Heterogeneous and partially uncoupled muscle degeneration patterns in older adults: a 3-year longitudinal MRI study.

BACKGROUND: Age-related skeletal muscle degeneration involves both muscle atrophy and increased intramuscular adipose tissue (intraMAT); however, their longitudinal relationship across multiple muscles remains unclear. This study aimed to clarify longitudinal patterns of muscle degeneration in the hip and thigh and their functional relevance. METHODS: This 3-year longitudinal study included 70 community-dwelling older adults (75.0 ± 7.0 years; 57.1% women). MRI was performed at baseline and 3-year follow-up to quantify muscle volume and intraMAT fraction across 19 hip and thigh muscles using automated segmentation. Coupling between muscle atrophy and intraMAT was assessed using linear mixed-effects models and participant-specific slopes. Principal component analysis (PCA) was used to identify patterns of multi-muscle degeneration. Associations of PCA-derived patterns with changes in vertical jump and 30-s sit-to-stand performance were evaluated using multiple linear regression. RESULTS: Muscle volume decreased and intraMAT fraction increased during follow-up, with heterogeneity across muscles. Coupling between muscle atrophy and intraMAT differed significantly among muscles. Directionally coupled degeneration was observed in 38.6% of participants, but robust coupling in only 4.3%. PCA identified distinct patterns of muscle degeneration, with PC1 representing coordinated increases in intraMAT and PC2 representing muscle atrophy. Higher PC1 scores were independently associated with greater declines in lower-limb power, whereas PC2 was not. CONCLUSIONS: Age-related changes in muscle volume and intraMAT were heterogeneous and partially uncoupled across muscles and individuals. Coordinated increases in intraMAT, rather than muscle atrophy alone, were associated with declines in lower-limb power, highlighting muscle composition as an important dimension of skeletal muscle aging.