Factors associated with early-stage frailty (pre-frailty) in patients with chronic obstructive pulmonary disease (COPD) remain unestablished. In addition to skeletal muscle quantity, skeletal muscle dysfunction can be estimated using an angular metric from bioelectrical impedance analyzer (BIA), termed the phase angle, that reflects cell membrane reactance representing the structural stability. This study examined whether the phase angle was more closely associated with pre-frailty compared with skeletal muscle quantity in patients with COPD.

This cross-sectional analysis included stable smokers with and without COPD whose frailty status was assessed using the Japanese version of the Cardiovascular Health Study criteria. The phase angle and skeletal muscle index (SMI) were measured using BIA, and physical activity over one week was assessed using triaxial accelerometers.

A total of 159 patients were categorized into robust, pre-frail, and frail groups (n = 38, 92, and 29, respectively). The phase angle was significantly smaller in the pre-frail and frail groups than in the robust group after adjusting for age, sex, height, body mass index, smoking history, and lung function. In contrast, SMI did not differ between the robust and pre-frail groups. When combining the pre-frail and frail groups into a non-robust group, 4.8° was determined as the cutoff phase angle value to identify non-robust status. A phase angle <4.8° was associated with shorter durations of moderate-intensity physical activity but not with light physical activity.

A smaller phase angle was associated with pre-frailty and impaired moderate-intensity physical activity in smokers with and without COPD.