Differential Effects of Low and High Caffeine Doses on Bench Press Muscular Endurance: A Randomized, Double-Blind, Placebo-Controlled Crossover Study

Background: Caffeine contributes to improvements in physical performance by enhancing muscular strength and endurance. However, it is necessary to evaluate the effects of different dosages on resistance training (RT) performance. Therefore, this study aimed to compare the effects of different caffeine doses (i.e., 3 mg·kg−1 and 6 mg·kg−1) on the maximum number of repetitions in a muscular endurance test. Methods: The study included 11 male participants (25.7 ± 5.9 years) who completed six in-person visits. During the first visit, a 24 h dietary recall (24HDR) was administered, anthropometric measurements were assessed, and one-repetition maximum (1RM) was determined in the flat bench press (BP). The second visit (baseline; BL) included a new 24HDR, assessment of muscle thickness using portable ultrasound (pre- and post-test), and a muscular endurance test in the BP at 80% of 1RM performed until concentric failure. The four subsequent visits followed the same protocol, with the administration of caffeine or placebo capsules 60 min before testing in a randomized, double-blind manner: low-dose caffeine (3 mg·kg−1; LC), high-dose caffeine (6 mg·kg−1; HC), low-dose placebo (3 mg·kg−1; LP), and high-dose placebo (6 mg·kg−1; HP). The first three interventions were conducted with 48 h intervals, and the remaining interventions were separated by a 7-day interval. Results: The number of repetitions and total workload (TWL) increased in all conditions compared with baseline; however, no significant differences were observed (p > 0.05). LC and HP achieved the highest repetition values (LC: 12.09 ± 3.33 reps; HP: 12.27 ± 2.72 reps). Muscle thickness was greater in all conditions in the post-test assessment, showing a significant increase (p < 0.05). Conclusions: Low- or high-dose caffeine supplementation appears to moderately influence responses in a muscular endurance test, suggesting that caffeine may be a potential supplement for resistance training.