Effects of Caffeine Supplementation on Blood Glucose and Electromyographic Activity During Resistance Exercise: A Randomized, Double-Blind, Placebo-Controlled Crossover Study

Background/Objectives: Caffeine is a widely used ergogenic aid; however, evidence regarding its effects on neuromuscular and metabolic responses during resistance training remains limited. Therefore, this study aimed to analyze the effects of caffeine supplementation on blood glucose concentration and neuromuscular activation, assessed through electromyographic activity, during a muscular endurance test. Methods: Eleven resistance-trained men (25.7 ± 5.9 years) participated in this randomized, double-blind, crossover trial. Six laboratory visits were conducted, including one-repetition maximum (1RM) and maximal voluntary isometric contraction (MVIC) testing in the bench press exercise. From the second visit onward (baseline; BL), participants completed a 24 h dietary recall (24hDR), pre- and post-exercise blood glucose assessments, and a muscular endurance test performed at 80% of 1RM until concentric failure, while electromyographic activity was recorded from the clavicular and sternal portions of the pectoralis major, triceps brachii, and anterior deltoid. From the third visit onward, participants received one of the following randomized interventions 60 min before testing: low-dose caffeine (LC = 3 mg·kg−1), high-dose caffeine (HC = 6 mg·kg−1), low-dose placebo (LP = 230 mg), or high-dose placebo (HP = 460 mg). Results: There was an increase in the number of repetitions in all conditions vs. BL, with no significant differences (p > 0.05). Electromyographic activity did not differ between conditions (p > 0.05). Blood glucose decreased post-test in BL and placebo conditions (p < 0.05) but remained stable with caffeine (p > 0.05). Conclusions: Low and high doses of caffeine did not significantly affect muscular endurance performance or neuromuscular activation. However, caffeine supplementation appeared to attenuate the post-exercise decline in blood glucose concentration following exercise performed to concentric failure.