Abstract
ObjectivesMilitary personnel are required to train and operate in challenging multi-stressor environments, which can affect hormonal levels, and subsequently compromise performance and recovery. The aims of this project were to 1) assess the impact of an eight-day military training exercise on salivary cortisol and testosterone, 2) track the recovery of these hormones during a period of reduced training.MethodsThis was a prospective study whereby 30 soldiers (n = 27 men, n = 3 women) undergoing the Australian Army combat engineer ‘Initial Employment Training’ course were recruited and tracked over a 16-day study period which included an eight-day military training exercise. Non-stimulated saliva samples were collected at waking, 30 min post waking, and bedtime on days 1, 5, 9, 13, 15; measures of subjective load were collected on the same days. Sleep was measured continuously via actigraphy, across four sequential study periods; 1) baseline (PRE: days 1–4), 2) field training with total sleep deprivation (EX-FIELD: days 5–8), 3) training at simulated base camp with sleep restriction (EX-BASE: days 9–12), and 4) a three-day recovery period (REC: days 13–15).ResultsMorning cortisol concentrations were lower following EX-FIELD (p<0.05) compared to the end of REC. Training in the field diminished testosterone concentrations (p<0.05), but levels recovered within four days. Bedtime testosterone/cortisol ratios decreased following EX-FIELD and did not return to pre-training levels.ConclusionsThe sensitivity of testosterone levels and the testosterone/cortisol ratio to the period of field training suggests they may be useful indicators of a soldier’s state of physiological strain, or capacity, however inter-individual differences in response to a multi-stressor environment need to be considered.
Highlights
Military personnel are required to work in challenging multi-stressor environments that typically involve a combination of physical, psychological, and cognitive demands, sleep loss, and caloric restriction [1,2]
The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript
Data collection occurred as a part of a wider study designed to investigate the impact of an 8-day capstone assessment task for Initial Employment Training, on cognition and heart rate variability (HRV), which was embedded within a 16-day study period
Summary
Military personnel are required to work in challenging multi-stressor environments that typically involve a combination of physical, psychological, and cognitive demands, sleep loss, and caloric restriction [1,2] Exposure to these stressors across military training and operations needs to be balanced with adequate recovery to maintain health and performance [1,3]. Training-induced changes in circulating levels of cortisol and testosterone, and the testosterone/cortisol ratio (T:C; representing the balance between anabolic and catabolic activity), have shown potential in non-military populations as markers of excessive training stress, physiological strain and inadequate recovery [5,10,11] These hormones are sensitive to various forms of military training stress involving prolonged physical activity and sleep loss for periods lasting 10 days to 11 weeks, with cortisol concentrations increasing during and following training [12–14], and total and free testosterone concentrations exhibiting decreases [15–17]. Decreases in the free T:C ratio have been associated with overtraining following training [6]
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