Exercise is a major component of a healthy lifestyle, and the benefits of regular physical activity are well established. However physical activity sometimes is accompanied by certain arrhythmias – premature beats or atrial fibrillation (AF). Less often such activity may result in life-threatening arrhythmias – specifically ventricular fibrillation (1). The relationship between physical activity and risks of AF continues to be a subject of scientific debate. Sub sole nihil novi(lat.) – in a broad sense, it would be wrong characteristics of the article by Nielsen et al. (published in this issue of JAFIB). In fact, numerous studies have been conducted regarding (AF) occurrence and risks in healthy people or in people with undefined health status in statistically meaningful groups. Large-scale meta-analysis of long-term studies and remarkable comparative insights highlight an ambiguity of AF behavior in different groups of individuals (adults, athletes and non-athletes) with their different intensity levels of physical activity. Authors have revealed that both long-term vigorous physical training as well as a lack of physical activity are related to increased risk of AF. Their discrete hint on the presence of U-shaped relationship between physical activity and risks of outbreak of AF is very important for comprehension of AF. Conclusions in fact elegantly reflect true reality emphasizing that we deal with volatile and heterogeneous nature of AF. At large, irrespective of the intensity of physical activity, the new onset of AF paroxysm is unpredictable. Deductively, the results of this remarkable study reveal the presence of individual cardiac/atrial exercise tolerance or individual sensitivity to physical stress. In other words, there probably is some individual stress level threshold above which the risk of AF paroxysm in a specific lifetime may culminate.