Repeatability of heart-rate variability and baroreflex sensitivity as metrics of cardiac autonomic function during exercise in young females and males

Purpose: We examined the inter-day repeatability of cardiovagal baroreflex sensitivity (BRS) and heart rate variability (HRV) metrics during a dual-intensity cycling protocol in healthy young males and females. Methods: Forty young adults (20 males, 20 females; age: 18–31 years) completed two randomized reclined cycling bouts at a moderate (50% heart rate reserve [HRR]) and vigorous (80% HRR) exercise intensity on separate days. HRV metrics and spontaneous BRS were analyzed over 2-min bins, using RR intervals and beat-to-beat systolic pressure obtained in a semi-recumbent position. The repeatability of BRS and HRV metrics was examined using coefficients of variation (CV), intra-class correlation coefficients (ICC), and Bland–Altman plots with 95% limits of agreement. Results: BRS showed overall good-to-excellent inter-day repeatability, which was similar across sex and exercise intensities (Females: 50%HRR: CV = 14%, ICC = 0.86, 95% LOA: -1.85 to 1.48 ms/mmHg; 80% HRR: CV = 20%, ICC = 0.86, 95% LOA: -2.24 to 1.98 ms/mmHg; Males: 50% HRR CV = 15%, ICC = 0.82, 95% LOA: -2.10 to 1.78 ms/mmHg; 80% HRR CV = 22%, ICC = 0.77, 95% LOA: -2.69 to 2.07 ms/mmHg). Vagal-related HRV metrics (high-frequency power, SD1, SDNN) exhibited good inter-day repeatability, not altered by vigorous-intensity exercise, with females showing lower repeatability for SD1 and SDNN compared to males (female: CV ~ 20–25%, ICC:0.70–0.85; male: CV ~ 15–20%; ICC:0.75–0.90). The root-mean square of successive differences exhibited the greatest repeatability for HRV metrics, being similar across sex and exercise intensity (CV ~ 13–16%, ICC ~ 0.90). Conclusions: BRS and vagal-related HRV metrics were repeatable during cycling across intensities and sex, supporting their use in detecting cardiac autonomic changes in clinical and active populations. Clinical trials.gov registration ID: NCT06617117.