Association Between Lower-Limb Lean Soft Tissue Assessed by DXA and A-Mode Ultrasound and the Functional Performance of Older Adults

Abstract

<title>Abstract</title> <p> <bold>Background/Objectives:</bold> Sarcopenia is a public health concern linked to frailty. Accurate assessment is essential, yet standardization remains a challenge. This study assessed the agreement between Dual-energy X-ray Absorptiometry (DXA) and A-mode Ultrasound (US), evaluating their independent capacity to explain functional performance in community-dwelling older adults. <bold>Subjects/Methods</bold> : This cross-sectional study included 136 adults (60 years). Body composition was measured via DXA (Appendicular Lean Soft Tissue Index, ALST/h²) and A-mode US (Anterior Thigh Muscle Thickness, MT). Functional performance was assessed using the Sit-to-Stand (STS) test (low performance: 15 seconds). Multiple linear regression models, adjusted for age, BMI, and sex, identified predictors of STS time. <bold>Results</bold> : ALST/h² and anterior thigh MT were correlated with STS time (rho=0.317, =<0.001;rho=0.293, =0.002) but weakly correlated with each other (r=0.088, =0.310). The adjusted model explained 25.5% of STS variance (R² Adjusted=0.255, p<0.001). Age was the strongest independent predictor (ß=0.405,p<0.001). The independent contribution of morphological measures was attenuated when age was included in the model. No significant interaction was found between age and ALST/h² (p=0.465) or age and MT (p=0.859). <bold>Conclusion</bold> : Chronological age associates more strongly with functional performance than muscle quantity. Weak agreement between DXA and A-mode US suggests these methods are not interchangeable. While muscle mass and thickness explain little functionality (25%), morphology alone is insufficient to predict performance, likely due to age-related declines in muscle quality (e.g., myosteatosis) that A-mode US cannot distinguish. </p>