As a consequence of hypoxemia or detraining, a deficit in voluntary activation may contribute to the reduction of strength in chronic obstructive pulmonary disease (COPD). This was studied in 13 severely deconditioned COPD patients and eight healthy subjects. Exercise tolerance, fat-free mass, and quadriceps maximal voluntary contractions (MVC) were evaluated. Magnetic stimulation was used to measure maximal quadriceps twitch tension (TwQp) at rest and during a voluntary contraction (superimposed twitch: TwQs). A maximal recruitment fraction (MRF) was assessed by the ratio of the true MVC to the maximal achievable strength (using regression of TwQs vs. volitional strength at four submaximal intensities). MRF was significantly lower in COPD than in controls (89+/-20 vs. 109+/-6%, P<0.01). Seven COPD patients completed 3-month endurance training. After rehabilitation, patients had greater MVC (+24+/-26 N), associated with a slightly greater MRF (+10+/-11%). Changes in MRF were independently associated with changes in MVC (r=0.85, P=0.03). Deficit in voluntary activation of quadriceps muscle may therefore contribute to muscle weakness in severe COPD patients and is reversed by training.