The availability of drinking water on postural sway following a simulated military march in the heat

This study will examine the effect of heat stress and dehydration from a self-paced 60 min march in the heat. Descriptive characteristics of the participants will be measured (height, mass, body composition by DEXA). Peak power output and oxygen consumption will be determined on a treadmill using a standard ramped protocol and standard metabolic cart methods available in our laboratory. Participants will be initially randomly assigned to a dehydration (control - DEH) or hydration (intervention - HYD) group. The experiment will be conducted in climate chamber with temperature set at ~30 +/-2°C, relative humidity set at 50 +/-2%. Participants will undertake two trials (DEH & HYD) wearing standard military combat gear. The goal of the march will be to complete a 60 min march. Participants in the HYD trial will consume plain water ad libitum. The time between the cross over for trials for each participant will be a minimum of 3 days and no more that 7 days. Measurements: The following measurements will be taken for the purpose of further understanding the key responses during heat stress with and without hydration. A finger prick blood sample will be collected before, each 20 min interval and at the completion of the test to assess blood lactate, glucose, haemoglobin and hematocrit. To assess the level of hydration, a urine sample will be collected before and at the completion of the test which will be checked for osmolality. Core temperature will be continuously monitored and recorded at 5 min intervals via radio telemetry pill ingested 5 h before the trial. Skin temperature at four sites will be monitored continuously and recorded at similar intervals with the use of iButtons. Heart rate will also be continuously monitored and recorded at 15 s intervals. Rating of perceived exertion (RPE), thermal comfort, thermal sensation and thirst sensation will be measured and recorded a 10 min intervals. To further understand and evaluate the effect of dehydration during long marches in the heat, urinary samples will be taken prior to and immediately on completion of the trial and will be analysed for osmolality (Advanced Micro Osmometer 3300, Advanced Instruments Inc, MA, USA). This will allow the evaluation of hydration and its potential effects on body fluid compartments. To examine the potential combined effect of hydration, heat stress and a long march on postural stability and sway, participants will immediately undertake a NeuroCom test each 10 min. Using a NeroCom Balance (VSR SPORT; NeuroCom International, Clackamas, OR, USA), each participant’s static control will be assessed by performing a stability evaluation test and limits of stability test. A stability evaluation test will be used to assess an individual’s postural sway velocity during double-tandem. Limits of stability will assess the ability of the participant to move their centre of gravity location accurately and quickly voluntarily to predetermined targets without losing balance. The postural sway that might eventuate will be no more than what is experienced following normal exercise. It is not detectable by the participant. The apparatus that they will stand on will not cause sway. It will measure the amount of force that changes from one foot to the other over the 20 s measurement period. Immediately following each postural sway measurement, the participant will be handed a simulation laser guided pistol and requested to aim the laser at predetermined targets on the wall in front of them. They will randomly fire at the targets which will measure their accuracy at various stages of the exercise trial. The simulated pistol is produced and identified as a plastic toy, and operates similar to common toy pistols used for playing video games. Consequently, licensing and permits are not required. Operating the simulated pistol is akin to playing a video game, no prior experience with a firearm is necessary. Nevertheless, participants will be instructed on how to use the simulated pistol before any data collection. This will include instructions to only point the simulated pistol at the targets, and never at a person, including themselves or the researchers. No protective eyewear is required, as the simulated pistol is produced as a toy and emits a very low-intensity light that is safe for the naked eye.