Purpose To investigate whether caffeine ingestion offsets the circadian-related decline in morning neuromuscular performance in females. Methods Thirteen healthy females completed three experimental trials: i) evening placebo (PM PLAC ), ii) morning placebo (AM PLAC ), and iii) morning caffeine ingestion (6 mg·kg⁻¹; AM CAFF ). A within-subject, repeated-measures, crossover, single-blind placebo design was used, with all subjects serving as their own controls. Each trial consisted of a maximal voluntary isometric contraction (MVIC) of the knee extensors, followed by intermittent isometric contractions (6 s contraction, 4 s rest) until task failure, and a post-fatigue MVIC. Time to exhaustion, torque and peak force, surface electromyography (RMS amplitude, MDF frequency), rate of perceived exertion, and tympanic temperature were all recorded across conditions. Results During PM PLAC , peak force and time to exhaustion were significantly greater (p 0.05) across trials. AM CAFF also improved (p 0.05) in temperature. Conclusions Morning ingestion of 6 mg·kg⁻¹ of caffeine effectively reverses diurnal reductions in neuromuscular performance of females by enhancing peak force and time to exhaustion to evening levels. No change in MDF, but an increased RMS suggests a central rather than peripheral mechanism for caffeine’s ergogenic effects.
Singh et al. (2026) studied this question.