Abstract
INTRODUCTION: Heart rate variability (HRV), typically measured at rest, is a measure of autonomic balance sensitive to physical and psychological stress. Testosterone supplementation during multistressor military operations may maintain exercise HRV by preventing a decrease in cardiac autonomic tone. The purpose of this study was to assess exercise HRV as a marker of physiological and psychological stress and determine if testosterone administration maintains HRV during extended simulated military operations. METHODS: Thirty-two physically active males were enrolled in this parallel arm, randomized, placebo-controlled, double-blind study. Participants were randomized to receive a single injection of testosterone undecanoate (TEST, n = 16) or an isovolumetric placebo (PLA, n = 16) and completed 20 d of simulated operations cycles comprised of low-stress and high-stress days. Simulated operations were comprised of 4 cycles of two low-stress days (LS, ~1000 kcal of exercise-induced energy expenditure [EIEE] and 8 h of sleep) and three high-stress days (HS, ~3000 kcal EIEE, 4 h of sleep). Heart rate was collected during daily load carriage (~30% of body weight) exercise between 0500 and 0800 h. Short-term exercise HRV during this load carriage exercise was assessed through standard deviation of normal-to-normal beats (SDNN), root-mean-square of successive differences (RMSSD) between normal heart beats, low-frequency (LF) and high-frequency (HF) heartbeat sequences, and the ratio of LF to HF (LF/HF). RESULTS: SDNN and RMSSD increased from the first to the last LS day ( P = 0.018 and P = 0.003, respectively) and from the first to the last HS day ( P = 0.006 and P < 0.001, respectively). LF and HF also increased from the first to the last LS day ( P = 0.027 and P = 0.028, respectively) and from the first to the last HS day ( P = 0.004 and P = 0.002, respectively), whereas LF/HF was unchanged across these time points ( P = 0.800 and P = 0.643, respectively). TEST showed higher LF/HF ratio on LS days ( P = 0.006) and HS days ( P < 0.001) compared with PLA. CONCLUSIONS: Exercise HRV metrics suggested parasympathetic hyperactivity despite concomitant increases in stress across the extended simulated military operations. A single dose of TEST does not appear to negatively affect the autonomic response to high stress in young health physically active males.