Six weeks of high‐intensity interval training improve autonomic and metabolic responses in active postmenopausal women

Abstract 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), ( 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 were negatively correlated with age ( R 2 = 0.34), whereas net and gross efficiency improvements were positively correlated with this latter ( R 2 = 0.23 and 0.26). Six weeks of HIIT elicited safe, time‐efficient metabolic and autonomic adaptations in postmenopausal women. While power output, , 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.