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 is localized to the 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 both upper and lower limbs of COPD patients compared with controls.

Methods: Oxidative capacity of the forearm and medial gastrocnemius were measured using near-infrared spectroscopy to determine the muscle O2 consumption recovery rate constant (k, min-1) in 20 COPD (GOLD 2/3/4, n=7/7/6) and 20 smokers with normal spirometry (CON). Muscle k is linearly proportional to oxidative capacity. Steps/day and vector magnitude units (VMU)/min were assessed using triaxial accelerometry. Differences between group and limb were assessed by 2-way ANOVA.

Results: There was a significant main effect of group (F=11.2, ηp2=0.13, P=0.001): k was lower in both upper and lower limb muscles in COPD (1.01±0.17, 1.05±0.24 min-1) compared with CON (1.29±0.49, 1.54±0.60 min-1). There was no effect on k of limb (F=1.8, ηp2=0.02, P=0.18) or group x limb interaction (P=0.35). VMU/min was significantly lower in COPD (-38%; P=0.042). Steps/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/day).

Conclusion: 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.