The Mechanism of Bradycardia Evoked by Physical Training

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Abstract
The effect of training on heart rate was investigated in 14 young volunteers. A group of 11 active endurance athletes was also included to represent characteristics inherent to high working capacity. An approach to the mechanism of the heart rate changes was made through combined parasympathetic and beta-receptor blockade. The training period of 2 months'' duration reduced the heart rate only slightly at rest, but markedly during short term muscular exercise on an ergometer. The athletes had a low heart rate at rest and during exertion. Experiments with the blockade revealed that the bradycardia was mediated through different mechanisms at rest and during exercise. The bradycardia at rest was due to parasympathetic inhibition, whereas slow heart rate, upon exertion was due to a decreased sympathetic drive. Other possible mechanisms are briefly discussed.