Muscle Oxidative Capacity Is Reduced in Both Upper and Lower Limbs in COPD

Introduction Skeletal muscle atrophy, weakness, mitochondrial loss, and dysfunction are characteristics of chronic obstructive pulmonary disease (COPD). It remains unclear whether muscle dysfunction occurs in both upper and lower limbs, because findings are inconsistent in the few studies where upper and lower limb muscle performance properties were compared within an individual. This study determined whether muscle oxidative capacity is low in upper and lower limbs of COPD patients compared with controls. Methods Oxidative capacity of the forearm and medialgastrocnemiuswas measured using near-infrared spectroscopy to determine the muscle O(2)consumption recovery rate constant (k, min(-1)) in 20 COPD (Global Initiative for Chronic Obstructive Lung Disease (GOLD) 2/3/4,n= 7/7/6) and 20 smokers with normal spirometry (CON). Musclekis linearly proportional to oxidative capacity. Steps per day and vector magnitude units per minute (VMU center dot min(-1)) were assessed using triaxial accelerometry. Differences between group and limb were assessed by two-way ANOVA. Results There was a significant main effect of group (F= 11.2, eta(2)(p)= 0.13,P= 0.001):kwas lower in both upper and lower limb muscles in COPD (1.01 +/- 0.17 and 1.05 +/- 0.24 min(-1)) compared with CON (1.29 +/- 0.49 and 1.54 +/- 0.60 min(-1)). There was no effect onkof limb (F= 1.8, eta(2)(p)= 0.02,P= 0.18) or group-limb interaction (P= 0.35). (VMU center dot min(-1)) was significantly lower in COPD (-38%;P= 0.042). Steps per day did not differ between COPD (4738 +/- 3194) and CON (6372 +/- 2107;P= 0.286), although the difference exceeded a clinically important threshold (>600-1100 steps per day). Conclusions Compared with CON, muscle oxidative capacity was lower in COPD in both upper (-20%) and lower (-30%) limbs. These data suggest that mitochondrial loss in COPD is not isolated to locomotor muscles.