Caffeine’s ergogenic potential has been extensively studied in the sports science literature, with research dating back to 1907 . From investigating caffeine’s effects on aerobic exercise, in recent years the research focus has shifted to anaerobic exercise performance outcomes, such as muscular endurance, muscle strength, and jumping tasks that require muscle power. While caffeine has been found to significantly enhance muscular endurance , the effects of caffeine ingestion on maximal muscle strength (commonly operationalized as one repetition maximum [1RM]) and muscle power (commonly operationalized as vertical jump) remain unclear, and the practical utility of caffeine ingestion for enhancing performance in such physical tasks has not been fully elucidated.
The pioneering work on caffeine’s effects on strength by Astorino et al.  reported no significant strength-enhancing effects with caffeine ingestion in a group of resistance trained men. Recent work by Grgic and Mikulic , however, found a significant 3% increase in lower body strength with caffeine ingestion using the barbell back squat 1RM as a measure of maximal strength. Goldstein et al.  reported a significant increase in upper body strength with caffeine ingestion, while Williams et al.  reported no ergogenic effect. The inconsistent results of individual studies prevent drawing sound conclusions regarding the ergogenic potential of caffeine for maximal strength outcomes.
Equivocal findings have also been presented for the effects of caffeine intake on muscle power. A recent study by Ali et al.  reported no effect on countermovement jump height with caffeine ingestion. However, the findings of Bloms et al.  support conclusions about caffeine as an effective ergogenic aid for achieving acute improvements in countermovement jump height and peak force. Given the importance of jumping abilities for many common sports, it would be of both scientific and practical significance to determine a reasonably precise estimate regarding the potential performance-enhancing impact of caffeine ingestion on muscle power.
Several aspects that vary between studies, including the exercise used, participants’ characteristics (e.g., age, sex, and training experience), and caffeine form, might be responsible for the inconsistency of findings. Most importantly, small sample sizes often limited the statistical power to detect significant effects . A meta-analysis of individual studies is needed to circumvent these issues and provide in-depth, evidence-based scrutiny of the current body of evidence. The first meta-analytic investigation on the topic of caffeine and strength was performed by Warren et al. , who found a mean increase of approximately 7% in lower body maximal voluntary contraction with caffeine ingestion. A limitation of the meta-analysis is that only two of the included studies tested the effects of caffeine ingestion on 1RM, which significantly restricted the findings to isometric and isokinetic strength outcomes.
The latest meta-analysis on the topic, done by Polito et al. , found no significant effect of caffeine intake on performance in 1RM strength tests. However, only three studies met the inclusion criteria for the meta-analysis. The total number of pooled participants was relatively low (n = 46), potentially indicating issues with the statistical power of the analysis. Furthermore, the small number of included studies prevented subgroup analyses for possible moderators that may potentially impact the ergogenic potential of caffeine. Since the review by Polito et al. , a number of experimental trials have been published [4, 11–16], presenting novel findings for females , trained [4, 16] and untrained men [11, 13], athletes , and adolescents ; as such, an updated review appears to be warranted.
No previous meta-analyses have pooled the results of individual studies on the effects of caffeine on muscle power. The aim of this systematic review was, therefore, twofold: (a) to perform an updated meta-analysis of the acute effects of caffeine ingestion on maximal muscle strength; and (b) to conduct the first meta-analysis of acute effects of caffeine ingestion on muscle power assessed by vertical jump tests. The results may benefit athletes and practitioners in a variety of sports in which muscle strength and/or power are important determinants of performance.