Objective physical activity level is associated with rectus femoris muscle echo-intensity in patients with chronic obstructive pulmonary disease.

Introduction: Skeletal muscle dysfunction is one of the major extrapulmonary complications of chronic obstructive pulmonary disease (COPD). Some studies have reported a relationship between physical activity (PA) level and skeletal muscle quality assessed by echo-intensity (EI) in healthy individuals but not in patients with COPD.

Objectives: The aim of this study is to investigate the relationships between PA level and both skeletal muscle EI and skeletal muscle mass in patients with COPD.

Methods: We employed a cross-sectional design. Forty male outpatients with stable COPD were enrolled. Using B-mode ultrasonography, we measured the rectus femoris muscle cross-sectional area (RF-CSA) and EI (RF-EI). The RF-CSA and RF-EI were measured on frozen images using an electronic caliper and 8-bit gray-scale analysis, respectively. The objective PA level was determined by monitoring daily step counts and moderate-to-vigorous physical activity time (MVPA) with an activity monitor. A general regression model was used to assess the relationships between PA level and both RF-CSA and RF-EI. Age and body mass index (BMI) were adopted as confounding variables.

Results: Twenty-five outpatients with stable COPD (age, 70 ± 7 years old; forced expiratory volume in 1 s, 55.0 ± 24.9% of predicted values) were finally enrolled in the present study. Even after adjusting for age and BMI, the daily step counts and MVPA were significantly associated with RF-EI, and knee extensor force was associated with RF-CSA.

Conclusion: The present study showed that PA level was associated with RF-EI in patients with COPD. In addition, RF-CSA was associated with knee extensor force. When assessing skeletal muscle using ultrasonography in patients with COPD, we should also assess EI.