Upper and lower limb muscles in patients with COPD: similarities in muscle efficiency but differences in fatigue resistance.

BACKGROUND Peripheral muscle dysfunction is a common finding in patients with COPD; however, the structural adaptation and functional impairment of the upper and lower limb muscles do not seem to be homogenous. We compared muscle fatigue and recovery time between 2 representative muscles: the middle deltoid and the quadriceps femoris. METHODS Twenty-one subjects with COPD (FEV1 46.1 ± 10.3% of predicted) underwent maximal voluntary isometric contraction and an endurance test (60% of maximal voluntary isometric contraction, to the limit of tolerance). The maximal voluntary isometric contraction test was repeated after 10 min, 30 min, 60 min, and 24 hours for both the quadriceps femoris and middle deltoid. Surface electromyography was recorded throughout the endurance test. RESULTS Maximal voluntary isometric contraction significantly decreased only for the middle deltoid between 10 and 60 min after the endurance test. A significant increase of the root mean square and a greater decline in median frequency throughout the endurance test occurred for the middle deltoid, compared with the quadriceps femoris. When dyspnea and fatigue scores were corrected by endurance time, higher values were observed for the middle deltoid (0.07 and 0.08, respectively) in relation to the quadriceps femoris (0.02 and 0.03, respectively). CONCLUSIONS Subjects with COPD had a higher fatigability of a representative upper limb muscle (middle deltoid) than a lower limb muscle (quadriceps femoris).