Uncoupling psychological from physiological markers of heat acclimatization in a military context

Heat acclimatization may help personnel who travel to areas with a hot climate (WBGT > 27 °C), making them operationally more efficient and performant through improvements in physiological and psychological parameters. Their work-related physical activities may aid active heat acclimatization. However, it is unknown whether adding physical training to improve adaptation is effective, particularly if there is sufficient time for full acclimatization, classically reached after 15 days. Thirty French soldiers (Training group, T) performed a progressive and moderate (from three to five 8-min running sets at 50-60% of their speed at VO2max with 4-min periods of active recovery in between) aerobic training program upon arriving at their base in United Arab Emirates (~40 °C and 20% RH). A control group (30 soldiers; No Training, NT) continued to perform only their usual outdoor military activities (~5 h d-1). A field heat stress test (HST: three 8-min running sets at 50% of the speed at VO2max) was performed before (D0), during (D10), and after (D15) the heat acclimatization period to assess physiological and psychological changes. An 8-km trial in battledress was then performed at D17. Although physiological modifications were mostly similar (p < 0.001 for all) for both groups (rectal temperature at the end of the HST: -0.58 ± 0.51 vs -0.53 ± 0.40 °C, HR at the end of the HST: -21 ± 12 vs -19 ± 9 bpm, and sweat osmolality: -47 ± 30 vs -26 ± 32 mOsmol.l-1 between D15 and D0 for T and NT groups, respectively), thermal discomfort (-31 ± 4 vs -11 ± 5 mm between D15 and D0, p = 0.001) and rates of perceived exertion (-3.0 ± 0.4 vs -1.4 ± 0.3 D15 and D0, p = 0.001) were much lower in the T than NT group during the HST. HST-induced modifications in facial temperature only decreased in the T group (-1.08 ± 0.28 between D15 and D0, p < 0.001). Moreover, there was a difference in perceived thermal discomfort during the 8-km trial (40 ± 20 vs 55 ± 22 mm for the T and NT groups, respectively, p = 0.010). Thus, a 15-day, low-volume training regimen during a mission in a hot and dry environment has a modest impact on physiological adaptation but strongly decreases the perceived strain of exertion and climate potentially via greater reductions in facial temperature, even during a classical operational physical task in a military context.