Exercise training bradycardia: the role of autonomic balance.

We used an algebraic model of resting heart rate (HRr), HRr-mn (HRo), to compare resting parasympathetic (n) and sympathetic (m) influence, intrinsic heart rate (HRo), and resting autonomic balance (Abal) in ten endurance-trained (ET) and ten nontrained (NT) men. The values of m, n, and Abal were determined by selective pharmacological blockade with atropine and metoprolol. HRo was obtained during double blockade with atropine and metoprolol. HRo and HRr were significantly lower (P less than or equal to 0.04 and P less than or equal to 0.01, respectively) in the ET subjects (79.5 +/- 2.8 beats.min-1 and 54.7 +/- 3.0 beats.min-1, respectively) when compared to the NT subjects (86.6 +/- 2.5 beats.min1 and 70.2 +/- 3.1 beats.min-1, respectively). Parasympathetic influence (n) was greater in the ET subjects (P less than or equal to 0.04), while sympathetic influence (m) was slightly (P less than or equal to 0.05) less in the ET subjects. Consequently, the value of Abal was significantly less in the ET subjects (P less than or equal to 0.02), indicating that resting parasympathetic predominance was significantly greater in the ET subjects. We concluded that the exercise training bradycardia, observed in this group of subjects, was due to both a lower HRo and an Abal with an augmented parasympathetic dominance.