Sports Med 2003; 33 (12): 889-919
0112-1642/03/0012-0889/

Heart Rate Variability in Athletes

Andre E. Aubert, Bert Seps and Frank Beckers
Laboratory of Experimental Cardiology, School of Medicine, K.U. Leuven, Leuven, Belgium

This review examines the influence on heart rate variability (HRV) indices in athletes from training status, different types of exercise training, sex and ageing, presented from both cross-sectional and longitudinal studies. The predictability of HRV in over-training,  athletic condition and athletic performance is also included. Finally, some recommendations concerning the application of HRV  methods in athletes are made.

The cardiovascular system is mostly controlled by autonomic regulation through the activity of sympathetic and parasympathetic  pathways of the autonomic nervous system. Analysis of HRV permits insight in this control mechanism. It can easily be determined from ECG recordings, resulting in time series (RR-intervals) that are usually analysed in time and frequency domains. As a first approach, it can be assumed that power in different frequency bands corresponds to activity of sympathetic (0.04–0.15Hz) and  parasympathetic (0.15–0.4Hz) nerves. However, other mechanisms (and feedback loops) are also at work, especially in the low frequency band.

During dynamic exercise, it is generally assumed that heart rate increases due to both a parasympathetic withdrawal and an augmented sympathetic activity. However, because some authors disagree with the former statement and the fact that during exercise there is also a technical problem related to the non-stationary signals, a critical look at interpretation of results is needed.

It is strongly suggested that, when presenting reports on HRV studies related to exercise physiology in general or concerned with athletes, a detailed description should be provided on analysis methods, as well as concerning population, and training schedule, intensity and duration. Most studies concern relatively small numbers of study participants, diminishing the power of statistics. Therefore, multicentre studies would be preferable.

In order to further develop this fascinating research field, we advocate prospec- tive, randomised, controlled, long-term studies using validated measurement methods. Finally, there is a strong need for basic research on the nature of the control and regulating mechanism exerted by the autonomic nervous system on cardiovascular function in athletes, preferably with a multidisciplinary approach between cardiologists, exercise physiologists, pulmonary physiologists, coaches and biomedical engineers.

The manner in which the intact organism (in general) and the cardiovascular system (more specifically) responds to the stress of exercise has intrigued sports physiologists for the past century. The cardiovascular adjustments necessary to meet the extraordinary demands of the working musculature, which begins even before the onset of exercise, remains an area of intense investigation and speculation.[1,2] Also, anatomical geometry and cardiovascular function of the heart are altered after chronreic
physical activity.[3] For example, on the one hand, persistent volume load such as that elicited after endurance training (or its pathological equivalent after aortic or mitral insufficiency) leads to enlargement of the left ventricular internal diameter and a
advanceproportional increase in wall thickness.[4,5] This type  of adaptation is called eccentric left ventricular hypertrophy. On the other hand, a pressure load such as elicited after power training (or its pathological equivalent of aortic stenosis or hypertension) leads to a thickening of the ventricular wall and an unchanged internal dimension. This type of adaptation is called concentric left ventricular hypertrophy. An essential difference between exercise and pathological conditions is that the load on the heart is continuous in the latter case and intermittent in the former.

Other adjustments take place in almost every organ system of the body and involve all aspects of cardiac and peripheral vascular control, including regulation by the autonomic nervous system (ANS). Neural mechanisms appear to be of great importance in mediating the initial response to exercise, which involves very rapid changes in heart rate and blood pressure. All these phenomena involving heart rate and blood pressure are described as ‘cardiovascular variability’. Both phenomena covered in this review, exercise training and its relation to control and regulation of the cardiovascular function by the ANS, also have the following important clinical aspects: (i) can exercise training be used to retard the advance of coronary and other heart diseases?; and (ii) can HRV be used as a predictor or as a marker of  the progression of cardiovascular disease?

Understanding interactions between cardiovascular function, activity of the ANS and exercise training, remains a difficult problem. The disciplines of medicine, exercise and environmental physiology, physical education and biomedical engineering are all closely allied to study the effects of exercise and other stresses on cardiac structure and function.

This review discusses how some of the consequences of exercise training on the cardiovascularsystem can be deducted from measured basic experimental data of heart rate variability (HRV), aortic blood pressure variability (BPV) and baroreflex sensitivity (BRS). More specifically, time and frequency  analysis of heart rate will  be  described as a valuable tool to investigate the reflex mechanisms of cardiovascular regulation in active athletes in a fully non-invasive way.

The  parameters  of  HRV,  BPV  and  BRS  can simply  be  obtained  from  the  measurement of the ECG (and heart rate) and (non-invasive) blood pressure as will be shown in section 2. Indices from HRV and BPV can be studied in time (statistical studies)  and  frequency  domain (power spectrum). These indices can be a valuable non-invasive tool to investigate the reflex mechanisms of cardiovascular regulation   during   and   after   exercising, for  de-training and over-training, sex differences  and the effects of ageing.

This review discusses consecutively:  (i) control mechanisms of heart rate and blood pressure and the role of the ANS; (ii) how to measure experimentally and analyse HRV and BPV, starting from the ECG and (non-invasive) blood pressure signals; (iii) correlation between HRV and physical and physiologi cal parameters; and (iv) HRV data obtained from studies on athletes and related to training, training overload, and age and sex differences.

Read the full study: Heart Rate Variability in Athletes

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