Paraspinal Fat Quantity and Three-Dimensional Spatial Distribution in Relation to Isokinetic Trunk Strength: An Exploratory Cross-Sectional Study

Background/Objectives: Fat infiltration describes how much fat is present but not its location within a muscle compartment. This exploratory study characterized regional fat distribution in older adults and explored whether spatial descriptors provide information about trunk strength beyond fat infiltration (FI). Methods: 79 community-dwelling healthy adults underwent CT-based thoracic and lumbar posterior paraspinal assessment and isokinetic strength testing. Fat infiltration and direction-corrected, size-normalized fat-to-whole-compartment centroid offsets were examined using correlations and multivariable linear regression adjusted for age, sex, and corresponding FI. Results: Fat centroids were consistently medial and predominantly anterior relative to their compartment references, with superior thoracic and inferior lumbar positions. Right-sided FI was higher than left-sided FI, with mean differences of 1.93 percentage points in the lumbar region and 0.58 percentage points in the thoracic region. Higher FI was associated with smaller medial offsets. Larger medial offsets tended to accompany greater extension and flexion strength after adjustment, with global medial offset providing additional explanatory information beyond FI. Adding this feature increased the explained variation in extension and flexion strength by approximately 6.7 and 6.2 percentage points, respectively. The positive medial-offset pattern was retained across sensitivity analyses, although no primary offset association met the false discovery rate criterion. Conclusions: Posterior paraspinal fat showed distinct regional spatial patterns and bilateral differences in fat quantity. Medial centroid offsets emerged as candidate features for functional assessment alongside FI. However, the positive medial-offset estimates did not establish strength associations after multiplicity adjustment, and inference was sensitive to covariance specification. Potential applications to functional assessment and rehabilitation monitoring require prospective validation.