1 Introduction

The term heart rate variability (HRV) conventionally describes the beat-to-beat fluctuations in the heart rate or the variations in consecutive RR intervals. The HRV is mainly caused by efferent modulations of the sinus node which is considered as the pacemaker of the heart rate.

Casting a retrospective glance, the existence of physiological rhythms in the beat-to-beat heart rate signal was attended twenty years prior to the first clinical application of the HRV which was appreciated in 1965 by Hon and Lee [32]. Twelve years later, Wolf et al. [96] found associations between a higher risk of post infarction mortality and reduced HRV. For many years, the HRV has only been expressed in mean values and standard deviations as a representation of the time domain analysis until Akselrod et al. [1] described the relation between quantitative evaluations of the beat-to-beat cardiovascular control by the power spectral analysis of the heart rate fluctuations.

Nowadays, the frequency domain analysis obtained by mathematical processing of the RR intervals is well accepted to assess the neural mechanisms controlling the heart rate. Thereby two main spectral components which are considered as markers of the sympathetic and parasympathetic control of the heart have been discriminated: a high frequency component (HF) which ranges from 0.4-0.15 Hz and a low frequency one (LF) ranging from 0.15-0.04 Hz. Furthermore, the HRV measure is apparently easy to derivate because of the availability of new, digital, high-frequency, long- and short-term multichannel electrocardiogram (ECG) recordings. Due to this, the HRV is considered as a useful method for both clinical and research studies to examine the autonomic nervous modulation of the heart. Especially specific experimental conditions as awake [page 3↓]and sleeping situations, different body positions, physical training as well as pathological conditions provide a good insight into the vegetative control of the heart. Nevertheless the susceptibility of the autonomic nervous system by factors such as respiration, internal and external influences have to be considered. To avoid affected HRV results, standardized measurements conditions in accordance to the guidelines of the Task Force [84] have to be established.

In animal model researches, dogs with acute myocardial ischemia were observed to show an increased HRV after 6 weeks of exercise training [84]. Based on these findings, exercise training was thought to accelerate the recovery of the physiological sympathovagal interaction and to decrease cardiovascular mortality and sudden cardiac death. Moreover, endurance trained athletes have been noted to show profound bradycardia (lower resting heart rate) compared with sedentary control which implicated enhanced vagal and/or diminished sympathetic activity [13, 46, 51,57, 70, 74, 85, 89]. Thus, physical activity was thought to affect positively the indexes of the HRV in healthy humans as well as in patients of specific pathologies.

Still conflicting results exist in literature concerning the effects of aerobic training on the HRV. Some studies have reported an increase in the magnitude of the HRV in the time and the frequency domain while others have reported absence of modifications of the sympathovagal balance in the sinus node. Most studies involve only male subjects whereas studies comparing trained with untrained females remain still inconsistent. Furthermore, few authors supposed modulation of the vegetative control of the heart in relation to the menstrual cycle in women whereas others could not find any affected HRV by endogenous female hormones [30, 50, 76, 75, 97].

On the basis of these considerations, one purpose of the present study was to investigate the effects of long term endurance training on the efferent autonomic cardiac control of the heart at rest in male and female athletes compared with sedentary controls. In addition another aim was to assess the possible influence of endogenous hormones on the vegetative control of the heart in endurance trained and untrained females. Therefore, the HRV was investigated throughout the menstrual cycle in women.

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