Effects of acute caffeine supplementation on muscle strength and muscular endurance: a randomized, double-blind, placebo-controlled crossover trial in recreationally active women and men

Abstract Background Caffeine is widely used as an ergogenic aid; however, evidence regarding its effects on muscular strength and muscular endurance, particularly in women, remains inconsistent. This study investigated the acute effects of caffeine supplementation on muscle strength and muscular endurance in recreationally active women and men. Methods In this randomized, double blind, placebo controlled, crossover clinical trial, 14 recreationally active adults (7 women and 7 men; 18 to 35 years) completed two experimental sessions separated by 7 days, ingesting caffeine (6 mg·kg⁻¹) or placebo (Ethics Committee Nº: 5.951.416). Muscle strength was assessed using handheld dynamometry for multiple muscle groups, including handgrip, quadriceps, gluteus maximus and medius, biceps, deltoid, and pectoralis major. Muscular endurance was evaluated using the static squat and push up tests. Results No significant condition effects were observed for any strength or performance outcomes ( p  = 0.21). Handgrip strength showed no differences between caffeine and placebo for the right hand ( p  = 0.40; η² p  = 0.06) or left hand ( p  = 0.99; η² p  = 0.00). Knee extension strength was unaffected by caffeine ( p  = 0.45; η² p  = 0.04), as were gluteus maximus ( p  = 0.213) and gluteus medius strength ( p  = 0.70). Upper limb strength measures also showed no condition effects for biceps ( p  = 0.67), deltoid ( p  = 0.424), or pectoralis major ( p  = 0.62). Muscular endurance outcomes were similarly unchanged, with no caffeine effect on static squat time ( p  = 0.52) or push up repetitions ( p  = 0.71). A consistent main effect of sex was observed across most outcomes, with men demonstrating greater absolute strength than women, including handgrip (η² p  = 0.62 left hand to 0.65 right hand), deltoid (η² p  = 0.71), pectoralis major (η² p  = 0.73), and biceps (η² p  = 0.61). No meaningful sex by condition interactions were detected, except for a modest interaction in pectoralis major strength ( p  = 0.037; η² p  = 0.31). Conclusions Acute caffeine supplementation at 6 mg·kg⁻¹ did not enhance muscle strength or muscular endurance in recreationally active adults, regardless of sex. Although men exhibited greater absolute strength, the magnitude of response to caffeine was similar between women and men. These findings reinforce the concept of substantial interindividual variability in caffeine ergogenicity and highlight the importance of considering exercise modality, training status, and individual characteristics when evaluating caffeine’s performance effects.