Abstract:
When humans are exposed to harsh environments, physical and mental performance is often substantially degraded. Few practical strategies are available to sustain performance under such conditions. The first of three studies evaluated the possible diuretic effect of caffeine following exercising in the heat. This is important for many U.S. military personnel who are engaged in combat situations in the Middle East, as dehydration may pose serious health issues. Eight participants completed four trials either with or without caffeine prior to the exercise and undertook a 3-h recovery period with or without fluid replenishment. Results revealed that ingestion of caffeine did not increase urine volume and did not increase fluid requirement during rest. In the second study, we examined the effects of menthol and caffeine on ventilatory and perceptual responses during simulated firefighting in the heat. Ten participants completed three trials, either with caffeine, menthol lozenges, or placebo. Contrary to the literature, menthol actually increased ventilatory loads (i.e., V_E , VO_2 ) and did not improve thermal or breathing comfort. The third study examined the effects of repeated dynamic exercise in the heat on mood and cognitive performance. Exercise in the heat substantially deteriorated mood states, but cognition was well maintained despite being at or near maximal heart rate, hypohydration, and hyperthermic state, suggesting possible coping mechanisms while working in multi-stressors situations. Neither caffeine nor menthol reduced the adverse impact of exercise-heat stress on mood. Current results provided information for military and occupational personnel performing duties in hot environments: (1) there was no evidence that caffeine ingestion in moderation would impair fluid balance during prolonged exercise in the heat and recovery; (2) perceptual and mood state degraded to a similar extent as physical performance; (3) caffeine and menthol exerted no ergogenic effect for firefighting in the heat. More efforts are needed to help those at-risk workers to cope with multi-stressors environments in order to maintain operational efficiency.
