7-Day Caffeine and Upper-Body Power in Active Women

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).

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