BACKGROUND: The aim of this study was to investigate whether electromyographic manifestations of fatigue and exercise tolerance were related to stage of disease in men with a COPD diagnosis. METHODS: Fourteen men with COPD with a diagnosis of mild to severe air flow obstruction were involved in 2 separate testing sessions. The first one consisted of a pulmonary function (FEV 1 and FEV 1 /FVC) and an exercise tolerance assessment using the 6-min walk test. During the second session, a multichannel surface electromyography was recorded from vastus medialis and vastus lateralis muscles during an isometric knee extension at 70% of maximum voluntary contraction. The slope of muscle fiber conduction velocity during the contraction was calculated as the index of fatigue. RESULTS: Conduction velocity slope significantly correlated with FEV 1 (vastus medialis: r ؍ 0.86, P < .001; vastus lateralis: r ؍ 0.68, P ؍ .01), FEV 1 /FVC (vastus medialis: r ؍ 0.70, P ؍ .006), and 6-min walk test (vastus medialis: r ؍ 0.72, P ؍ .005; vastus lateralis: r ؍ 0.80, P ؍ .001). CONCLUSIONS: The electromyographic manifestations of fatigue during sustained quadriceps contraction significantly correlated with disease severity and exercise tolerance in moderate to severe COPD.