Autonomic Response to the Harvard Step Test in Normal BMI and Overweight Young Adults: A Heart Rate Variability Study

Background Excess adiposity in young adults is associated with early autonomic imbalance, preceding the development of overt cardiovascular disease. Heart rate variability (HRV) is a simple, non-invasive measure of cardiac autonomic modulation. The present study aimed to compare resting and early post-exercise HRV between normal body mass index (BMI) and overweight young adults using the Harvard Step Test. Methodology This comparative, analytical, observational study with repeated measures included 200 young adults aged 17-25 years, divided into a normal BMI group (n = 100) and an overweight group (n = 100). After anthropometric assessment, baseline electrocardiography (ECG) was recorded following 5 minutes of supine rest. Participants then performed the Harvard Step Test on a 23-cm stool at a rate of 40 steps per minute for a maximum of 5 minutes or until fatigue, whichever occurred earlier. Post-exercise ECG recordings were obtained for 15 minutes, and the first 5 minutes were used for early recovery HRV analysis. Time-domain HRV parameters included mean RR interval, mean heart rate (HR), standard deviation of normal-to-normal intervals (SDNN), root mean square of successive differences (RMSSD), number of pairs of adjacent normal-to-normal intervals differing by more than 50 ms (NN50 count), and percentage of NN50 counts (pNN50). Frequency-domain parameters included high-frequency (HF) power and low-frequency/high-frequency (LF/HF) ratio. The Physical Fitness Index (PFI) was also calculated. Between-group and within-group comparisons were performed, along with adjusted regression, mixed-effects modelling, and sensitivity analyses. Results Age and sex distribution were comparable between groups. At baseline, the overweight group demonstrated significantly lower mean RR interval, SDNN, RMSSD, NN50, and pNN50, along with higher mean HR compared with the normal BMI group (all p < 0.001). Following the exercise, both groups exhibited significant autonomic changes. However, post-exercise SDNN, RMSSD, NN50, pNN50, and HF power remained significantly lower in the overweight group. In adjusted regression analysis, overweight status was independently associated with lower baseline RMSSD (β = -20.23 ms), lower post-exercise RMSSD (β = -5.40 ms), lower post-exercise HF power (β = -82.64 ms²), and lower PFI (β = -11.36) (all p ≤ 0.001). Mixed-effects modelling demonstrated a significant group × time interaction for SDNN, RMSSD, and HF power, while mean HR did not show a significant interaction effect. Conclusions Overweight young adults exhibit reduced vagal-mediated HRV at rest and a less favorable early post-exercise autonomic recovery profile compared with normal BMI individuals. Among HRV indices, RMSSD and SDNN showed the most consistent alterations and may serve as useful early markers of autonomic dysfunction in this population.