THE OXYGEN COST OF BREATHING DURING VIGOROUS EXERCISE

Abstract

The oxygen cost of breathing has been measured in 10 sedentary subjects during treadmill exercise at a known fraction (80 per cent) of their maximal aerobic work capacity. Each subject carried out 16 fifteen‐minute treadmill runs over 6–8 weeks; runs were arranged in a “cross‐over” design to minimize learning and training effects. Increments of respiratory minute volume were produced by voluntary hyperventilation throughout four runs, and by inhalation of 2 per cent CO 2 in air throughout four runs. Since airway resistance is influenced by changes in alveolar CO 2 tension, the true oxygen cost of respiratory work during near maximal exercise lies between estimates based on the two techniques of hyperventilation. The estimated oxygen cost for the spontaneously selected ventilation (average 89 1./min.) was 110 ml. STPD/min. The additional cost of ventilation in excess of 89 1./ min. was ∼4·4 ml./l. at a controlled frequency of 50 breaths/ min., and ∼4·3 ml./l. at 100 breaths/min. These values closely matched the possible added intake of oxygen with hyperventilation during maximal exercise; the useful limit of ventilation was ∼120 1./min. in these subjects. The efficiency of non‐elastic work was in the range 7–8 per cent.