The consequences of chronic hypoxemia on maximal force and endurance time to sustained 80% of maximal isometric contraction of two skeletal muscles (adductor pollicis and vastus lateralis) and the diaphragm were studied in patients with chronic obstructive pulmonary disease (COPD). Compared to normal subjects, COPD patients have lower values of Fmax for the two skeletal muscle groups and Pmax (diaphragm). Endurance time was also shorter for the diaphragm and adductor pollicis. Chronic hypoxemia was associated with an accentuation in integrated EMG changes in both low and high frequency bands for adductor pollicis and diaphragm. Inhalation of oxygen enriched gas mixture for a 15-min period increased markedly Fmax and PImax values, prolonged the endurance time to sustained thumb adduction, and reduced the EMG changes in the low frequency band for adductor pollicis. The present observations provide evidence for altered maximal performances of skeletal muscles in chronic hypoxemic patients and also point out the virtues of oxygen breathing in these patients.