The HRV analysis, which gives a good insight into the autonomic control of the heart, has been rarely investigated in athletes; data particularly of female athletes are still missing. Due to the unclear physiological significance in athletes, the HRV analysis is not yet practicable in relation to different training interventions, which may cause short and long time disturbances beside acute or chronically adaptations of the autonomic nervous system. Nevertheless, short time recording of the HRV as well as the orthostatic reflex control are assumed to be a valuable tool to supervise and/or control the training process in male and female athletes to prevent overstrain and overtraining syndromes.
Therefore, these two main questions in relation to the HRV have been investigated in the present study:
HRV measurements repeated five times during one month, i.e. one menstrual cycle, were examined by short time ECG recordings of 20 min at rest as well as during an orthostatic test consisting of three parts: 20 min supine, 10 min standing and 20 min [page 113↓]supine. On one hand the HRV was compared between 24 long term endurance trained (≥2 years of training) athletes who were involved in marathon running, cycling and/or triathlon and 27 sedentary controls. And on the other hand, the HRV was investigated in trained (n=11) and untrained (n=11) normal ovulatory women in course of five different menstrual cycle phases which were individually determined based on the basal body temperature of the preceding month.
The measurement conditions of the ECG recording were standardized for each study day because of the known effects on the HRV of internal and external stimuli. The ECG recordings were done by commercial equipment designed to analyse short time HRV. Prior to the evaluation, the ECG signals at rest were shortened to 10 min sequences. First the correction of the ECG signals was done by hands followed by the application of an automatic detection algorithm implemented by RASCHlab; no interpolation was used. Finally the HRV was calculated in the frequency domain by a Fast Fourier Transform after a band pass filtering by a Hamming window and in the time domain by descriptive mathematical methods. Due to the missing golden standard in the analysis of the HRV, the guidelines of the Task Force  were followed in the present study.
In the present study, male and female athletes showed significantly higher HRV which explained enhanced vagal activity in the time domain whereas an increased HF power in the frequency domain was missing. However, enhanced LF power was noted in male and female athletes. Moreover, male athletes showed a BF which had its main frequency inside the LF power band. These resulted in an overlapping of the HF inside the LF power band which leaded to an increased LF power in trained subjects. Based on these findings, we conclude that we did not fail to demonstrate an increased vagal activity in trained males due to the missing enhanced HF power in athletes compared with controls. However, trained females did not show an affected HF power by slow BF. The missing HF as well as the augmented LF power was suggested to be induced by other mechanisms. Therefore, the influence of the individual daily training program during the study was taken into consideration. Several authors [28, 35, 57] described depressed vagal and enhanced sympathetic activity which lasted up between 24-48h after trainings of higher work load. The individually different training pattern of each athlete was led by daily protocol during the study. On the basis of these data, athletes trained in average 4 times per week with an individual variance between 3-7 times per [page 114↓]week. Based on the above mentioned studies [28, 35, 57] this would imply, that male and female athletes would have a training induced modulation of the vegetative control of the heart due to lacking rest prior to the HRV measurements. Nevertheless the orthostatic provocation was similar in trained and untrained subjects with a vagal predominance at rest and an augmented sympathetic and/or reduced parasympathetic activity while standing induced by an active body position change.
In course of the menstrual cycle, hormonal fluctuations of LH, FSH, E2 and P could be demonstrated without any differences in athletes and sedentary women. Still the parameter of the time as well as the ones of the frequency domain remained similar in the five phases. No differences were noted between the HRV of trained and untrained women in course of one menstrual cycle. Even the orthostatic provocation reacted not significantly different between the menstruation, the middle of the follicular, the ovulation, the middle of the luteal and the pre menstruation phase. These findings which are consistent with Leicht et al.  induce, that the hormonal fluctuation in normal ovulatory active and sedentary women did not directly influence the vegetative control of the heart. Although an enhanced BF throughout the menstrual cycle was noted, no affected HRV was found because the BF remained inside the HF power band in women. Therefore, the hormonal induced modulation of the BF did not affect the HRV results in the present study.
In summary, male and female athletes showed enhanced HRV parameters in the time domain compared with sedentary subjects. Nevertheless, we failed to demonstrate enhanced HF power in athletes at rest primarily because of respiration and secondarily because of training induced modulations of the autonomic nervous control of the heart. Still, the orthostatic provocation was similar in both groups. Furthermore, no menstrual cycle related HRV changes could be found at rest or during the orthostatic provocation in normal ovulatory females. Based on these findings, a direct influence of the endogenous hormones on the autonomic nervous control of the heart can be excluded.
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