Metabolic responses to exercise in three thermal environments.

Male and female "physically active" but untrained subjects walked on a treadmill grade approximating 58% vo2 max as determined in 25 degrees C under three environmental conditions. The environments represented thermal neutrality (Ta = 25 degrees C), a "warm" environment (Ta = 32 degrees C), and a "hot" environment (Ta = 40 degrees C). Relative humidity was 50 to 55%. Exercise vo2 values were significantly influenced by increased environmental temperature. In 40 degrees C, male and female vo2 increased 12% above values in 25 C. VE increase 25% for the female and 32% for the male subjects during exercise in 40 degrees C. In both male and female subjects, true O2 increased slightly in 32 degrees C. but decreased in 40 degrees C from values obtained in 25 degrees C. It was postulated that as Ta increased, blood flow was diverted from metabolically active muscle to nonmetabolically active tissue to enhance heat dissipation via conduction and convection. As a result, oxygen extraction declined, and an increased respiratory drive was incurred.