Heat acclimatization during summer running in the northeastern United States

Five highly trained distance runners (DR) were observed during controlled 90-min thermoregulation trials in spring (T1) and late summer (T2) to document the nature of heat acclimatization in the northeastern United States. These trials simulated environmental (30.3 +/- 0.1 degrees C dry bulb, 34.9 +/- 0.5% relative humidity, 4.47 m X s-1 wind speed) and exercise (treadmill running at 80, 120, 160, and 200 m X min-1) stresses encountered by DR during summer training in the northeastern United States. Between T1 and T2, DR trained outdoors for 14.5 +/- 0.4 wk, but consequently exhibited few physiological adaptations classically associated with heat acclimatization. Statistical comparison of T1 and T2 indicated no significant differences in mean heart rate, rectal temperature, sweat Na+ and K+, plasma Na+ and K+, or change in plasma volume during exercise. Mean weighted skin temperature was unchanged except at 50 min of exercise, and sweat rate was also unchanged except during the initial 30 min segment: 73 +/- 6 vs 93 +/- 8 ml X m-2 X h-1. Significant decreases (P less than 0.05) in submaximal VO2 were observed: T1 vs T2 values were 13.97 +/- 0.27 vs 10.19 +/- 1.19, 31.38 +/- 1.15 vs 27.91 +/- 1.45, and 44.97 +/- 0.85 vs 41.24 +/- 0.97 ml X kg-1 X min-1, at treadmill speeds of 80, 120, and 200 m X min-1, respectively. We conclude that DR did not require 14.5 wk of summer training to maintain safe rectal temperatures (less than or equal to 38.4 degrees C) during T1, which simulated the hottest days of summer in the northeastern United States.