Autonomic nervous system response to close quarter battle training: Is there a threshold of stress adaptation?

Currently, there are no objective indices of cognitive or physiological responses to stress during close quarters combat (CQC) training. While shot accuracy and marksmanship reaction time are routinely evaluated during such training, tactical performance errors (i.e., loss of situational awareness, unnecessary threat exposure, failure to support battle buddy) may precede these behavioral end-states and be associated with the physiological stress response. Thus, this study’s purpose was to characterize CQC training effects on autonomic nervous system (ANS) activity and to determine the relationship between ANS activity and performance errors during a CQC simulation. Forty elite military personnel wore heart rate monitors during simulated CQC training. A standardized scenario was established, and operators were monitored during weeks 1 and 3 of training to determine changes in heart rate variability (HRV), heart rate, and respiratory rate. The training cadre collected performance metrics and provided them to research staff. Primary findings are heart and respiratory rates decreased while measures of HRV increased following CQC training. This study also found that higher high-frequency normalized units and low frequency/high frequency were associated with more safety violations and tactical performance errors. The novel finding of this study indicates that there is a positive correlation between vagal tone during the anticipation and execution phase of CQC training, and error rate; thus, indicating those who can regulate their sympathetic nervous system and exhibit flexibility between arousal and reset perform better in high-stress scenarios. This knowledge will allow for the identification of individuals who may not be adapting to the stress load as reflected by decrements in performance and maladaptation to stress.