Thermoregulatory boundaries to exercise in hot, humid environments

Vita.The purpose of this study was to investigate the thermoregulatory mechanisms associated with exercise of long duration in a hot, humid environment. The specific problem was to describe the upper limit s of environmental heat stress which are acceptable for safe exercise performance by individuals working at 60 to 70 percent of their maximal oxygen uptake. The subproblems were to: (1) explore the mathematical model developed by Stolwijk (1971), which simulated physiological temperature regulation in man by comparing responses predicted by the model with criterion responses measured in selected subjects, and (2) investigate the relationships between and among body core temperature, ambient temperature, water vapor pressure, total water loss, oxygen uptake, and heart rate. Eight male volunteers were selected for their known participation in long-distance running. Maximal oxygen uptake (MAX VO2) was determined as a criterion for selection as a highly-fit subject for the study (MAX VO2 > 57 ml*kg^-1 * min^-1). Basal values for body core (rectal) temperature, heart rate, and oxygen uptake were measured at 22.6°C. Testing sessions were conducted in the environmental chamber of the Texas A&M University Industrial Engineering Department. Air temperature and humidity were controlled within +0.5°C/+1 percent relative humidity in the chamber during exercise bouts. Airflow and treadmill speed were identical. All subjects were exercised at 65 percent of their calculated MAX VO2 at a 3° uphill grade. Each exercise session was scheduled for a maximum of 90 minutes treadmill running with a 15 minute recovery period. The exercise bout was terminated for any one of the following reasons: (1) a steady-state response (measured by < 0.2°C increase in rectal temperature over a 15 minute period), (2) the heart rate exceeded 180 beats per minute and/or rectal temperature exceeded 39.5°C, or (3) the subject terminated the bout due to unwillingness to continue. The descriptive results of this study were presented as response points on a psychrometric chart. The points indicated the subject's stable or nonstable responses. The results were also presented by a curve on a psychrometric chart which follows the points defined by nonstable exercise performance