Fig 1.
Schematic of experimental design and procedures.
Thirteen healthy female participants completed three conditions in a repeated-measures, single-blind, placebo-controlled crossover design. Three trials were completed: morning placebo (AMPLAC) at 0800, morning caffeine (AMCAFF) at 0800 (6 mg·kg⁻¹), and evening placebo (PMPLAC) at 1700. Anthropometric measures and core temperature were taken at the beginning of each trial. Each trial consisted of a 5 minute cycling warm-up at 90 rpm, 80 ± 10 W. The first (pre-) maximal voluntary isometric contraction (MVIC) was then performed for 5 seconds at 90 degrees knee flexion with 2 min rest periods between 3 efforts. A fatiguing protocol consisting of 6 second maximal isometric contractions interspersed by 4 seconds rest was then implemented, until peak torque declined to 50% of pre-MVIC levels, and then post-fatigue MVICs were performed. Surface electromyography and perceived exertion were measured throughout.
Table 1.
Outcomes measures across experimental conditions.
Fig 2.
Effect of time-of-day and caffeine ingestion on knee extensor isometric strength.
Morning caffeine (AMCAFF) increased torque and time to exhaustion compared with morning placebo (AMPLAC), reaching values comparable to the evening condition (PMPLAC). Data are presented as mean ± SD with individual values that represent single participants plotted. Trials were conducted in the morning (08:00) with placebo (AMPLAC), with caffeine ingestion (6 mg.kg⁻¹; AMCAFF), and in the evening (17:00; PMPLAC). Red bars represent data for AMPLAC trial, blue bars for AMCAFF, and grey bars for PMPLAC. Solid bars indicate pre-fatigue MVIC, and hatched bars indicate post-fatigue MVIC. (A) Peak torque normalised to body mass (N·m/kg) during pre-fatigue MVIC. (B) Peak torque normalised to body mass (N·m/kg) during post-fatigue MVIC. (C) Comparison of pre- and post-fatigue MVIC torque. (D) Time to exhaustion in seconds, defined as the time until <50% of pre-MVIC or task failure. Statistical analysis was performed using one-way and two-way repeated-measures ANOVA with Tukey’s post hoc tests. Significance is indicated as *p < 0.05, **p < 0.01, ***p < 0.001, ****p < 0.0001.
Fig 3.
Effect of time-of-day and caffeine ingestion on neuromuscular activation.
Morning caffeine (AMCAFF) increased EMG amplitude compared with morning placebo (AMPLAC), with no clear changes in median frequency, indicating enhanced neural drive without alterations in frequency-domain fatigue characteristics. Data are presented as mean ± SD with individual values that represent single participants plotted. Trials were conducted in the morning (08:00) with placebo (AMPLAC), with caffeine ingestion (6 mg.kg⁻¹; AMCAFF), and in the evening (17:00; PMPLAC). Red bars represent AMPLAC, blue bars AMCAFF, and grey bars PMPLAC. Solid bars indicate pre-fatigue MVIC, and hatched bars indicate post-fatigue MVIC. (A-C) Root mean square EMG amplitude measured in µV during pre- and post-fatigue MVIC. (D-F) Median frequency measured in Hz during pre- and post-fatigue MVIC. Statistical analysis was performed using one-way and two-way repeated-measures ANOVA with Tukey’s post hoc tests. Significance is indicated as *p < 0.05, **p < 0.01, ***p < 0.001, ****p < 0.0001.
Fig 4.
Effect of time-of-day and caffeine ingestion on perception of effort and core temperature.
Morning caffeine (AMCAFF) reduced perceived exertion compared with morning placebo (AMPLAC), with no meaningful differences in core temperature between conditions. Data are presented as mean ± SD with individual values that represent single participants plotted. Trials were conducted in the morning (08:00) with placebo (AMPLAC), with caffeine ingestion (6 mg.kg⁻¹; AMCAFF), and in the evening (17:00; PMPLAC). Red bars represent AMPLAC, blue bars AMCAFF, and grey bars PMPLAC. (A) Rating of perceived effort (scale 1-10) was taken during each trial, and (B) tympanic temperature was measured in degrees Celsius. Statistical analysis was performed using one-way repeated-measures ANOVA with Tukey’s post hoc testing. Significance is indicated as *p < 0.05, **p < 0.01, ***p < 0.001, ****p < 0.0001.