Fig. 4. (A) Data from subjects exercising at sea level (filled circles) and 5050m altitude (open squares) demonstrating the maximal oxygen cost of breathing (_O2rm,max) during maximal ventilation at physiological levels of mechanical efficiency (range 5–20%). (B) Similar data except that _O2rm,max is expressed as a percentage of the total maximal rate of O₂ uptake (_O2tot,max). These data show that the work of breathing is substantially higher at this altitude than at sea level (from Cibella et al., 1999, with permission). Values are means + S.E.M.