Submaximal and maximal exercise tests in twelve healthy young subjects under ordinary and hyperbaric conditions have shown: 1. 1) at a low relative work intensity the heart rate was 12.9 per cent lower during oxygen breathing at 3 ata than air breathing at a corresponding work intensity. 2. 2) This difference was smaller at a higher work intensity. 3. 3) The maximal heart rate was lower during oxygen than air breathing. It is concluded that the maximal utilization rate of oxygen in active muscle tissue determines the maximal heart rate. 4. 4) In spite of the lower maximal heart rate during oxygen breathing, indicating that the maximal cardiac output was not utilized, the performance time at the work intensity leading to exhaustion in 6-8 min was not significantly prolonged during oxygen breathing at 3 ata. This seems to indicate that the oxygen transport capacity of the circulatory system is not the main limiting factor for the aerobic working capacity. © 1970.
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