Resting heart rate variability as a predictor of exercise response in mild post-COVID: insights from a six-minute step test

Background COVID-19 may impair autonomic and cardiorespiratory function, even in mild cases, resulting in reduced heart rate variability (HRV) and diminished functional capacity. Given their shared regulatory pathways, resting HRV may serve as a non-invasive predictor of oxygen uptake (V̇O 2 ) during the six-minute step test (6MST). Objective To investigate whether resting short-term HRV can predict V̇O 2 during the 6MST in individuals recovering from post-COVID. Methods In this cross-sectional study, adults recovering from mild COVID underwent assessment of autonomic modulation via short-term HRV and cardiorespiratory response during 6MST. HRV was recorded under standardized resting conditions. Gas exchange was measured throughout the 6MST. Spearman correlation and multiple linear regression analyses were performed to test associations between HRV parameters and V̇O₂ expressed in milliliters per kilogram per minute (mL·kg −1 ·min −1 ). Results Data from 45 participants were analyzed. Several HRV variables demonstrated statistically significant correlations with V̇O₂ and were therefore included in the simple linear regression analysis: SDNN (ms) (rho = 0.587), RMSSD (ms) (rho = 0.430), RR Tri (rho = 0.594), TINN (rho = 0.596), SD1 (ms) (rho = 0.431), SD2 (ms) (rho = 0.609), ApEn (rho = − 0.388), and DFA α2 (rho = − 0.404). Multiple linear regression showed that SD2 (ms) and sex were significant predictors of V̇O₂ (mL·kg −1 ·min −1 ) at the peak of the 6MST, while weight (kg) and age (years) were not. The model explained 50.7% of the variance (adjusted R 2 = 0.507, p < 0.001). Conclusion Several HRV parameters were significantly correlated with V̇O₂, indicating associations between cardiac autonomic modulation and aerobic performance. Among these, SD2 together with sex, emerged as significant predictors of VO₂ at the peak of the 6MST. Future studies will be needed to combine HRV indices with clinical outcomes in order to determine the mechanisms of V̇O₂ variability in post-COVID populations.