Low Phase Angle as a Clinical Marker of Sarcopenia in Older Adults: Exploring the Structure–Quality–Strength–Mobility (SQSM) Framework

Objectives: This study aimed to evaluate phase angle (PhA) as a clinical marker of sarcopenia within the multidimensional Structure-Quality-Strength-Mobility (SQSM) framework and to compare its associations with muscle strength and physical performance against sarcopenia defined by appendicular skeletal muscle mass index (ASMI). Methods: A cross-sectional study was conducted on 87 older adults (mean age 68.8 years; 70.1% female) recruited into a structured exercise program. PhA and ASMI were measured via bioelectrical impedance analysis in the program's final week. Muscle strength and physical performance were measured using the Asian Working Group for Sarcopenia definitions. The SQSM-operationalised definition(OD) of sarcopenia was classified as low PhA combined with either low handgrip strength (HGS) or gait speed(GS). Logistic regression assessed associations. Results: Phase angle ≤4.25° was associated with low HGS (OR = 2.51, p = 0.058), GS (OR = 3.96, p = 0.010), and falls (OR = 4.20, p = 0.024). SQSM-OD of sarcopenia showed higher prevalence and stronger association with falls (OR = 5.77, p = 0.008) than ASMI-defined sarcopenia. Conclusion: PhA was associated with low muscle strength, gait speed and falls supporting its role as a marker of muscle quality. SQSM-OD identified more at-risk individuals highlighting the utility of incorporating muscle quality into sarcopenia assessment.