Many people believe that the frequency of our heart beats follows a fixed rhythm, but that is not the fact. Measurement of the electrical signals from the heart show that tiny differences occur between each heartbeat, also called heart rate variability (HRV). An average heart rate of 60 beat per minute does not mean that the interval between successive heartbeats would be exactly one second. In fact the heart beats may vary from 0.5 to 2.0 second. The interplay between the circulatory system organs and the autonomic nervous system is affected by complex biosignals (such as heart rate) which in turn contribute to a dynamic balance between the brain and the cardiovascular system. HRV is used as an indicator of the activity of the autonomic nervous system. A high HRV, which is evaluated to be associated with good cardiovascular health, indicates dominance of the parasympathetic response, the side of the autonomic nervous system that promotes relaxation, digestion, sleep, and recovery. The research literature has also established that individuals with a range of psychiatric disorders have reduced HRV, but more research in this field is needed.
The response to exercise training is often described in general terms, with the assumption that the group average represents a typical response for most individuals. However, in reality, it is more common for individuals to show a wide range of responses to identical exercise programs. In 1999, a large study published by Claude Bouchard and colleagues, reported that 20 % of us show little or no gain in maximal oxygen consumption (VO2max) with exercise training. This is a concern, since a high VO2max is associated with decreased rates of cardiovascular morbidity and mortality. Exploring the phenomenon of high responders and low responders following the same exercise program may provide helpful insights into mechanisms of training adaptation and methods of training prescription.
It is well established that aerobic exercise training causes reduction in the resting heart rate. This has traditionally been associated with autonomic signals from the nervous system. A new study recently published in Nature Communications challenges this view by demonstrating that reduction in resting heart rate is a result of training-induced changes in the heart itself, specifically, changes in the heart’s natural pacemaker, the sinus node.
In this study which is performed by CERG, NTNU, The University of Manchester and The University in Milano, the researchers showed in rodent models of endurance training that even the absence of stimulation from the autonomous nervous system, a reduced resting heart rate was still present. This result led to further investigation of mechanisms underlying training induced reduction in resting heart rate. The researchers demonstrated that the heart’s pacemaker changes in response to training and in particular there is a downregulation of an important protein known as the funny channel (or HCN4) which is responsible for controlling the rhythm of the sinus node and thereby responsible for the low heart rate.
Article about the study in BBC News: Endurance exercise interferes with heart rythm.
Athletes are considered the healthiest members of the society and aerobic exercise training is undoubtedly beneficial for the heart. However, elderly athletes with a lifelong history of training and competing in endurance events such as cross country skiing and marathons are prone to heart rhythm disturbances. The most frequent rhythm disturbance in response to exercise training is reduced resting heart rate, less than 50-60 beats per min. While normal adults have resting heart rates between 60-100 beats per min, it has been reported resting heart rates as low as 30 beats per min in elite endurance athletes, for example Bjørn Dæhlie had a heart rate of about 30 beats per min. Low heart rate in athletes is usually a benign physiological adaptation in the heart, however, it can cause problems especially in elderly athletes with a lifelong training history and veteran athletes are more likely to need an electronic pacemaker implantation (to keep the heart in regular rhythm).
Although exercise training is beneficial, intense endurance training can have harmful effects and in the referred study the authors have highlighted potential pathological remodeling in the sinus node. The volume and intensity of exercise training may be of critical importance for remodeling in the heart and identification of harmful training induced changes in the heart may lead to changes in training regimes to avoid such changes. In addition it may suggest potential therapeutic targets for veteran athletes which are prone to heart rhythm disturbances.
The positive effects exercise training has on the heart, more than outweigh the harmful once. So keep on exercising!
Anne Berit Johnsen, researcher at CERG