Abstract

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) than for AM PLAC . Peak force was higher (p < 0.01) in AM CAFF in both pre- (+33%) and post- (+45%) fatigue MVIC trials compared to PM PLAC . Similarly, RMS was higher (p < 0.01) during both AM CAFF and PM PLAC compared to AM PLAC , whereas MDF did not change (p > 0.05) across trials. AM CAFF also improved (p < 0.01) time to exhaustion by 43% compared to AM PLAC , and significantly reduced (p < 0.01) rate of perceived exertion, but without any change (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.