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.