Effects of caffeine intake on muscle strength and power: a systematic review and meta-analysis

Abstract

Background: Caffeine is commonly used as an ergogenic aid. Literature about the effects of caffeine ingestion on muscle strength and power is equivocal. The aim of this systematic review and meta-analysis was to summarize results from individual studies on the effects of caffeine intake on muscle strength and power.

Methods: A search through eight databases was performed to find studies on the effects of caffeine on: (i) maximal muscle strength measured using 1 repetition maximum tests; and (ii) muscle power assessed by tests of vertical jump. Meta-analyses of standardized mean differences (SMD) between placebo and caffeine trials from individual studies were conducted using the random effects model.

Results: Ten studies on the strength outcome and ten studies on the power outcome met the inclusion criteria for the meta-analyses. Caffeine ingestion improved both strength (SMD = 0.20; 95% confidence interval [CI]: 0.03, 0.36; p = 0.023) and power (SMD = 0.17; 95% CI: 0.00, 0.34; p = 0.047). A subgroup analysis indicated that caffeine significantly improves upper (SMD = 0.21; 95% CI: 0.02, 0.39; p = 0.026) but not lower body strength (SMD = 0.15; 95% CI: -0.05, 0.34; p = 0.147).

Conclusion: The meta-analyses showed significant ergogenic effects of caffeine ingestion on maximal muscle strength of upper body and muscle power. Future studies should more rigorously control the effectiveness of blinding. Due to the paucity of evidence, additional findings are needed in the female population and using different forms of caffeine, such as gum and gel.

Keywords: Data synthesis; Ergogenic aid; Performance; Power.

Fig. 1

Flow diagram of the search and study selection process

Fig. 2

Forest plot showing differences between the effects of placebo and caffeine trials on measures of maximal muscular strength. The size of the plotted squares reflects the relative statistical weight of each study. The numbers on the x-axis denote the standardized mean differences expressed as Hedge’s g. The horizontal lines denote the respective 95% confidence intervals (CI)

Fig. 3

Forest plot showing differences between the effects of placebo and caffeine trials on measures of upper-body maximal muscle strength. The size of the plotted squares reflects the relative statistical weight of each study. The numbers on the x-axis denote the standardized mean differences expressed as Hedge’s g. The horizontal lines denote the respective 95% confidence intervals (CI)

Fig. 4

Forest plot showing differences between the effects of placebo and caffeine trials on measures of lower-body maximal muscle strength. The size of the plotted squares reflects the relative statistical weight of each study. The numbers on the x-axis denote the standardized mean differences expressed as Hedge’s g. The horizontal lines denote the respective 95% confidence intervals (CI)

Fig. 5

Forest plot showing differences between the effects of placebo and caffeine trials on measures of muscle power expressed as vertical jump height. The size of the plotted squares reflects the relative statistical weight of each study. The numbers on the x-axis denote the standardized mean differences expressed as Hedge’s g. The horizontal lines denote the respective 95% confidence intervals (CI)