Aim: We hypothesize that training-induced changes in muscle oxygen saturation (StO2) assessed by near-infrared spectroscopy (NIRS) during constant work rate cycling exercise (CWRE) may be a useful marker of the effects of training at ‘vastus medialis’ of the quadriceps in patients with chronic obstructive pulmonary disease (COPD). Methods: Incremental exercise [peak oxygen uptake (VO2)] and CWRE at 70% pretraining peak VO2, before and after 8-w training, were done in 10 healthy age-matched subjects (H) [80% men, 65(11) years, FEV1 105(14)%] and 16 COPD patients [94% men, 70(5) years, FEV1 46(11) %] encompassing the entire spectrum of disease severity, recruited in the outpatient clinics. NIRS was used to assess StO2 in the ‘vastus medialis’ of the left quadriceps. Results: Pretraining CWRE decreased StO2 (P<0·05) and generated marked StO2 rebound (P<0·001) after unloading in the two groups. After training, VO2 peak increased in H [253(204) ml min−1] (P<0·01) and in COPD [180(183) ml·min−1] (P = 0·01) and blood lactate fell [−4·4 (2·7) and −1·6(2·3) mmol·m−1] (P<0·05 each). Training generated a further fall in StO2 during CWRE [−10(12)% and −10(10)%, P<0·05] and increased StO2 rebound after unloading [8(7)% and 5(9)%, P<0·05] in both groups. Conclusion: Endurance training further decreased StO2 during CWRE, similarly in both groups, likely due to training-induced enhancement of muscle O2 transfer and utilization. Training-induced StO2 fall during CWRE may be useful individual marker for non-invasive assessment of enhanced muscle aerobic post-training function.