Cross-sectional evaluation of leg-predominant relative segmental muscle mass differences in hospitalized older adults with sarcopenia

Background Skeletal muscle mass index (SMI) summarizes total muscle mass but does not reflect regional distribution. This study investigated whether sarcopenia is associated with a preferential segmental pattern of lower muscle mass and whether regional fluid distribution contributes to this pattern. Methods This cross-sectional study included 231 hospitalized adults aged ≥60 years, including 60 with sarcopenia. Segmental muscle mass and extracellular water-to-total body water ratio (ECW/TBW) were assessed using multifrequency bioelectrical impedance analysis. Sex-specific standardized segmental muscle mass scores were calculated using participants without sarcopenia as the reference. The primary analysis evaluated the interaction between sarcopenia status and body segment using generalized estimating equation models. Supporting analyses included within-group segmental comparisons and segment-specific regression models with adjustment for clinical covariates. Results Among participants without sarcopenia, no consistent segmental distribution pattern was observed (Friedman p = 0.994). In contrast, participants with sarcopenia showed relatively lower standardized leg muscle mass compared with the arms and trunk (adjusted p = 0.037 and p < 0.001). After adjustment for clinical covariates, sarcopenia was associated with an additional relative reduction in leg muscle mass compared with the arm segment ( β = −0.427; 95% CI, −0.708 to −0.147; p = 0.003). This association remained unchanged after further adjustment for segmental ECW/TBW ( β = −0.425; 95% CI, −0.708 to −0.142; p = 0.003). Conclusion In this hospitalized cohort, sarcopenia was associated with a greater relative muscle mass deficit in the legs than in the arms or trunk. This pattern was not substantially explained by regional fluid distribution. Segmental muscle mass assessment may provide additional information beyond conventional SMI, warranting prospective evaluation of its functional and prognostic relevance.