Differential factors associated with muscle quantity and quality in older adults

Sarcopenia involves a progressive decline in skeletal muscle mass and strength. Appendicular skeletal muscle mass (ASMM) is a traditional diagnostic marker, while phase angle (PhA) has emerged as an indicator of cellular integrity and physiological resilience. This study aims to identify factors associated with muscle quantity (ASMM) and muscle quality (PhA) in older adults.

A multicenter cross-sectional analysis was conducted using two cohorts from Ireland and Spain. Assessments included ASMM and PhA via bioelectrical impedance analysis (BIA), rectus femoris (RF) thickness through ultrasound, and handgrip strength (HGS) via dynamometry. Two crosswise multivariable linear regression models explored independent predictors of ASMM and PhA.

Seventy-four participants were included (83.8% women; mean age 78.7 years [SD 8.67]). Sarcopenia prevalence was 27% by HGS and 50% by ASMM. Polypharmacy was associated with lower PhA (4.57° [SD 0.79], p=0.021), reduced RF thickness (1.05cm [SD 0.38], p<0.001), and weaker HGS (19kg [SD 10.2], p=0.020). The ASMM model (adjusted R²=0.725) identified sex, body mass index (BMI), HGS, comorbidity burden and PhA as significant predictors. The PhA model (adjusted R²=0.440) showed age, comorbidity burden, RF thickness, HGS, and ASMM as independent determinants.

RF thickness was independently associated with PhA, suggesting it reflects muscle quality and cellular integrity beyond ASMM. ASMM depends mainly on anthropometric characteristics. Although interrelated, PhA and ASMM capture distinct domains, supporting the complementary role of ultrasound in geriatric musculoskeletal and nutritional assessment.