Seven-Day Caffeine Supplementation Enhances Upper-Body Peak Anaerobic Power Without Altering Cardiac Autonomic Function or Cognitive Performance in Active Women: A Double-Blind, Placebo-Controlled Trial
Seven-Day Caffeine Supplementation Enhances Upper-Body Peak Anaerobic Power Without Altering Cardiac Autonomic Function or Cognitive Performance in Active Women: A Double-Blind, Placebo-Controlled Trial
This study investigated the effects of a 7-day caffeine supplementation on heart rate variability, cognitive performance, and upper-body anaerobic power in active women. Thirty healthy active women aged 25-30 years participated in this double-blind, placebo-controlled trial. Participants were randomly assigned to receive either 5 mg/kg of caffeine or a placebo daily for 7 days. Heart rate variability, cognitive function (measured by the Stroop test), and upper-body anaerobic power (measured by a 20-second Wingate test) were assessed at baseline, 90 minutes after supplementation, and 30 minutes after exercise. The primary outcome was peak anaerobic power. The study aimed to determine whether short-term caffeine habituation enhances exercise performance without altering cardiac autonomic function or cognitive performance.
Inclusion Criteria:
Active women aged 25 to 30 years
Body mass index (BMI) between 18 and 25 kg/m²
Physical activity readiness (based on the PAR-Q questionnaire)
No injuries within the last month
Non-smoking
No cold or COVID-19 infection in the past four weeks
No cardiovascular, metabolic, or respiratory infections in the last three months
Normal blood pressure
No use of anabolic steroids, central nervous system stimulants, or other sports supplements in the past six months
Exclusion Criteria:
Inability to follow the protocol
Daily caffeine intake exceeding 300 mg (assessed using an adapted FFQ)
Consumption of alcohol or intense aerobic activity within 48 hours prior to testing
Resistance training with weights within 72 hours prior to testing
Caffeine consumption within 24 hours prior to testing
This was a double-blind, placebo-controlled, parallel-group study conducted at the University of Birjand, Iran. Thirty active women (aged 25-30 years, BMI 18-25 kg/m²) were randomly assigned to either a caffeine group (n=15) or a placebo group (n=15). Participants received 5 mg/kg of caffeine powder or placebo (flour) in gelatin capsules daily for 7 days.
Heart rate variability (HRV) parameters including RMSSD (Root Mean Square of Successive Differences), SDNN (Standard Deviation of Normal-to-Normal Intervals), LF (Low-Frequency) power, HF (High-Frequency) power, and LF/HF ratio were measured using a Polar H10 heart rate sensor. Cognitive performance was assessed using the Golden Stroop Test (color-word interference task). Upper-body anaerobic power was measured using a 20-second Wingate test on a cycle ergometer (Monark 891 E) with a workload of 50 g/kg body weight.
Measurements were taken at three time points: baseline (after 10-hour overnight fast), 90 minutes post-ingestion, and 30 minutes post-exercise. The protocol was repeated after 7 days of daily supplementation. Data were analyzed using repeated-measures ANOVA with Bonferroni post-hoc tests. The primary outcome was peak power output (W·kg-¹) during the Wingate test. Secondary outcomes included HRV parameters, cognitive performance scores, and other anaerobic performance indices (mean power, minimum power, fatigue index).