AbstractThe energy cost of walking up and down a A‐shaped, 1 m high staircase was studied in 10 male and 10 female subjects in terms of the O2requirement for performing the work under truly aerobic conditions. Two different speeds of walking were used, yielding external work loads ofk× 14.7 andk× 26.7 kpm/min (k= body weight of individual in kg) in both the positive and negative phases. At the lower speed the net O2cost per kg b. w. averaged 16.8 ml/min, independent of sex; at the higher speed the corresponding value was 33.0 ml/min in the men, and 32.0 ml/min in the women. From these values the average physiological load was estimated tok× 8.2 kpm/min for men and women at the lower speed, and tok× 16.2 andk× 15.7 kpm/min for men and women, respectively, at the higher speed. The ratio of O2costs for the positive and negative works approximated 8: 1 at the lower speed, and 5: 1 at the higher speed. During each period of positive work the calculated O2demand of a 70 kg man exceeded the O2supply to the working muscles by about 60 ml at both speeds. This O2debt of 60 ml could be contracted without any increment in the lactacid O2debt and supports the notion that a certain amount of O2can be released from the myoglobin O2store before anaerobic processes become involved. The O2capacity of this aerobic debt mechanism was calculated to be in the order of 3 ml per kg of active muscle tissue. Evidence is presented that, in the steady state, the upper limit for performing work under truly aerobic conditions corresponds to about 70 per cent of the maximal O2consu