Purpose: Various measures of autonomic function have been developed, and their applicability and significance during exercise are controversial. Methods: Physiological data were therefore obtained from 23 sport students before, during, and after exercise. Measures of R-R interval variability, QT variability index (QTvi), and electrodermal activity (EDA) were calculated. We applied an incremental protocol applying 70%, 85%, 100%, and 110% of the individual anaerobic threshold for standardized comparison. Results: Although HR increased stepwise, parasympathetic parameters such as the root mean square of successive differences were not different during exercise and do not mirror autonomic function satisfactorily. Similar results were observed with the approximate entropy of R-R intervals (ApEnRR). In contrast, the increase in sympathetic activity was well reflected in the EDA, QTvi, and ApEn of the QT interval (ApEnQT)/ApEnRR ratio. Conclusion: We suggest that linear and nonlinear parameters of R-R variability do not adequately reflect vagal modulation. Sympathetic function can be assessed by EDA, QTvi, or ApEnQT/ApEnRR ratio.
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