Studies on heart rate variability (HRV) hold increasing interest because of the predictive association between reduced HRV and mortality risks (1). While there is little doubt that endurance training increases vagal-related HRV indexes and thus has a protective effect against lethal arrhythmias, it has not been established whether a higher training load would increase this beneficial effect. During intensive training associated with fatigue or the stress of a world competition, studies have reported a conversion from vagal to sympathetic predominance. We decided to study a population of young students without any stressful stimuli which can obscure the effect of training per se. Autonomic control of the heart, as inferred from HRV analysis, was assessed in three groups of participants with distinct training states on the basis of their weekly training load evaluated by the Baecke questionnaire sport score. We examined HRV levels during slow wave sleep which is characterized by a pronounced vagal tone, and offers high stationarity for optimal HRV analysis (2). The results were confronted to sleep quality indexes and the profile of mood state (POMS) (3) assessing fatigue and stress level. Methods HRV was analyzed in three groups of subjects with different training states (sedentary, SED; moderately-trained, MT; and highly-trained, HT) in a period free of stressful stimuli or overload. HRV was analyzed in 5-min segments during slow wave sleep (SWS) identified by visual analysis of sleep recordings. ANOVA and post hoc Tuckey tests were made on log-tranformed values to be able to assume normality of the distribution for statistical analysis. Results We found that SWS, when the sinus node is mainly under vagal influences, allows discrimination of the changes in autonomic balance in subjects with different training states. HR was significantly lower in higly-trained athletes than in sedentary and middle-trained subjects (Figure). The moderately-trained athletes showed significantly higher HF/(LF+HF) than sedentary subjects. However, this training-induced effect was reversed in highly-trained athletes who presented significantly lower HF/(LF+HF) than moderately-trained students. Sleep parameters did not significantly differ among each group despite a tendency for sleep duration to be longer for MT than SED and HT. Concerning the POMS test, no differences in global measure of mood or in any intermediate score were noticed between the three groups (in particular, fatigue score was 43.7 ± 5.3, 44.5 ± 1.3 and 47.1 ± 2.0 for SED, MT, and HT, respectively). Discussion/Conclusion With a novel approach using slow wave sleep as an optimal condition for investigating autonomic cardiac control without any confounding influences, our study demonstrates that the increase in vagal-related HRV indexes after moderate training load is reversed in highly-trained athletes even in the absence of competition, fatigue or overload. These present results lend support to guidelines that recommend a moderate but not too high a training load for promoting health and preventing the age-related decline in vagal control of the heart.
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