Possible Sex Differences in the Morphological and Functional Muscle Quality Reflected by Phase Angle: Implications for Muscle Quality Assessment

Background and Purpose Phase angle (PhA), derived from bioelectrical impedance analysis, is a noninvasive indicator of muscle quality. It is unclear whether PhA better reflects the morphological or functional muscle quality in community‐dwelling older adults, and whether these relationships differ by sex. Therefore, this study aimed to investigate sex differences in the relationships between PhA and morphological and functional muscle quality in community‐dwelling older adults. Methods This cross‐sectional study enrolled 323 participants from the Health Check Program. PhA was measured using a multifrequency bioelectrical impedance analyzer. Morphological muscle quality was evaluated based on the echo intensity of the rectus femoris (RF) obtained via ultrasound. Functional muscle quality was calculated as the isometric knee extension strength divided by lower extremity muscle mass or RF muscle thickness. Multiple regression analyses were performed using morphological and functional muscle quality as dependent variables and lower extremity PhA as the independent variable, stratified by sex. Results A total of 231 participants were included in the analyses. Based on lower extremity muscle mass, PhA was more strongly associated with functional muscle quality ( β = 0.48, p < 0.001) than with morphological muscle quality in men ( β = −0.23, p = 0.044), whereas in women, PhA was more strongly associated with morphological muscle quality ( β = −0.52, p < 0.001) than with functional muscle quality ( β = 0.22, p = 0.030). No significant association between PhA and muscle quality based on muscle thickness was detected in either sex. Conclusions In community‐dwelling older adults, there are possible sex differences in the muscle quality reflected by PhA; therefore, when using PhA to assess muscle quality and interpret the results, these possible sex differences must be taken into account.