Six weeks of high-intensity interval training improve autonomic and metabolic responses in active postmenopausal...

This study analysed the effects of a supervised high-intensity interval training (HIIT) programme on metabolic flexibility and autonomic function in active postmenopausal women. Twenty women (66.6 ± 6.2 years; 67.9 ± 11.1 kg) completed a 6-week cycling HIIT intervention consisting of three weekly sessions (20 min/session). Each session included five 1-min bouts at 170-184% of the power output achieved at maximal carbohydrate oxidation (MCO) determined during a submaximal graded FAT_max test, interspersed with 1-min active recovery at 0.5 W/kg. The initial 2-week period was included to progressively increase training intensity. Pre- and post-intervention assessments comprised a 30-min basal metabolic rate evaluation with continuous heart rate variability and gas-exchange measurements. Paired comparisons with bar and scatter plots were performed. At MCO intensity, significant improvements were observed in MCO (d = 0.76), ${\stackrel{.}{V}}_{O_2}$ (d = 1.04), gross and net (d = 1.63; d = 1.35) efficiency, and relative power (d = 2.67), with higher Stress Index (d = 0.72) and blood lactate (d = 0.55). Maximal fat oxidation (MFO) increased from 0.28 to 0.32 g/min (d = 0.51) alongside higher sample-entropy and reduced perceived exertion (d = 0.65, d = 0.80) at MFO intensity, despite no changes in basal metabolic rate or body composition. Changes in ${\stackrel{.}{V}}_{O_2}$ were negatively correlated with age (R² = 0.34), whereas net and gross efficiency improvements were positively correlated with this latter (R² = 0.23 and 0.26). Six weeks of HIIT elicited safe, time-efficient metabolic and autonomic adaptations in postmenopausal women. While power output, ${\stackrel{.}{V}}_{O_2}$ , carbohydrate oxidation, efficiency and Stress Index improved at MCO intensity, enhanced fat oxidation accompanied by greater autonomic complexity and reduced perceived effort reflect key findings at MFO.