Oxygen is required to generate chemical energy (ATP) to allow muscle contraction. The amount of chemical energy required is directly proportional to the work rate performed. Early in exercise, the muscle creatine phosphate and oxygen stores, primarily in the form of oxymyoglobin and oxyhemoglobin (oxygen content of venous blood decreases), are used for energy. This allows time for cardiac output and ventilation to increase to satisfy the total O2 requirement. Steady-state time depends on the level of work relative to the anaerobic threshold (AT). For work rates below the AT, steady-state for VO2 is achieved by 3 min and by 4 min for VCO2 and VE. For work rates above the AT, steady-states are considerably delayed or not achieved. For purposes of description of the pattern of external respiration (gas exchange at the lungs), three phases are defined. Phase I is the initial increase in VO2 and VCO2 at the start of exercise, lasting approximately 15 s. Because the gas exchange ratio (R) typically doesn't change during Phase I, the initial increase in VO2 and VCO2 must be due primarily to an increase in pulmonary blood flow and proportional increase in ventilation (cardiodynamic phase). Phase II is the exponential-like increase in VO2 and VCO2, which follow Phase I and terminates in a steady-state or asymptotic value (Phase III). At moderate work, VO2 increases more rapidly than VCO2 during Phase II (CO2 stores increase). Therefore, R decreases before it increases to the steady-state. Below the AT, the rate of external respiration equals the rate of internal respiration during Phase III.(ABSTRACT TRUNCATED AT 250 WORDS)