Assessment of Limb Muscle Function

Abstract

Limb muscle dysfunction is frequent in COPD and contributes to its morbidity and mortality. Limb muscle dysfunction encompasses several manifestations including muscle atrophy and weakness, susceptibility to muscle fatigue and reduced oxidative capacity and mitochondrial function. Depending on the criteria used, up to a third of patients with COPD expresses some form of muscle dysfunction, including atrophy and weakness [1]. Although the extent of muscle atrophy and weakness is greater in more advanced disease, it is important to recognize that muscle dysfunction may also occur in early disease [1, 2]. The typical patients with COPD entering a pulmonary rehabilitation program have already lost about 30% of muscle mass and strength [3]. One important aspect of limb muscle dysfunction is that it is amenable to therapy, the most effective for this problem being exercise training [4–7]. Arguably, the most perverse consequence of muscle dysfunction is its negative effect on life expectancy. Parameters such as reduced mid-thigh cross-sectional area [8], fat-free mass [9], lower quadriceps strength [10] and vastus lateralis fibre-type shift [11] are predictors of mortality in subjects with COPD. Beyond its negative impact on survival, limb muscle dysfunction also contributes to exercise intolerance in COPD and poor quality of life in this disease. For example, quadriceps strength is a strong determinant of exercise capacity [12]. Premature leg fatigue reduces the ability of bronchodilators to enhance exercise tolerance [13, 14]. The links that exist between limb muscle function and relevant clinical outcomes in COPD stress out the importance for clinicians to carefully monitor body composition and muscle function when evaluating a patient with COPD, particularly before pulmonary rehabilitation where one goal of the intervention is to improve limb muscle function.