Load and muscle group size influence the ergogenic effect of acute caffeine intake in muscular strength, power and endurance

Introduction Although acute caffeine intake seems to improve muscular strength–power–endurance performance, there is scarce evidence evaluating upper vs lower-body exercises at different loads. Thus, this study aimed to examine the effects of acute caffeine intake on upper and lower-body muscular strength, power and endurance performance at different loads. Methods Twenty resistance-trained athletes (male/female: 10/10; age: 23 ± 4 years; body mass: 70.6 ± 15.1) participated in a double-blind, placebo-controlled, cross-over and randomized study. Participants were provided with either 3 mg/kg of body mass of caffeine or maltodextrin (placebo). Sixty minutes after ingestion, they performed muscular strength and power assessment for bench press and back squat exercise at 25%, 50%, 75% and 90% 1-repetition-maximum (1RM), performing 3, 2, 1 and 1 repetitions respectively, followed by muscular endurance assessment for both exercises at 65% and 85% 1RM performing until task failure. Isometric handgrip, isometric mid-thigh pull and vertical jump tests were also performed. Results In muscular strength and power, compared to placebo, caffeine improved mean velocity ( P = 0.045; p η 2 = 0.101), mean power ( P = 0.049; p η 2 = 0.189) and rate of force development (RFD, P = 0.032; p η 2 = 0.216), particularly in back squat exercise at 75% and 90% 1RM where mean velocity increased by 5–7% ( P = 0.48–0.038; g = 0.348–1.413), mean power by 6–8% ( P = 0.050–0.032; g = 0.547–0.818) and RFD by 17–97% ( P = 0.042–0.046; g = 1.436–1.196). No differences were found in bench press exercise. In muscular endurance, caffeine improved the number of repetitions in all exercises and loads ( P = 0.003; p η 2 = 0.206), but only in back squat exercise at 85% 1RM, caffeine increased mean and peak velocity (8–9%, P = 0.006–0.004; g = 2.029–2.075), mean and peak power (10–13%, P = 0.006–0.003; g = 0.888–1.151) and force peak (3%, P = 0.009; g = 0.247). Conclusions Acute caffeine intake (3 mg/kg) improved muscular strength, power and endurance performance, revealing a more pronounced effect at high-loads (≥ 75% 1RM) and in lower-body (back squat) than in upper-body exercise (bench press) according to muscle group size.