BACKGROUND Several studies have indicated that skeletal muscle is important in determining the exercise capacity of patients with chronic heart failure (CHF). However, this theory has been investigated only in experiments based on local exercise involving a small muscle mass. We investigated skeletal muscle metabolism during maximal systemic exercise to determine whether muscle metabolism limits exercise capacity in patients with CHF. We also studied the relationship between muscle metabolic abnormalities during local and systemic exercise. RESULTS Skeletal muscle metabolism was measured during maximal systemic exercise on a bicycle ergometer by a combination of the metabolic freeze method and 31P magnetic resonance spectroscopy in 12 patients with CHF and 7 age- and size-matched normal subjects. We also evaluated skeletal muscle metabolism during local exercise while subjects performed unilateral plantar flexion. Muscle phosphocreatine (PCr) was nearly depleted during maximal systemic exercise in patients with CHF and normal subjects (12.5+/-0.04% and 12.3+/-0.07%, respectively, of initial level). PCr depletion occurred at a significantly lower peak oxygen uptake (peak VO2) in patients with CHF than in normal subjects (CHF, 20.2+/-3.0 versus normal, 31.8+/-3.7 mL . min-1 . kg-1, P<0. 0001). Muscle metabolic capacity, evaluated as the slope of PCr decrease in relation to increasing workload, was correlated with peak VO2 during maximal systemic exercise in patients with CHF (r=0.83, P<0.001). Muscle metabolic capacity during local exercise was impaired in patients with CHF and was correlated with capacity during systemic exercise (r=0.76, P<0.01) and with peak VO2 (r=0. 83, P<0.001). CONCLUSIONS These results suggest that impaired muscle metabolism associated with early metabolic limitation determines exercise capacity during maximal systemic exercise in patients with CHF. There was a significant correlation between muscle metabolic capacity during systemic and local exercise in patients with CHF.