For exercise modalities such as cycling which recruit a substantial muscle mass, muscle oxygen uptake (&OV0312;O2) is the primary determinant of pulmonary &OV0312;O2. Indeed, the kinetic complexities of pulmonary &OV0312;O2associated with exercise onset and the non-steady states of heavy (>lactate threshold) and severe [>asymptote of power-time relationship for high intensity exercise (&OV0312;)] exercise reproduce with close temporal and quantitative fidelity those occurring across the exercising muscles. For moderate (<lactate threshold) exercise and also rapidly incremental work tests, pulmonary (and muscle) &OV0312;O2increases as a linear function of work rate (≈9 to 11 ml O2/W/min) in accordance with theoretical determinations of muscle efficiency (≈30%). In contrast, for constant load exercise performed in the heavy and severe domains, a slow component of the &OV0312;O2response is manifest and pulmonary and muscle &OV0312;O2increase as a function of time as well as work rate beyond the initial transient associated with exercise onset.In these instances, muscle efficiency is reduced as the &OV0312;O2cost per unit of work becomes elevated, and in the severe domain, this &OV0312;O2slow component drives &OV0312;O2to its maximum and fatigue ensues rapidly. At pulmonary maximum oxygen uptake (&OV0312;O2max) during cycling, the maximal cardiac output places a low limiting ceiling on peak muscle blood flow, O2delivery and thus muscle &OV0312;O2. However, when the exercise is designed to recruit a smaller muscle mass (e.g. leg extensors, 2 to 3kg), mass-specific muscle blood flow and &OV0312;O2at maximal exercise are 2 to 3 times higher than during conventional cycling. Consequently, for any exercise which recruits more than ≈5 to 6kg of muscle at pulmonary &OV0312;O2max, there exists a mitochondrial or &OV0312;O2reserve capacity within the exercising muscles which cannot be accessed due to oxygen delivery limitations. The implications of these latter findings relate to the design of exercise tests. Specifically, if the purpose of exercise testing is to evaluate the oxidative capacity of a small muscle mass (<5 to 6kg), the testing procedure should be designed to restrict the exercise to those muscles so that a central (cardiac output, muscle O2delivery) limitation is not invoked. It must be appreciated that exercise which recruits a greater muscle mass will not stress the maximum mass-specific muscle blood flow and &OV0312;O2but rather the integration of central (cardiorespiratory) and peripheral (muscle O2diffusing capacity) limitations.