The aim of this study was to test the possibility of the ultra-short-term lnRMSSD (measured in 1-min post-1-min stabiliza-tion period) to detect training induced adaptations in futsal players. Twenty-four elite futsal players underwent HRV as-sessments pre- and post-three or four weeks preseason training. From the 10-min HRV recording period, lnRMSSD was ana-lyzed in the following time segments: 1) from 0-5 min (i.e., stabilization period); 2) from 0-1 min; 1-2 min; 2-3 min; 3-4 min; 4-5 min and; 3) from 5-10 min (i.e., criterion period). The lnRMSSD was almost certainly higher (100/00/00) using the magnitude-based inference in all periods at the post- moment. The correlation between changes in ultra-short-term lnRMSSD (i.e., 0-1 min; 1-2 min; 2-3 min; 3-4 min; 4-5 min) and lnRMSSDCriterion ranged between 0.45 - 0.75, with the highest value (p = 0.75; 90% CI: 0.55 - 0.85) found between ultra-short-term lnRMDSSD at 1-2 min and lnRMSSDCriterion. In conclu-sion, lnRMSSD determined in a short period of 1-min is sensi-tive to training induced changes in futsal players (based on the very large correlation to the criterion measure), and can be used to track cardiac autonomic adaptations.
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