Increased incidence of arrhythmias in the athlete's heart. There is a well demonstrated association between atrial fibrillation and/or flutter and endurance exercise training. There is also an increase in premature ventricular beats, although this tends to be benign in most athletes. Although there is some speculation that extreme exercise might cause serious arrhythmias in some cases, these events remain very uncommon. PPM, permanent pacemaker; RV, right ventricular.