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

Abstract

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 medial gastrocnemius was measured using near-infrared spectroscopy to determine the muscle O2 consumption recovery rate constant (k, min) 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). Muscle k is linearly proportional to oxidative capacity. Steps per day and vector magnitude units per minute (VMU·min) 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, ηp = 0.13, P = 0.001): k was lower in both upper and lower limb muscles in COPD (1.01 ± 0.17 and 1.05 ± 0.24 min) compared with CON (1.29 ± 0.49 and 1.54 ± 0.60 min). There was no effect on k of limb (F = 1.8, ηp = 0.02, P = 0.18) or group-limb interaction (P = 0.35). (VMU·min) 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.

FIGURE 1

Changes in TSI during the NIRS muscle test. Three protocol phases are shown. The gray shading indicates muscle contractions, always preceded by a resting (i.e., baseline) phase. Data are from the medial gastrocnemius muscle of a control individual.

FIGURE 2

Representative COPD and CON responses for the muscle oxidative capacity assessments. mV˙O₂ recovery after brief contractions is shown, with a monoexponential fit (dashed line), for the UL (medial forearm muscle, left) and LL (medial gastrocnemius, right). τ, mV˙O₂ recovery time constant; k, mV˙O₂ recovery rate constant (k = (1/τ).60, min⁻¹).

FIGURE 3

Median and interquartile ranges of medial forearm (UL) and medial gastrocnemius (LL) skeletal muscle V̇O₂ recovery rate constant (k) in the COPD and CON groups. Dotted line indicates the group mean. *Main effect of condition (COPD vs CON), P ≤ 0.001.