Inclusion Criteria:  healthy non-smoking physically fit, including top-level athletes  Exclusion Criteria:  no reported history of a cardiopulmonary or renal diseases, and other chronic diseases no major orthopedic injury or illness resulting in an inability to run no medications that could affect cardiopulmonary functions normal resting electrocardiogram body mass index (BMI) below 30.0 kg·m-2 normal erythrocyte count and Hb content no pathological states known of significantly elevated adenylate pool and concentration of ATP, e.g. sickle cell disease, diabetes, leukemia, sepsis, tuberculosis, meningococcal infection, or renal insufficiency