Heart Rate Variability Recordings are a Valid Non- Invasive Tool for Evaluating Soldiers ’ Stress

The purpose of the present study was to investigate physiological responses and to evaluate heart rate variability as a non-invasive stress indicator during a 72-hour military field training (MFT). Ten healthy male soldiers (age 20 ± 1 yr.) participated in MFT. They slept approximately 2 h/day and ate only army field rations. During MFT, the soldiers ’ mean (±SD) energy expenditure was 4646 ± 674, energy intake 2200 ± 326, and energy deficit (ED) 2405 ± 890 kcal⋅day-1. Throughout the entire training period, serum total testosterone (TES) reduced from 19.0 ± 3.0 to 12.6 ± 6.2 nmol⋅l-1 (p<0.001). Mean HR during the entire MFT was 85 ± 6 bpm. RMSSD, which reflects cardiac vagal activity, decreased from 54 ± 19 to 42 ± 12 ms (p<0.05). No changes (p>0.05) were observed in HF and LF power, but changes in HF power correlated with baseline serum TES (r=-0.72, p<0.05). Based on the present findings, working for 72 h at a relatively low level of cardiovascular strain with ED and sleep deprivation can individually modify hormonal responses in association with cardiac vagal outflow. This suggests that HRV can be used as a non-invasive tool to measure stress in soldiers during MFT