Abstract
Special operations forces (SOF) soldiers are elite fighters and tactical professionals who perform in high-stress environments. SOF selection processes aim to identify candidates who can sustain performance in high-stress and changing conditions. The vagal nerve is a crucial moderator of stress responses, and its activity (indexed by heart rate variability, HRV) has been shown to predict performance and psycho-physiological resilience in various settings. However, its predictive validity needs to be clarified. This study examined the relationship between HRV and success in an intensive selection procedure. In a historical prospective study, we derived an HRV parameter (root mean square of successive differences between normal heartbeats, RMSSD) from a 10-second ECG of 365 candidates for an SOF naval unit. The ECG was taken approximately two months before the selection procedure. The predictive validity of other routinely obtained measures was also considered. High RMSSD was significantly associated with success, but this relation disappeared after controlling for confounders (e.g., running score). However, after matching pairs of successful and non-successful candidates on confounders, HRV was again significantly related to course performance. The results of this study support the predictive value of HRV for tactical professionals. Given the high cost of training elite soldiers and the burden they undergo, improving accuracy of the selection processes may reduce the burden on candidates and lead to resource savings. Future studies should measure HRV at several time points with longer ECG records.