Acute caffeine intake improves muscular strength, power, and endurance performance, reversing the time-of-day effect regardless of muscle activation level in resistance-trained males: a randomized controlled trial

Abstract

Introduction This study examined the effects of acute caffeine intake on muscular electrical activity during strength, power, and endurance performance tests at different times of day in bench press and back squat exercises. Methods Thirteen resistance-trained men participated in a triple-blind, cross-over, randomized controlled trial with four conditions: (a) morning with caffeine, (b) morning with placebo, (c) evening with caffeine, and (d) evening with placebo. Trials were conducted at 9:00 AM and 6:00 PM, with participants consuming caffeine or placebo (3 mg/kg) 60 min prior. Muscular strength/power was tested at 25%, 50%, 75%, and 90% one-repetition maximum (1RM) for bench press and back squat, while muscular endurance was assessed at 65% 1RM through a set-to-failure. Surface electromyography (EMG) measured muscle electrical activity. Results In muscular strength/power tests, caffeine increased mean velocity (V mean ) and power output (W mean ) in the back squat at 75% (P = 0.012–0.001, g = 3.84–2.71) and 90%1RM (P = 0.043–0.009, g = 2.77–2.46) in both morning and evening trials. At 25%1RM, caffeine counteracts morning performance decline in bench press (10–11%, P = 0.001, g = 2.62–1.68) and back squat (8–11%, P = 0.010–0.003, g = 2.22–1.64). In muscular endurance tests, caffeine increased V mean and W mean in bench press in the morning (11–12%, P = 0.003–0.005, g = 2.55–1.89) and back squat in both morning and evening trials (6–9%, P = 0.001–0.027, g = 2.79–1.73). EMG activity remained unchanged in all conditions. Conclusions Acute caffeine intake enhances muscular strength/power at moderate-to-high loads (75%- 90%1RM) and endurance performance (65%1RM) in the back squat while counteracting morning declines at light-load (25%1RM) for both exercises without altering muscle electrical activity.

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