BACKGROUND: Virtual reality (VR) technology enables safe simulation of hazardous water environments and may provide a novel approach for risk assessment. Autonomic nervous system responses, particularly heart rate variability (HRV), reflect stress responsiveness and physiological adaptability and may serve as potential indicators of dropout risk during long-distance swimming. This study examined the relationship between cardiac autonomic responses during swimming VR exposure and dropout occurrence during an actual long-distance swim. METHODS: Thirty-one healthy adults (16 men, 15 women) participated in a target 3-hour long-distance sea swim and were classified as Finishers (n = 22) or Did Not Finish (DNFs; n = 9). Before the event, participants completed a 400-m breaststroke time trial, an exercise stress test to assess aerobic capacity, and a swimming VR viewing session using a head-mounted display. HRV was assessed during 2 min of baseline rest and 2 min of VR viewing. The change from baseline to VR viewing was calculated for heart rate, lnLF/lnHF and lnHF. RESULTS: There were no differences between the groups in physical characteristics, aerobic capacity, or 400-m swimming performance. However, DNFs showed a lower level in *lnLF/lnHF from baseline to VR viewing compared with Finishers (p < 0.05). Additionally, no abrupt changes in heart rate were observed at the time of dropout. CONCLUSIONS: Participants with lower HRV responses during swimming VR may be more likely not to complete a long-distance swim. However, HRV alone is not enough to predict this risk and should be combined with other measures.
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