Exercise Heart Rate Variability Suggests Parasympathetic Hyperactivity during Simulated Military Operations Irrespective of Testosterone Administration.

INTRODUCTION

Heart rate variability (HRV), typically measured at rest, is a measure of autonomic balance sensitive to physical and psychological stress. Testosterone supplementation during multi-stressor 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 days of simulated operations cycles comprised of low-stress and high-stress days. Simulated operations were comprised of 4 cycles of 2 low-stress days (LS, 1,000 kcals of exercise-induced energy expenditure [EIEE] and 8 hours of sleep) and 3 high-stress days (HS, ~3,000 kcals EIEE, 4 hours of sleep). Heart rate was collected during daily load carriage (30% of body weight) exercise between 0500-0800 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 between normal heart beats (RMSSD), low- (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, p = 0.003; respectively) and from the first to the last HS day (p = 0.006, p < 0.001; respectively). LF and HF also increased from the first to the last LS day (p = 0.027, p = 0.028; respectively) and from the first to the last HS day (p = 0.004, p = 0.002; respectively), while LF/HF was unchanged across these timepoints (p = 0.800, p = 0.643; respectively). TEST showed higher LF/HF ratio on LS days (p = 0.006) and HS days (p < 0.001) compared to PLA.

DISCUSSION

Exercise HRV metrics suggested parasympathetic hyperactivity despite concomitant increases in stress across the extended simulated military operations. A single dose of testosterone undecanoate does not appear to negatively affect the autonomic response to high stress in young health physically active males.