The first aim of the study was to verify if in individuals with different physical fitness levels the Recurrence Quantification Analysis (RQA) of Heart Rate Variability (HRV) time series could be an alternative to Gas Exchange (GE) analysis in the determination of metabolic thresholds. The second aim was to investigate the validity of the RQA method compared to the GE method in thresholds detection. The two metabolic thresholds were estimated in thirty young individuals during Cardiopulmonary Exercise Testing on a cycle-ergometer and HR, VO2 and Workload were measured by the two different methods (RQA and GE methods). RM ANOVA was used to assess main effects of methods and methods-by-groups interaction effects for HR, VO2 and Workload at the aerobic (AerT) and the anaerobic (AnT) thresholds. Validity of the RQA at both thresholds was assessed for HR, VO2 and Workload by Ordinary Least Products (OLP) regression analysis, Typical Percentage Errors (TE), Intraclass Correlation Coefficients (ICC) and the Bland Altman plots. No methods-by-groups interaction effects were detected for HR, VO2 and Workload at the AerT and the AnT. The OLP regression analysis showed that at both thresholds RQA and GE methods had very strong correlations (r >0.8) in all variables (HR, VO2 and Workload). Slope and intercept values always included the 1 and the 0, respectively. At the AerT the TE ranged from 4.02% to 10.47% (HR and Workload, respectively) and in all variables ICC values were excellent (≥0.85). At the AnT the TE ranged from 2.61% to 6.64% (HR and Workload, respectively) and in all variables ICC values were excellent (≥0.89). Our results suggest that the RQA of HRV time series is a COVID-safe approach for the determination of metabolic thresholds in individuals with different physical fitness levels, therefore, it can be used as a valid method for threshold detection alternative to gas analysis.
