Abstract
<title>Abstract</title> <p> Noninvasive assessment of superficial muscle and adipose tissue composition may help characterize sex-related differences in body composition and their relevance to metabolic health. In this pilot study, we used broadband time domain diffuse optical spectroscopy (TD-DOS) to investigate <italic>in vivo</italic> soft-tissue composition in healthy adults at two anatomical sites: the thigh over the <italic>vastus lateralis</italic> muscle and the abdomen. Optical measurements at multiple source‒detector distances were combined with ultrasound assessment of skin and subcutaneous adipose tissue ( <italic>SAT</italic> ) thickness. Absorption and reduced scattering spectra were assessed and used to estimate lipid, water, collagen, total hemoglobin content, and tissue oxygen saturation. At the <italic>vastus lateralis</italic> site, females showed higher lipid content and lower water, collagen, and total hemoglobin content than males, consistent with thicker <italic>SAT</italic> and predominant sampling of the subcutaneous adipose layer. In males, optical signatures were more heterogeneous and suggested a detectable muscle contribution only in subjects with thin <italic>SAT</italic> . In the abdomen region, <italic>SAT</italic> thickness exceeded the estimated penetration depth in all subjects, and measurements were consistent with predominant adipose tissue sampling in both sexes. These findings indicate that multidistance broadband TD-DOS can detect non-invasively interindividual and sex-related differences in the composition of superficial soft tissues, while underscoring the need to account for <italic>SAT</italic> thickness when interpreting depth-dependent optical measurements. </p>