Different volumes of physical activity are associated with changes in the autonomic nervous system control: Possible role of a composite autonomic index

Background and Purpose The Autonomic Nervous System plays a central role in exercise-associated adaptation, and its dysregulation is linked to many chronic conditions, making it a key therapeutic target. Nevertheless, it is rarely used in clinical practice due to technical/interpretative issues. We hypothesized that a composite-derived Heart-Rate-Variability (HRV) Index (ANSI) might overcome some of these limitations and capture adaptive differences associated with higher levels of physical activity, characterizing subjects exceeding the World Health Organization physical activity recommendations (≥ 1 200 MET·min/week). Methods In this cross-sectional observational study, we evaluated cardiorespiratory fitness (estimated by peak exercise capacity), autonomic regulation, and hemodynamic and metabolic parameters in 179 adults without overt cardiometabolic disease, grouped by weekly physical activity volume (METsTOT). Those above the highest recommendation (ABOVE-1 200 METs) were compared to those below (BELOW-1 200 METs). Group differences were analyzed using Mann-Whitney U tests, GLMs, and false discovery rate correction. Multivariate regression analysis was also performed. Results The ABOVE-1 200 METs group had lower adiposity, reduced stress perception, better metabolic profile, higher estimated cardiorespiratory fitness, and improvements in some HRV indices, including ANSI. However, after sex and age adjustment and false discovery rate correction, the ANSI was the only autonomic variable that remained significant. METsTOT was independently associated with ANSI ( β = 0.316, p < 0.001), with each additional 1 000 MET·min/week associated with a 7-point increase in ANSI. Conclusions The ABOVE-1 200 METs group exhibited differences across multiple domains, particularly in cardiac autonomic regulation. Among HRV-derived variables, ANSI emerged as a robust autonomic index. ANSI may serve as a simple, cost-effective, and integrative marker with potential clinical utility for monitoring autonomic regulation associated with changes in physical activity.