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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