The cumulative physiological effects of consecutive days of physical work in hot conditions

INTERVENTION: Participants will perform three consecutive days of simulated work. Each simulated workday will start at 08.30am and will consist of four 2h blocks (8 h total). Each 2h block will comprise of two 30 min bouts of work, each followed by 30 min seated rest (i.e. a work‐to‐rest cycle of 30 min rest, 30 min work). The 30 min work bouts will involve 15 min of treadmill walking (4.5 km/h, 5% grade) followed by 15 min of manual handling, which will comprise lifting and carrying a 15‐kg sand bag for 4 m at a rate of 3 reps/min (for 8 min) and then moving a 15 kg sandbag between the ground and a 1.5 m platform at a rate of 3 reps/min (for 7 min). During each rest period, participants will have the opportunity to use the toilet and consume water and a self‐provided lunch and snacks as desired. All participants will complete the simulated workday within a hot‐humid environment (35°C and 63% relative humidity; treatment [hot]). The hot‐humid environment has been informed by the climate statistics from Darwin Airport, Northern Territory during February to simulate the conditions experienced by personnel deployed to this environment. The participant will be directly supervised throughout by at least one member of the research team to ensure adherence to the study protocol. CONDITION: Cardiovascular ‐ Normal development and function of the cardiovascular system Heat stress; ; Heat stress Mental Health ‐ Studies of normal psychology, cognitive function and behaviour Musculoskeletal ‐ Normal musculoskeletal and cartilage development and function Neurological ‐ Studies of the normal brain and nervous system PRIMARY OUTCOME: Body core temperature, as assessed using an ingestible pill thermometer (BodyCap, e‐Celsius Performance, France).[ E nd of each 8‐h simulated work day (immediately at the end of the final 2 h work block) on days one, two and three.] INCLUSION CRITERIA: Participants will be physically active males and females ages 18 – 40 years and BMI <35 kg/m2 as this is a representative sample of military personnel completing physically demanding workloads in the hot humid environments. Prospective participants will complete a pre‐assessment screening to confirm eligibility for inclusion. This will involve a DEXA scan and an aerobic and strength assessment to determine if they meet the minimum entry fitness standards for the Australian Army (i.e., equivalent to a multi‐stage shuttle run result = 6.1; 2 min maximum push ups = 4 for females, 8 for males; 2 min maximum sit ups = 20). These age range, BMI and physical fitness levels have been selected to ensure that participants are healthy and physically capable of completing the protocol. SECONDARY OUTCOME: Grip strength will be assessed with a hand‐grip dynamometer, as the maximum force (Newtons) produced.[ End of each 8‐h simulated work day (immediately at the end of the final 2 h work block) on days one, two and three.] Heart rate (beats/min), as assessed using the Polar Team2 System (Polar Electro Oy, Kempele, Finland).[ End of each 8‐h simulated work day (immediately at the end of the final 2 h work block) on days one, two and three.] Medicine ball throw (strength and power) will be measured as the maximal distance (m) that the participant can throw a 5 kg medicine ball from the seated position.[ End of each 8‐h simulated work day (immediately at the end of the final 2 h work block) on days one, two and three.] Neurobehavioural testing will be conducted at the start and end of each 8‐h simulated workday using the automated 'Cognition' test battery from Joggle Research (Joggle Research, Seattle, WA), presented to participants on an electronic tablet (Apple iPad Air, Cupertino, CA, USA),[ End of each 8‐h simulated work day (immediately at the end of the final 2 h work block) on days one, two and three.] Skin temperature will be measured at 4 sites (arm, chest, thigh, calf) using iButtons (DS1921 G‐F5 Thermochron, iButtonLink LLC, WI, USA) fixed to the skin with medical tape. Mean skin temperature will be approximated from the four local skin temperatures (arm: 30%, chest: 30%, thigh: 20%, calf: 20%).[ End of each 8‐h simulated work day (immediately at the end of the final 2 h work block) on days one, two and three.] Sweat loss (kg) from the start to end of each 8‐h simulated work day, as assessed by the change in body mass measured with digital weighing scales (corrected for fluid and food consumption and urine production). [ End of each 8‐h simulated work day (immediately at the end of the final 2 h work block) on days one, two and three. ] Venous blood will be collected at the start and end of each 8‐h simulated work day via venepuncture into K2EDTA 5.4 mg BD Vacutainer® tubes (BD, Franklin Lakes, NJ, USA) and used to measure haemoglobin and haematocrit concentrations (HemoCue AB, Ängelholm, Sweden) to determine the change (%) in plasma volume (Dill and Costill method). [ End of each 8‐h simulated work day (immediately at the end of the final 2 h work block) on days one, two and three.] Vertical jump performance (strength and power) will be assessed with a force plate as the flight time (seconds) during a maximal vertical jump. [ End of each 8‐h simulated work day (immediately at the end of the final 2 h work block) on days one, two and three.] Whole‐body strength will be assessed using the mid‐thigh pull, the participant will pull on a fixed barbell with a maximal effort, with force (Newtons) measured with a force plate (Fitness Technology 400S Performance Force Plate, Fullarton, SA, Australia) underneath the participant. [ End of each 8‐h simulated work day (immediately at the end of the final 2 h work block) on days one, two and three.]