Context: The lack of consensus on the best approach to assess the fatigue and readiness status of swimmers has led to an increased risk of maladaptation, overtraining, and suboptimal readiness to perform. Subjective athlete monitoring is prominent in swimming but is prone to bias, distortion, and misinterpretation. Heart-rate variability (HRV) has gained attention from swimming practitioners as an objective monitoring tool for assessing physiological status. Objectives: The aim of the study was to investigate the validity of HRV in detecting changes in physiological status between overload, recovery, and taper training phases in elite swimmers. Methods: Eleven state- to international-level swimmers were recruited for the study. Retrospective routine HRV, training, and competition data were accessed for 5 months leading into the Tokyo 2020 Olympics national trials. Linear mixed models examined the relationship between training phase, swimming performance, and 3 HRV metrics (average of all normal R-R intervals [AVNN], root mean square of successive R-R intervals [rMSSD], and rMSSD/AVNN). Results: rMSSD/AVNN was significantly higher in overload training compared with regular training (mean difference = 0.020, P = .002; effect size = moderate). No statistical difference was found for rMSSD or AVNN between training phases (P > .05). Descriptive analyses revealed intraindividual and interindividual variability in HRV response to training phase. Successful swimmers reported higher parasympathetic dominance in early taper followed by increased sympathetic activity leading into competition. Conclusion: The findings suggest that a ratio measure between parasympathetic tone and modulation may provide a more comprehensive evaluation of an athlete`s physiological status compared with a sole parasympathetic modulation or tone marker. Further research is recommended to improve our understanding of the autonomic nervous system`s response to overload and taper training phases.
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