ObjectiveTo elicit a criterion elevation (>10%) in resting heart rate (HR) with training overstress, and subsequently test the hypothesis that such “reversed bradycardia” (RB) negatively affects running performance.DesignProspective before-and-after intervention with a comparison group.SettingGeneral community.Participants21 healthy male marathon runners.InterventionVoluntary doubling of training miles on 14 consecutive days.Main Outcome MeasuresLeft ventricular (LV) function by echocardiography, HR, and plasma epinephrine (PE) at rest and during submaximal exercise, and 15 km road run performance.ResultsTwo days after the training overstress, 12 runners met the criterion (RB group), showing an average elevation in resting HR of 16% (range: 11 to 23%). The RB group also exhibited hyperkinetic LV shortening (p < 0.05), elevated exercise HR (p < 0.001), increased PE at rest and during exercise (p < 0.05), and reduced 15 km performance (p < 0.05). The other nine runners who maintained a stable resting HR during the intervention showed no significant outcome changes.ConclusionsIn addition to muscular overuse, heightened sympathetic drive likely contributed to the observed reversal of bradycardia. The development of this stress-related cardiac perturbation was associated with a decrement in running performance, confirming the hypothesis.