Pulmonary and leg V̇O₂ during submaximal exercise: Implications for muscular efficiency

Insights into muscle energetics during exercise (e.g., muscular efficiency) are often inferred from measurements of pulmonary gas exchange. This procedure presupposes that changes of pulmonary O₂ (V̇O₂) associated with increases of external work reflect accurately the increased muscle V̇O₂. The present investigation addressed this issue directly by making simultaneous determinations of pulmonary and leg V̇O₂ over a range of work rates calculated to elicit 20-90% of maximum V̇O₂ on the basis of prior incremental (25 or 30 W/min) cycle ergometry. V̇O₂ for both legs was calculated as the product of twice one-leg blood flow (constant-infusion thermodilution) and arteriovenous O₂ content difference across the leg. Measurements were made 3-5 min after each work rate imposition to avoid incorporation of the V̇O₂ slow component above the lactate threshold. For all 17 subjects, the slope of pulmonary V̇O₂ (9.9 ± 0.2 ml O₂ · W^-1 · min^-1) was not different (P > 0.05) from that for leg V̇O₂ (9.2 ± 0.6 ml O₂ · W^-1 · min^-1). Estimation of 'delta' efficiency (i.e., delta work accomplished divided by delta energy expended, calculated from slope of V̇O₂ vs. work rate and a caloric equivalent for O₂ of 4.985 cal/ml) using pulmonary V̇O₂ measurements (29.1 ± 0.6%) was likewise not significantly different (P > 0.05) from that made using leg V̇O₂ measurements (33.7 ± 2.4%). These data suggest that the net V̇O₂ cost of metabolic 'support' processes outside the exercising legs changes little over a relatively broad range of exercise intensities. Thus, under the conditions of this investigation, changes of V̇O₂ measured from expired gas reflected closely those occurring within the exercising legs.