Physiologic tolerance to uncompensable heat: intermittent exercise, field vs laboratory

Purpose: This study determined whether exercise (30 min)-rest (10 min) cycl es alter physiologic tolerance to uncompensable heat stress (UCHS) when out doors in the desert. In addition, the relationship between core temperature and exhaustion from heat strain previously established in laboratory studi es was compared with field studies. Methods: Twelve men completed four tria ls: moderate intensity continuous exercise (MC), moderate intensity exercis e with intermittent rest (MI), hard intensity continuous exercise (HC), and hard intensity exercise with intermittent rest (HI). UCHS was achieved by wearing protective clothing and exercising (estimated at 420 W or 610 W) ou tdoors in desert heat. Results: Heat Stress Index values were 200%, 181%, 4 17%, and 283% for MC, MI, HC, and HI, respectively. Exhaustion from heat st rain occurred in 36 of 48 trials. Core temperatures at exhaustion averaged 38.6 +/- 0.5 degrees, 38.9 +/- 0.6 degrees, 38.9 +/- 0.7 degrees, and 39.0 +/- 0.7 degreesC for MC, MI, HC, and HI, respectively. Core temperature at exhaustion was not altered (P > 0.05) by exercise intensity or exercise-res t cycles and 50% of subjects incurred exhaustion at core temperature of 39. 4 degreesC. These field data were compared with laboratory and field data c ollected over the past 35 years. Aggregate data for 747 laboratory and 131 field trials indicated that 50% of subjects incurred exhaustion at core tem peratures of 38.6 degrees and 39.5 degreesC, respectively. When heat intole rant subjects (exhaustion < 38.3<degrees>C core temperature) were removed f rom the analysis, subjects from laboratory studies (who underwent short-ter m acclimation) still demonstrated less (0.8 degreesC) physiological toleran ce than those from field studies (who underwent long-term acclimatization). Conclusion: Exercise-rest cycles did not alter physiologic tolerance to UC HS. In addition, subjects from field studies demonstrate greater physiologi c tolerance than subjects from laboratory studies.