BACKGROUND: Heart Rate Variability (HRV) is a non-invasive method of assessing the autonomic nervous system response during exercise and fatigue. OBJECTIVE: The aim of the study was to analyze the validity and feasibility of the stress score index (SS) calculated from SDNN values during exercise. METHODS: 18 Men performed 2 running tests: 1) incremental exercise test; 2) 10-minute constant load test. Subjects underwent HRV analysis during the constant load test, before both tests, and afterward in a seated position at 3 intervals (0`-5`, 5`-10`, 10`-15`). The relationship between SDNN and SD2 was analyzed before, during, and after the test. SS was calculated as 1/SD2*1000. The Bland-Altman test analyzed the reliability of ESS. The bias, limits of agreement (LoA), standard deviation of difference, intraclass correlation (ICC), and person coefficient were calculated. RESULTS: The bias was 0.15 ± 2.54 (UperLOA: 2.54; LowerLOA: -2.23). In all conditions, SD2 and SDNN showed a positive linear relationship (r2 = 0.986); SS and ESS were correlated; and SS and ESS described a positive linear relationship (r2 = 0.993). CONCLUSIONS: The SS index calculation from SDNN is a reliable alternative during exercise.
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