Cardiac Autonomic Function and Sports Activity Part 2

Behaviour of HRV During Exercise

During the performance of an exercise test HR and blood pressure increase constantly which is presumed to be due to higher sympathetic tone associated to withdrawal of the parasympathetic input. When using spectral HRV parameters during an acute physical effort, it would be expected to assist to an increase of the LF and a decrease of the HF components with a concommitant increase of the LF/HF ratio, consistent with sympathetic hyperactivity and parasympathetic drive during recovery. Although there is evidence to suggest a shift towards sympathetic dominance, particularly at the peak of highly strenuous training regimens [65-67] the expected behaviour of HRV parameters has not been systematically observed. In some studies of HRV absolute LF and HF power decreased during exercise. However when quantifying HF and LF power in normalized units, paradoxical results have been found. Indeed, normalized HF power, a marker of vagal activity, instead of diminishing, it increased gradually during exercise, whereas normalized LF power, a marker of sympathetic activity, decreased during exercise. [55, 68, 69]

One possible explanation for this finding is a decline in the periodic nature of the spectral peaks during exercise testing and, thus, of their density.

We performed in a group of 15 high-level male athletes (unpublisehd data) two maximal graded stress tests on a cycle ergometer, the first one at 8.30 am and the second one at 4.30 pm, and one submaximal one-hour exercise test (endurance test) at 12.30 pm with a constant workload of 75 watts. The athletes were composed of 2 weight-lifters, 3 cyclists, 2 longdistance runners, 1 soccer and 7 hockey players. Mean age was 29±6 years. Each athlete was under a continuous 24-hour Holter recording over the whole test day. Spectral HRV parameters were taken from these 24-hour Holter recordings. Spectral values were given in absolute logarithmic (ln ms2) units. During the morning and the afternoon maximal stress tests when reaching a maximal HR the LF and the HF components as well as their ratio dramatically decreased (Table 3). During the post-effort period both components began to recover already after the third post-effort minute and entirely recovered one hour after the morning and the afternoon stress tests were completed.

Table 3. Morning and afternnon maximal graded stress tests

PE = post-effort.

The behaviour of LF and HF components was somewhat different in the course of the endurance test performed at 12.30 pm with a constant workload of 75 watts (Table 4). Indeed, during this test a submaximal HR was achieved and this resulted only in a slight decrease of the LF component associated with a more marked decrease of the HF component. However, when compared with the findings during both maximal stress tests the changes were much less pronounced.

Table 4. Submaximal one-hour exercise test

PE = post-effort.

In another study recently published by our group [70] we found that during an endurance mountain running all spectral components of HRV, particularly VLF and LF power, dramatically decreased during the ascent, and progressively normalized during descent and arrival. We concluded that the behaviour of our HRV data was due to an extreme activation of the sympathetic nervous system. The physiological response of the heart in this situation was a down-regulation of the B-adrenergic receptors to protect the myocardial function with subsequent rise in parasympathetic tone, reflected by an increase of the high frequency (HF) power and a decrease of the LF/HF ratio.

Thus, when considering our data the decrease of LF and HF during strenuous exercise appears to correspond to a protective physiological response of the heart to the high level of circulating catecholamines. The rise in vagal tone after the effort expresses in a healthy population expresse an .integer autonomic function.

Conclusions

Regular sports activity increases HRV, mainly by enhancing the parasympathetic tone, suggesting thereby a beneficial influence on cardiac autonomic activity and playing thereby a cardio-protective role. The type of sports discipline performed may also play a role and have a variable effect on HRV parameters. Thus, endurance disciplines, such as running and cycling, may have a particularly beneficial effect on the global cardiac autonomic activity. During strenuous exercise spectral HRV parameters expressed in absolute values decrease, however when expressed in normalized units a paradoxical increase of the normalized HF power has been found.