Fatigued muscles in COPD but no finishing line in sight

Abstract

Decreased exercise capacity is a prominent symptom leading to progressive disablement in chronic obstructive pulmonary disease (COPD). Exercise capacity is frequently measured by means of incremental cycle ergometry. Although a well-established test, studies are ongoing to improve test design, test variables and data interpretation. For example, a high-intensity constant-work rate cycle ergometer test appears to be more informative than an incremental test to evaluate the outcome of rehabilitative interventions. At present, however, incremental cycle ergometry is considered the gold standard for evaluating the pathophysiology of exercise impairment in COPD 1. Besides decreased ventilatory function, skeletal muscle dysfunction has been identified as an important determinant of exercise limitation in moderate-to-severe COPD. Impaired muscle function can manifest itself as the loss of strength and/or as reduced endurance ( i.e. fatigue). Both are obviously involved in COPD but their relative contribution to exercise intolerance in individual patients is as yet unclear. In healthy subjects as well as in COPD patients, muscle strength depends largely on muscle mass or fat-free mass; fat-free mass has indeed been found to be a strong predictor of exercise capacity 2. However, the relationship between fat-free mass ( i.e. muscle strength) and exercise capacity was much weaker in COPD patients as compared with healthy controls, which may point to a more prominent role for muscle fatigue in COPD. But how to proceed? First of all, there is an urgent need for adequate muscle function tests and reference values. The determination of muscle fatigue, however, is very complicated since there is no uniform definition of muscle fatigue and available tests …