Heart Rate Responses in the Diving Reflex among Aerobically Trained and Untrained Men

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The mechanisms of the diving reflex involve the simultaneous activation of the sympathetic and parasympathetic nervous systems (PNS). The enhanced PNS capabilities as achieved through aerobic training has been investigated in the diving reflex, specifically vagally-mediated bradycardia, and the research remains controversial. PURPOSE: The current study was conducted to evaluate heart rate responses associated with the diving reflex in aerobically trained and untrained men. METHODS: Using 2 x 3 mixed factorial ANOVA, lowest heart rate achieved, percent heart rate reduction, and time course of heart rate reduction were compared between aerobically untrained (n = 7) and trained (n = 9) men while breath holding in air, water at 15 °C, and water at 0 °C. RESULTS: Results showed that trained men did not differ in percent heart rate reduction or lowest heart rate achieved, although there was a tendency for lowest heart rate achieved in the trained group (p = .06). A significant interaction was found for time course (p = .01), where trained men took significantly longer to reach a plateau in heart rate in the air compared to untrained men. No differences in training status were found for time course in the two water conditions. However, a linear increase in time course for untrained men in the water conditions indicated a potentially delayed response by the PNS with the diving reflex. CONCLUSION: No additional clarification on the impact of training status on the diving reflex has been made except that an effect may lie in the timing of the response of the PNS.