effects of acute ingestion of caffeine capsules on muscle strength and muscle endurance. We searched the PubMed, Web of Science, Cochrane, Scopus, and EBSCO databases. Data were pooled using the weighted mean difference (WMD) and 95% confidence interval. Fourteen studies fulfilled the inclusion criteria. The acute ingestion of caffeine capsules significantly improved muscle strength (WMD, 7.09,p< 0.00001) and muscle endurance (WMD, 1.37; p< 0.00001), especially in males (muscle strength, WMD, 7.59,p< 0.00001; muscle endurance, WMD, 1.40,p< 0.00001). Subgroup analyses showed that≥6 mg/kg body weight of caffeine (WMD, 6.35, p< 0.00001) and ingesting caffeine 45 min pre-exercise (WMD, 8.61,p< 0.00001) were more effective in improving muscle strength, with the acute ingestion of caffeine capsules having a greater effect on lower body muscle strength (WMD,
0.00001), especially in males (muscle strength, WMD, 7.59,p< 0.00001; muscle endurance, WMD, 1.40,p< 0.00001). Subgroup analyses showed that≥6 mg/kg body weight of caffeine (WMD, 6.35, p< 0.00001) and ingesting caffeine 45 min pre-exercise (WMD, 8.61,p< 0.00001) were more effective in improving muscle strength, with the acute ingestion of caffeine capsules having a greater effect on lower body muscle strength (WMD, 10.19,p< 0.00001). In addition, the acute ingestion of caffeine capsules had a greater effect in moderate-intensity muscle endurance tests (WMD, 1.76,p< 0.00001). An acute ingestion of caffeine capsules significantly improved muscle strength and muscle endurance in the upper body and lower body of males. Keywords:caffeine capsules; muscle strength; muscle endurance; systematic review; meta-analysis 1. Introduction Caffeine is widely available in a variety of beverages and nutritional supplements as a central nervous stimulant with energizing effects. Since it was delisted by the World Anti-Doping Agency (WADA) in 2004, elite athletes have been extensively utilizing caffeine as a tool to boost their training and competitive performance [1]. A previous study showed that around 73.8% of athletes ingest caffeine prior to or during competition, with a higher percentage of endurance athletes [2]. As of 2015, caffeine is used by up to 76% of elite athletes, with the most widespread use among athletes in track and field, cycling, and rowing [3]. The potentiating effect of caffeine on sports performance is well recognized. Typically, athletes consume a dose of 3–6 mg/kg body weight (BW) caffeine in pill or capsule form 30–90 min before exercise [4]. Muscle strength and endurance, the foundational abilities of athletes, are closely related to sports performance. Previous studies have shown that even low doses of caffeine ingestion can significantly enhance muscle strength [5], regardless of habitual caffeine consumption [6]. A previous study reported that acute ingestion of caffeine leads to a Nutrients2024,16, 1146.
Nutrients2024,16, 1146 2 of 20 significant enhancement in isokinetic strength [7]. However, Wilk et al. [8] showed that high acute caffeine ingestion did not result in any improvement in muscle strength, which may be due to the fact that caffeine tolerance varies in different individuals, and that high caffeine ingestion doses can lead to side effects, which may affect the potentiating effect. Furthermore, there is ongoing debate surrounding the impact of acute caffeine inges- tion on muscle endurance. Duncan et al. [9] demonstrated a marked rise in the number of repetitions performed, coupled with a substantial enhancement in both upper and lower body muscle endurance, subsequent to acute caffeine ingestion. Southward et al. [10] fur- ther verified the notable impact of caffeine ingestion on muscle endurance. This effect can be attributed to the mechanism where caffeine triggers the release of norepinephrine, acetyl- choline, dopamine, and serotonin by binding to adenosine receptors A1 and A2A [11]. This, in turn, enhances muscle tone [12] and diminishes the suppressive influence of adenosine on neurotransmission, excitation, and nociception [13]. In addition, caffeine delays fatigue by blocking the negative effects of adenosine receptors on the centralnervous system [14]. However, Beck et al. [15] observed no notable effect of consuming caffeine-containing sup- plements on upper body muscle endurance, which aligned with the results ofGrgic et al. [ . Their study revealed that caffeine ingestion failed to augment the number of repetitions during resistance training. Collectively, these findings imply that caffeine ingestion does not significantly boost muscle endurance. This variation may stem from the distinct muscle composition of the upper and lower body and the large differences in activated and mobilized muscle fibers, which affect the potentiation of caffeine [17]. Furthermore, different genotypes of individuals have different sensitivities to caffeine [18], which is also an important factor to consider. A prior study showed that caffeine ingestion led to an enhancement in both mus- cle strength and muscle endurance, but was limited to the muscle performance of knee extensors [19] . Moreover, Grgic et al. [20] exhibited that caffeine ingestion significantly bolstered muscle strength and endurance, albeit exclusively
sensitivities to caffeine [18], which is also an important factor to consider. A prior study showed that caffeine ingestion led to an enhancement in both mus- cle strength and muscle endurance, but was limited to the muscle performance of knee extensors [19] . Moreover, Grgic et al. [20] exhibited that caffeine ingestion significantly bolstered muscle strength and endurance, albeit exclusively in upper body muscle groups, with the subjects solely comprising females. Additionally, Southward et al. [10] revealed that caffeine ingestion notably enhanced muscle endurance, but specifically in the context of cycling tests. Caffeine is primarily administered in the form of capsules, and its efficacy in enhancing muscle strength and endurance is mostly established with this form. Therefore, this study aimed to explore the effects of an acute ingestion of caffeine capsules on muscle strength and endurance. 2. Materials and Methods 2.1. Design This study was conducted in accordance with the criteria and recommendations of the Preferred Reporting Items for Systematic Evaluation and Meta-Analysis [21]. The protocol was registered with PROSPERO under the registration number CRD42023424824. 2.2. Search Strategy We searched the PubMed, EBSCO, Cochrane, Web of science, and Scopus databases from the inception dates to 19 May 2023, using the following keywords and MESH terms: caffeine, muscle strength, and muscle endurance. We also manually searched references listed in the identified systematic reviews and meta-analyses. Two authors (W.W. and Z.C.) independently completed the article screening using a standardized form. 2.3. Eligibility Criteria Inclusion criteria were as follows: (1) RCTs; (2) including acute caffeine ingestion group and placebo ingestion group; (3) caffeine was provided using capsules; and (4) acute rather than long-term interventions. Exclusion criteria were as follows: (1) publications that were not in English; (2) conference papers; (3) review articles; and (4) studies conducted on animals.
Nutrients2024,16, 1146 3 of 20 2.4. Data Extraction WW and ZC independently conducted the data extraction process, primarily focusing on the following aspects: (1) study characteristics (first author’s surname, publication year); (2) subject characteristics (age, gender,n, training experience); (3) intervention characteristics (dosage and timing of caffeine ingestion); and (4) outcome characteristics (muscle group location, intensity of muscle endurance tests). 2.5. Outcomes The primary outcomes were muscle strength and muscle endurance. Muscle strength was measured using the maximum load lifted in a single effort, whereas muscle endurance was measured using the maximum number of repetitions achieved at a fixed load. 2.6. Methodological Quality Assessment The methodological quality of the included studies was evaluated using the Cochrane risk of bias tool [22,23]. In addition, we also used the Physiotherapy Evidence Database (PEDro) scale to further assess the quality of the included studies. For PEDro scale, 11 items were evaluated, where studies scoring <4 points, 4–5 points, 6–8 points, and >9 points are considered poor, average, good, and excellent quality, respectively [24]. 2.7. Statistical Analysis From each study, we extracted mean and standard deviation (SD) values pertaining to muscle strength and muscle endurance in both the caffeine and placebo ingestion groups. To estimate the impact of acute caffeine ingestion on these parameters, we employed the weighted mean difference (WMD) along with a 95% confidence interval (CI). For studies that reported standard errors (SE) and 95% CIs, we calculated the SD according to previous studies [19,20]. In cases where high heterogeneity was observed, we utilized subgroup analysis, meta-regression analysis, and sensitivity analysis to interpret the results [25,26]. For subgroup analyses, we tried to investigate the impact of acute ingestion of caffeine capsules on muscle strength using muscle group location (upper or lower body), dose of caffeine ingestion (<6 or≥6 mg/kg BW), timing of caffeine ingestion (45 min or 60 min pre-exercise), and participant gender (male or female). Similarly, we explored the effects of acute caffeine ingestion on muscle endurance using muscle group location (upper or lower body), intensity of the muscle endurance tests (low-, moderate-, or high-intensity), and participant gender (male or female). The
of caffeine ingestion (<6 or≥6 mg/kg BW), timing of caffeine ingestion (45 min or 60 min pre-exercise), and participant gender (male or female). Similarly, we explored the effects of acute caffeine ingestion on muscle endurance using muscle group location (upper or lower body), intensity of the muscle endurance tests (low-, moderate-, or high-intensity), and participant gender (male or female). The forest plots were generated using RevMan software (Version 5.4), and sensitivity analysis, funnel plot, and meta-regression were performed using Stata software (Version 15.0). Statistical significance was considered for outcomes with ap< 0.05. 3. Results 3.1. Studies Selection Figure of 1524 studies remained after excluding duplicates and 363 potentially eligible studies remained after the title and abstract screen. Upon reading the full text, 349 studies were excluded for the following reasons: (1) they investigated irrelevant outcomes (n= 256); (2) the caffeine ingestion group was combined with other interventions (n= 72); (3) they were animal studies (n= 17); and (4) they reported long-term interventions (n= 4). Finally, 14 studies [6,11,27–38] met the inclusion criteria.
Nutrients2024,16, 1146 4 of 20Nutrients 2024, 16, x FOR PEER REVIEW 4 of 20 3. Results 3.1. Studies Selection Figure 1 illustrates the initial retrieval of 2020 records from the databases. A total of 1524 studies remained after excluding duplicates and 363 potentially eligible studies re- mained after the title and abstract screen. Upon reading the full text, 349 studies were excluded for the following reasons: (1) they investigated irrelevant outcomes (n = 256); (2) the caffeine ingestion group was combined with other interventions (n = 72); (3) they were animal studies (n = 17); and (4) they reported long-term interventions (n = 4). Finally, 14 studies [6,11,27–38] met the inclusion criteria. Figure 1. PRISMA Flow Diagram of the Study Selection Process. 3.2. Characteristics of the Included Studies Table 1 presents the characteristics of caffeine ingestion and participants. Of the 14 studies, 11 studies involved only males [11,27–29,31–35,37,38] and 3 studies involved only females [6,30,36]. Eight studies provided data for muscle strength [6,27,28,30,33,34,36,38], which was tested using one repetition maximum (1 RM). In addition, 13 studies provided data for muscle endurance [6,11,28–38], which was measured using the maximum number of repetitions achieved at a fixed load. The dose of caffeine ingestion ranged from 2 to 11 mg/kg BW, and the timing of caffeine ingestion was 45 min or 60 min. In terms of the intensity of muscle endurance tests, three studies involved low-intensity muscle endur- ance tests (<60% 1 RM) [6,30,34], seven studies involved moderate-intensity muscle en- durance tests (60−85% 1 RM) [11,28,31,33,35,36,38], and three studies involved high-inten- sity muscle endurance tests (≥85% 1 RM) [29,31,32]. Figure 1.PRISMA Flow Diagram of the Study Selection Process. 3.2. Characteristics of the Included Studies Table 14 studies, 11 studies involved only males [11,27–29,31–35,37,38] and 3 studies involved only females [6,30,36]. Eight studies provided data for muscle strength [6,27,28,30,33,34,36,38], which was tested using one repetition maximum (1 RM). In addition, 13 studies provided data for muscle endurance [6,11,28–38], which was measured using the maximum number of repetitions achieved at a fixed load. The dose of caffeine ingestion ranged from 2 to 11 mg/kgBW, and
and 3 studies involved only females [6,30,36]. Eight studies provided data for muscle strength [6,27,28,30,33,34,36,38], which was tested using one repetition maximum (1 RM). In addition, 13 studies provided data for muscle endurance [6,11,28–38], which was measured using the maximum number of repetitions achieved at a fixed load. The dose of caffeine ingestion ranged from 2 to 11 mg/kgBW, and the timing of caffeine ingestion was 45 min or 60 min. In terms of the intensity of muscle endurance tests, three studies involved low-intensity muscle endurance tests (<60% 1 RM) [6,30,34], seven studies involved moderate-intensity muscle endurance tests (60−85% 1 RM) [11,28,31,33,35,36,38], and three studies involved high-intensity muscle endurance tests (≥85% 1 RM) [ Table 1.Characteristics of the studies included in this meta-analysis. Study Sample Size Age (y) Caffeine Dose (mg/kg BW) Timing of Caffeine Ingestion (min) Intensity of Muscle Endurance Tests (% 1 RM) Muscle Group Location Filip-Stachnik et al., 2020 [6] 13 23 ±0.8 6 45 50 Upper body Tamilio et al., 2022 [11] 22 20 ±2 3 45 - Upper body, lower body Ferreira et al., 2022 [27] 21 19.6 ±0.8 6, 8 45 - Upper body, lower body Berjisian et al., 2022 [28] 20 24 ±9 6 60 70 Upper body, lower body
Nutrients2024,16, 1146 5 of 20 Table 1.Cont. Study Sample Size Age (y) Caffeine Dose (mg/kg BW) Timing of Caffeine Ingestion (min) Intensity of Muscle Endurance Tests (% 1 RM) Muscle Group Location Grgic and Mikulic, 2021 [ 13 28.5 ±5 3 60 85 Upper body Filip-Stachnik et al., 2021 [30] 21 23 ±0.9 3, 6 60 - Upper body Grgic et al., 2020(1) [31] 20 29.3 ±4.8 3 60 85 Upper body Grgic et al., 2020(2) [32] 22 AA: 27.0±5.6 AC/CC: 29.8±3.6 3 60 85 Upper body Grgic et al., 2020(3) [33] 28 25 ±6 2, 4, 6 60 60 Upper body, lower body Wilk et al., 2019 [34] 16 24.2 ±4.2 9, 11 60 50 Upper body Richardson et al., 2016 [35] 9 24 ±2 5 60 60 Upper body, lower body Arazi et al., 2016 [36] 10 16.8 ±1.23 2, 5 60 60 Lower body Astorino et al., 2011 [37] 14 23.1 ±1.1 6 60 70, 80 Upper body, lower body Astorino et al., 2008 [38] 22 23.4 ±3.6 6 60 60 Upper body, lower body Abbreviations: y, year; AA, AA genotype at rs762551; AC/CC, AC/CC genotypes at rs762551; BW, body weight. 3.3. Meta-Analysis Compared with the placebo ingestion group, an acute ingestion of caffeine capsules significantly improved muscle strength (WMD, 7.09, 95% CI, 6.07 to 8.11,p< 0.00001, I 2 = 48%, Figure) and muscle endurance (WMD, 1.37, 95% CI, 0.98 to 1.77,p< 0.00001, I 2 = 4%, Figure).Nutrients 2024, 16, x FOR PEER REVIEW 6 of 20 Figure 2. Meta-analysis results of the effects of acute ingestion of caffeine capsules on muscle strength [6,27,28,30,33,34,36,38]. Diamonds indicated the effect size of each study summarized as WMD. The size of the shaded squares was proportional to the percentage weight of each study. Horizontal lines represented the 95% CI and the vertical line represented the overall effect. Figure 3. Meta-analysis results of the effect of acute ingestion of caffeine capsules on muscle endur- ance [6,11,28–38]. Diamonds indicated the effect size of each study summarized as WMD. The size of the shaded squares was proportional to the percentage weight of each
of each study. Horizontal lines represented the 95% CI and the vertical line represented the overall effect. Figure 3. Meta-analysis results of the effect of acute ingestion of caffeine capsules on muscle endur- ance [6,11,28–38]. Diamonds indicated the effect size of each study summarized as WMD. The size of the shaded squares was proportional to the percentage weight of each study. Horizontal lines represented the 95% CI and the vertical line represented the overall effect. Figure 2.Meta-analysis results of the effects of acute ingestion of caffeine capsules on muscle strength [6,27,28,30,33,34,36,38]. Diamonds indicated the effect size of each study summarized as WMD. The size of the shaded squares was proportional to the percentage weight of each study. Horizontal lines represented the 95% CI and the vertical line represented the overall effect.
Nutrients2024,16, 1146 6 of 20Nutrients 2024, 16, x FOR PEER REVIEW 6 of 20 Figure 2. Meta-analysis results of the effects of acute ingestion of caffeine capsules on muscle strength [6,27,28,30,33,34,36,38]. Diamonds indicated the effect size of each study summarized as WMD. The size of the shaded squares was proportional to the percentage weight of each study. Horizontal lines represented the 95% CI and the vertical line represented the overall effect. Figure 3. Meta-analysis results of the effect of acute ingestion of caffeine capsules on muscle endur- ance [6,11,28–38]. Diamonds indicated the effect size of each study summarized as WMD. The size of the shaded squares was proportional to the percentage weight of each study. Horizontal lines represented the 95% CI and the vertical line represented the overall effect. Figure 3.Meta-analysis results of the effect of acute ingestion of caffeine capsules on muscle en- durance [6,11,28–38]. Diamonds indicated the effect size of each study summarized as WMD. The size of the shaded squares was proportional to the percentage weight of each study. Horizontal lines represented the 95% CI and the vertical line represented the overall effect. 3.4. Meta-Regression Analysis Meta-regression analyses were conducted to explore the effects of the dose and the timing of caffeine ingestion. There was a significant association between the dose (p= 0.021) or the timing of caffeine ingestion (p= 0.003) and muscle strength (Figure S1). However, no significant associations were observed between the timing of caffeine ingestion (p= 0.966) or the dose of caffeine ingestion (p= 0.571) and muscle endurance (Figure S2). 3.5. Subgroup Analysis 3.5.1. Muscle Strength Stratifying the analysis by muscle group location, improvements in muscle strength remained significant in the upper body (WMD, 5.91, 95% CI, 4.72 to 7.11,p< 0.00001, I 2 = 13%) and the lower body (WMD, 10.19, 95% CI, 8.25 to 12.13,p< 0.00001,I 2 = 33%, Figure), while an acute ingestion of caffeine capsules exhibited a more pronounced effect on lower body muscle strength. Seven studies provided data for 6 mg/kg BW of caffeine ingestion, five studies pro- vided data for 3 mg/kg BW of caffeine ingestion, two studies provided
13%) and the lower body (WMD, 10.19, 95% CI, 8.25 to 12.13,p< 0.00001,I 2 = 33%, Figure), while an acute ingestion of caffeine capsules exhibited a more pronounced effect on lower body muscle strength. Seven studies provided data for 6 mg/kg BW of caffeine ingestion, five studies pro- vided data for 3 mg/kg BW of caffeine ingestion, two studies provided data for 2 mg/kg BW of caffeine ingestion, two studies provided data for 5 mg/kg BW of caffeine ingestion, and one study provided data for 4, 8, 9, and 11 mg/kg BW of caffeine ingestion. When analyzing the subgroup by dose of caffeine ingestion,≥6 mg/kg BW of caffeine signifi- cantly improved muscle strength (WMD, 6.35, 95% CI, 3.91 to 8.79,p< 0.00001,I 2 = 55%), while <6 mg/kg BW of caffeine did not significantly improve muscle strength (WMD, 1.29, 95% CI,−2.51 to 5.08,p= 0.51,I 2 = 0%, Figure).
Description
The study reviews the impact of caffeine on muscle performance.