Effect of caffeine on target detection and rifle marksmanship.

Thirteen healthy and rifle-trained male military reservists performed shooting sessions on two separate occasions 1 h following the ingestion of placebo or 300 mg of caffeine. Shooting included both friend-foe (FF) and vigilance (VIG) tasks, and were performed in the following order: two FF sequences (4 min each), four VIG sequences (30 min each), and two additional FF sequences. The shooting sessions lasted approximately 2.5 h under outdoor conditions (air temperature range from - 3 to 14 degrees C) and were held 48 h apart in a counter-balanced order. Performance measures during the shooting session included engagement time, friend-foe discrimination, and marksmanship accuracy and precision. Assessments of thermal comfort, tiredness, and debilitating symptoms preceded and followed the shooting session, while a self-assessment on performance was administered post-shooting only. Blood was sampled immediately prior to the beginning of the shooting session and was used to determine plasma caffeine, cortisol, and testosterone levels. Engagement times were faster and certain measures of accuracy and precision were impaired during the later FF and VIG sequences. However, caffeine ingestion had no affect upon any of the marksmanship measures, although it did alleviate cold stress and tiredness. That caffeine ingestion did not affect target detection and rifle marksmanship is a finding that differs from other studies, and is explained by a beneficial arousal caused by the mild level of cold stress experienced by the participants.

Changes in cold tolerance due to a 14-day stay in the Canadian Arctic.

Response to cold exposure tests both locally and of the whole body were examined in subjects who stayed in the arctic (average maximum and minimum temperatures -11 and -21 degrees C respectively) for 14 days of skiing and sleeping in tents. These changes were compared to responses in subjects living working in Ottawa, Canada (average max. and min. temperatures -5 and -11 degrees C respectively). The tests were done before the stay in the Arctic (Pre), immediately after the return (Post 1) and approximately 32 days after the return (Post 2). For the whole-body cold exposure each subject, wearing only shorts and lying on a rope mesh cot, was exposed to an ambient temperature of 10 degrees C. There was no consistent response in the changes of metabolic or body temperature to this exposure in either of groups and, in addition, the changes over time were variable. Cold induced vasodilatation (CIVD) was determined by measuring temperature changes in the middle finger of the nondominant hand upon immersion in ice water for 30 min. CIVD was depressed after the Arctic exposure whilst during the Post 2 testing, although variable, did not return to the Pre values; the responses of the control group were similar. These results indicate that normal seasonal changes may be as important in adaptation as a stay in the Arctic. Caution is advised in the separation of seasonal effects when examining the changes in adaptation after exposure to a cold environment.

Heat loss caused by cooling the feet.

The effect of cooling the feet to alleviate heat strain was examined. Subjects, wearing chemical protective clothing, immersed their feet in water at temperatures of 10, 15, 20, 25, and 30 degrees C after sitting for 120 min at 35 degrees C. Heat lost via the feet ranged from 151 +/- 15 to 55 +/- 5 W, being greater in the colder water. In a second experiment, subjects wearing chemical protective clothing and specially designed water-cooled socks walked on a treadmill at 5 km.h-1 and 2.5% grade for 90 min at 35 degrees C. Four conditions were examined: no cooling, cooling throughout the walk, cooling during the last 60 min, and cooling during the first 30 min. Rectal and skin temperatures and heart rates were monitored. Cooling for the first 30 min had little effect on the measured parameters, however, when core temperatures rose to over 37.5 degrees C, cooling during the last 60 min significantly attenuated the increase in body temperatures and heart rates. We conclude that this method could be used to alleviate heat strain.

Effect of working in hot environments on respiratory air temperatures.

The purpose of this study was to obtain an accurate measure of the temperature of respired air of subjects working on a treadmill at various ambient temperatures and ambient relative humidities (RH). The experiments were conducted in an environmental chamber at each of four different ambient temperatures (nominally 20, 30, 40 and 45 degrees C) and at two different ambient RH (20% and 100%) for a total of eight different conditions. Each experiment consisted of four tests at each ambient condition, these being: (A) standing quietly; (B) walking on a treadmill at 3 km.h-1, 0% grade; (C) walking on a treadmill at 3 km.h-1, 5% grade; and (D) walking on a treadmill at 5 km.h-1, 5% grade. It was found that under these conditions the maximum temperature of the expired air is independent of work rate and also ventilation rate but varies significantly with both the temperature and humidity of the inspired air. At low ambient RH the expired air temperature was [mean (SD)] 31.2 (1.3), 33.3 (0.7), 34.0 (1.7) and 35.6 (0.7) degrees C for ambient temperatures of 20, 30, 40 and 45 degrees C, respectively. At high ambient RH the expired air temperature was 32.0 (1.8), 35.3 (0.5), 37.6 (1.2) and 41.8 (0.8) degrees C at ambient temperatures of 20, 30, 40 and 45 degrees C, respectively. Thus the expired air temperature was higher at the higher ambient temperatures and ambient RH. While similar results have been reported before, the techniques used in this study should provide a more accurate measure of these effects than those previously reported.

Respiratory heat loss during work at various ambient temperatures.

The purpose of this investigation was to establish the temperature and humidity of the expired air of subjects working at various metabolic rates at ambient temperatures between -40 degrees C and 20 degrees C in order to calculate respiratory heat loss. Measurements of the respired air temperature and water vapour content were made for five subjects while they either stood or walked on a treadmill. The results indicated that the maximum respired air temperature varied slightly with the ambient air temperature but changes in metabolic rate, respiration rate and breathing frequency had no apparent effect on the expired air temperature under the conditions studied. The relative humidity of the respired air was found to be close to saturation in the extreme-cold environments. Heat loss due to respiration was between 25 and 30% of the resting metabolic and between 15 and 20% of the working metabolic rate.