Association between segmental and whole-body phase angle from bioelectrical impedance analysis and the physical function in community-dwelling older adults

Bioelectrical impedance analysis (BIA) and the derived phase angle (PhA) are recognized as a marker of cellular integrity and muscle quality, with emerging applications in the early detection of functional decline in older adults. However, evidence on the clinical utility of segmental PhA compared to whole-body measurements remains limited, particularly in community-dwelling populations. In contrast with previous Asian cohorts, this is the first study to systematically assess segmental and total PhA in a community-dwelling Spanish population. A cross-sectional study including 93 community-dwelling older adults aged ≥ 65 years was conducted in Southern Spain (median age: 75.5 years; 54.8% women and 45.2% men). Whole-body and segmental PhA (upper limbs, lower limbs, and trunk) were obtained using multifrequency BIA. Physical function was assessed using the Short Physical Performance Battery (SPPB), handgrip strength, gait speed (6-metre walk test), and lower-limb relative power (LL-rPOW). Multiple linear regression models adjusted by age, sex and BMI evaluated the associations between PhA and functional outcomes. Physical function outcomes (SPPB, LL-rPOW and Gait speed) showed numerically higher associations in an adjusted model with whole-body and lower-limb PhA. Lower-limb PhA demonstrated the highest association with SPPB (adjusted β = 0.538; p < 0.001) and was significantly associated with LL-rPOW (adjusted β = 0.356; p < 0.001) and gait speed (adjusted β = 0.335; p = 0.002). Upper-limb PhA had only significant association with handgrip strength (adjusted β = 0.220; p = 0.008). No significant associations were found for the PhA of the trunk. A clear anatomical specificity was observed, where the association strength of PhA increased when the anatomical region assessed matched the functional domain evaluated. Lower-limb PhA had higher numerically association with lower-extremity performance (SPPB, gait speed, LL-rPOW), whereas upper-limb PhA was primarily related to handgrip strength. Whole-body PhA was significantly associated with overall physical performance but lower-limb segmental PhA showed numerically higher standardized β values for lower-extremity outcomes, indicating greater regional specificity. These findings support segmental BIA as a rapid, non-invasive biomarker in primary care for stratification and monitoring functional status in community-dwelling older adults.