Figure 1.

Experimental protocol. Each experimental day the subjects performed two incremental exercise tests in random order. The incremental exercise test always began in severe hypoxia (P_IO₂ = 73 mmHg). At exhaustion (Exh1) the breathing gas mixture was swiftly changed to another one with a greater oxygen PO₂, except in one instance that the gas administered was the same one the subjects were breathing in severe acute hypoxia, i.e., P_IO₂ = 73 mmHg, to create a placebo condition. Subjects were asked to continue the exercise and after 2 min at the load eliciting Exh1, the load was increased by 20 or 30 W/2 min until exhaustion (Exh2). Once again, subjects were asked to keep pedaling while the gas mixture was swiftly changed to normoxia. After 2 min at the load eliciting Exh2, the intensity was increased by 20 or 30 W/2 min until the final exhaustion (Exh 3). Between Exh1 and Exh2, the breathing gas mixtures used corresponded to P_IO₂ of 73, 82, 92, 99, and 142 mmHg (Normoxia) and were administered following a double-blind design. Subjects were asked to pedal close to 80 rpm. However, when approaching exhaustion, the pedaling rate was always reduced.