Caffeine ingestion improves morning neuromuscular performance to evening levels in healthy females

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 (PMPLAC), ii) morning placebo (AMPLAC), and iii) morning caffeine ingestion (6 mg·kg⁻¹; AMCAFF). 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 PMPLAC, peak force and time to exhaustion were significantly greater (p < 0.05) than for AMPLAC. Peak force was higher (p < 0.01) in AMCAFF in both pre- (+33%) and post- (+45%) fatigue MVIC trials compared to PMPLAC. Similarly, RMS was higher (p < 0.01) during both AMCAFF and PMPLAC compared to AMPLAC, whereas MDF did not change (p > 0.05) across trials. AMCAFF also improved (p < 0.01) time to exhaustion by 43% compared to AMPLAC, 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.