In patients with congestive heart failure (CHF), the poor relationship between systemic exercise performance and cardiac function, together with morphologic and metabolic abnormalities in skeletal muscle, raises the possibility that skeletal muscle function may be impaired and limit systemic exercise performance. We assessed strength and endurance of the knee extensors during static and dynamic exercise in 16 patients with Class I-IV CHF and eight age-matched sedentary controls and related these measurements to systemic exercise performance. To assess skeletal muscle function independent of peripheral blood flow, endurance was repeated under ischemic conditions. Strength was not significantly different in the two groups. Dynamic endurance, quantified as the decline in peak torque during 15 successive isokinetic knee extensions, was significantly reduced in the patients compared to controls during aerobic (peak torque 65 vs. 86% of initial for exercise at 90 deg/s and 60 vs. 85% for exercise at 180 deg/s; P less than 0.002 for both), and during ischemic exercise (56 vs. 76% of initial torque; P less than 0.01). Static endurance, defined as the time required for force during a sustained maximal voluntary contraction to decline to 60% of maximal, was reduced in the patients compared to controls (40 +/- 14 vs. 77 +/- 29 s; P less than 0.02). There were highly significant relationships between systemic exercise performance and skeletal muscle endurance at 90 and 180 deg/s in the patients with CHF (r = 0.90 and 0.66, respectively). These findings indicate that skeletal muscle endurance is impaired in patients with CHF, that this abnormality is in part independent of limb blood flow, and that these changes may be important determinants of systemic exercise performance.