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The requirements to maintain a positive pressure with respiratory protection during heavy exercise and the effects on ventilation and feelings of discomfort were investigated. Eight male subjects participated, using the respirator system during rest and exercise at about 80% of their individual maximum power. A blower was used at maximum and medium capacity and at two pressure levels (3 and 15 mbar). Additionally, the mouth pressure was used as a feedback for the blower. The blower decreased the fraction of the breathing cycle with negative pressures from 50% (SD 4%) to 15% (SD 10%) during exercise. Negative pressures occurred at all settings of the blower during exercise. Thus, the currently available commercial blower systems do not supply a sufficient airflow to maintain a positive pressure during heavy exercise. Positive pressure breathing did not affect the ventilation and the circulation. But the oxygen consumption was higher with the blower and respirator than without. Keywords: Positive pressure breathing; Exercise; Performance