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