title: Heat strain during military training activities: The dilemma of balancing force protection and operational capability.
abstract: Military activities in hot environments pose 2 competing demands: the requirement to perform realistic training to develop operational capability with the necessity to protect armed forces personnel against heat-related illness. To ascertain whether work duration limits for protection against heat-related illness restrict military activities, this study examined the heat strain and risks of heat-related illness when conducting a military activity above the prescribed work duration limits. Thirty-seven soldiers conducted a march (10 km; ∼5.5 km h-1) carrying 41.8 ± 3.6 kg of equipment in 23.1 ± 1.8°C wet-bulb globe temperature. Body core temperature was recorded throughout and upon completion, or withdrawal, participants rated their severity of heat-related symptoms. Twenty-three soldiers completed the march in 107 ± 6.4 min (Completers); 9 were symptomatic for heat exhaustion, withdrawing after 71.6 ± 10.1 min (Symptomatic); and five were removed for body core temperature above 39.0°C (Hyperthermic) after 58.4 ± 4.5 min. Body core temperature was significantly higher in the Hyperthermic (39.03 ± 0.26°C), than Symptomatic (38.34 ± 0.44°C;P = 0.007) and Completers (37.94 ± 0.37°C;P<0.001) after 50 min. Heat-related symptom severity was significantly higher among Symptomatic (28.4 ± 11.8) compared to Completers (15.0 ± 9.8,P = 0.006) and Hyperthermic (13.0 ± 9.6,P = 0.029). The force protection provided by work duration limits may be preventing the majority of personnel from conducting activities in hot environments, thereby constraining a commander's mandate to develop an optimised military force. The dissociation between heat-related symptoms and body core temperature elevation suggests that the physiological mechanisms underpinning exhaustion during exertional heat stress should be re-examined to determine the most appropriate physiological criteria for prescribing work duration limits.
authors.affiliation: Land Division, Defense Science and Technology Group, Department of Defence , Fishermans Bend, Australia.
authors.forename: Andrew P
authors.initials: AP
authors.lastname: Hunt
authors.affiliation: Land Division, Defense Science and Technology Group, Department of Defence , Fishermans Bend, Australia.
authors.forename: Daniel C
authors.initials: DC
authors.lastname: Billing
authors.affiliation: Land Division, Defense Science and Technology Group, Department of Defence , Fishermans Bend, Australia.
authors.forename: Mark J
authors.initials: MJ
authors.lastname: Patterson
authors.affiliation: Land Division, Defense Science and Technology Group, Department of Defence, Fishermans Bend, Australia; Centre for Human and Applied Physiology, School of Medicine, University of Wollongong, Wollongong, NSW, Australia.
authors.forename: Joanne N
authors.initials: JN
authors.lastname: Caldwell
keywords: armed forces; body core temperature; exertional heat stress; heat strain; heat-related illness; military; work table