Dynamics of pulmonary gas exchange.

Power outputs that are below the anaerobic threshold (theta an) may be sustained for prolonged durations, whereas power outputs that are greater than theta an result in a significant reduction in the tolerable duration to fatigue. The theta an may therefore be considered to demarcate exercise intensity into moderate (below) and heavy (above) domains. O2 uptake (VO2) responds with linear first-order dynamics for sub-theta an power outputs with a time constant of approximately equal to 25 to 35 sec and a "delay" of 15 to 20 sec. A steady state is therefore normally achieved within 3 min. For supra-theta an exercise an additional, slower component of VO2 delays the steady state (if attainable). This slow phase of the VO2 response causes the VO2 to rise to values above the steady-state level attainable by fitter subjects at that work rate. The magnitude of this "excess" VO2 correlates highly with the increased arterial blood lactate [L-] and becomes marked when [L-] exceeds 4 to 5 meq/liter. The theta an may therefore be considered a crucial index for sustainable physical activity that is not--or is modestly--fatiguing.