Association with physical fitness, serum hormones and sleep during a 15-day military field training

Abstract

The present study aimed to investigate the association between physical fitness, sleep duration and hormonal responses during a 15-day military field training (MFT). The purpose of MFT was to practice offensive manoeuvres in a countryside area. Nine army officers volunteered to participate, and their daily working routine mainly consisted of tasks in the headquarters that required on-call-duty at all times. Physical fitness and body composition were measured just before MFT. Serum testosterone (TES) and cortisol (COR) concentrations and sex-hormone-binding globulin (SHBG) were measured before MFT, as well as 8 and 15 days after the beginning of MFT. Heart rate (HR) was recorded for approximately 24 h on days 8, 11 and 15 of MFT. Based on HR responses, there was no evidence of cardiorespiratory strain, hormonal responses or energy deficit during MFT. Although the changes in hormonal concentrations were insignificant, they were well correlated with physical fitness (r=0.67, p=0.05). Furthermore, the TES/SHGB ratio decreased by 28% in subjects whose VO2max was under 44 ml kg(-1) min(-1). On average, subjects slept for 6.20 h per day, but the sleeping rhythm was disturbed due to military tasks. This diurnal sleeping time was strongly associated with TES/COR ratio (r=0.78, p=0.01). These results indicate that MFT causes very individual stress reactions, despite the low levels of physical strain and energy deficit. We therefore concluded that the observed hormonal responses were mainly due to sleep deprivation and low physical fitness.