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We studied six patients with chronic obstructive pulmonary disease (COPD) (FEV₁ = 1.1 ± 0.2 L, 32% of predicted) and six age- and activity level–matched control subjects while performing both maximal bicycle exercise and single leg knee-extensor exercise. Arterial and femoral venous blood sampling, thermodilution blood flow measurements, and needle biopsies allowed the assessment of muscle oxygen supply, utilization, and structure. Maximal work rates and single leg V̇O₂max (control subjects = 0.63 ± 0.1; patients with COPD = 0.37 ± 0.1 L/minute) were significantly greater in the control group during bicycle exercise. During knee-extensor exercise this difference in V̇O₂max disappeared, whereas maximal work capacity was reduced (flywheel resistance: control subjects = 923 ± 198; patients with COPD = 612 ± 81 g) revealing a significantly reduced mechanical efficiency (work per unit oxygen …