Sympathetic and parasympathetic cardiac control in athletes and nonathletes at rest

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Abstract
A simple model to characterize sympathetic and parasympathetic effects on heart rate (R) was tested during rest in 10 nonathletes and 8 world-class oarsmen. The model states that R = mnRo, where Ro is the intrinsic cardiac rate, and m and n depend only on sympathetic and parasympathetic activity, respectively. The multipliers, m and n, were determined by dual pharmacological blockade in 2 sessions under similar conditions, but in 1 session propranolol and in the other atropine was given first. In agreement with the model, when corrections were made for atropine-induced blood pressure changes, m and n did not depend on which blocking agent was administered first. In athletes the control heart rate [55 .+-. 3.3 (SD) beats/min] and Ro (81 .+-. 8.3 beats/min) were lower than in nonathletes (62 .+-. 6.0, P < 0.01 and 102 .+-. 11, P < 0.001, respectively). The sympathetic multiplier, m, was similar (1.18 .+-. 0.06 vs. 1.20 .+-. 0.05, P > 0.4) in the 2 groups, but n, the parasympathetic multiplier, was closer to 1 in the athletes (0.57 .+-. 0.03 vs. 0.51 .+-. 0.05, P < 0.01). The model is suitable for the quantitative study of sympathetic/parasympathetic heart rate control in humans. The lower resting heart rate in oarsmen is solely due to a reduction in intrinsic cardiac rate, and not to an increase in parasympathetic tone.

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