Decreased Lower Limb Phase Angle in Older People Is an Indicator of Standing and Gait Function, Regardless of Age.

Background/Objectives: The phase angle (PhA), as measured using bioelectrical impedance analysis (BIA), indicates muscle mass and quality. However, its relationship with age-related muscle changes and motor function and effective BIA assessment/intervention sites remains unclear. Herein, we evaluated age-related changes in PhA, explored the relationship between PhA and muscle mass, and identified effective sites for BIA. Methods: We included 131 healthy community-dwelling adults divided into older (≥65 years) and control (≤65 years) groups. PhA and muscle mass were measured using BIA with a device determining electrical resistance by transmitting a weak alternating current <90 μA. Motor function was measured using ground reaction force index, gait function, and grip strength. The relationships between muscle mass, PhA, and motor function were analyzed. Results: All values, excluding upper limb muscle mass, were lower in the older group, as were the rates of change in muscle mass and PhA in the lower limbs vs. the upper limbs. Lower limb PhA showed negative and positive correlations with the Timed Up and Go test and standing function, respectively; it was significantly associated with all motor functions. Conclusions: Regardless of age, muscle mass and PhA measured by BIA were high and low in the lower limbs, respectively, and lower limb PhA was related to walking and standing function. The decrease in lower limb muscle mass and PhA may accelerate with age. Given global population aging, easy-to-assess lower limb PhA linked to the movements necessary for independent living may be an effective site for assessment and intervention in clinical practice.