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

Overview

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.

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).