Muscle Quality Deteriorates More Rapidly Than Muscle Quantity in Lumbar Paraspinal Muscles: A 4-Year Longitudinal MRI Study

OBJECTIVES: To compare longitudinal changes in muscle quantity and muscle quality over 4 years in community-dwelling older adults and to determine which better reflects age-related functional decline. DESIGN: Prospective longitudinal cohort study with baseline and 4-year follow-up assessments. SETTING AND PARTICIPANTS: Participants were community-dwelling adults aged ≥65 years enrolled in the SarcoSpine cohort. Of 111 participants initially enrolled, 66 participants, who completed both baseline and follow-up lumbar magnetic resonance imaging (MRI), were included in the longitudinal analysis. METHODS: Participants underwent anthropometric measurements, physical performance assessments (handgrip strength and gait speed), and lumbar MRI at baseline and after 4 years. Lumbar paraspinal muscle (LPM) quantity was evaluated using cross-sectional area and total muscle volume, while muscle quality was assessed using MRI-derived signal intensity (SI). Longitudinal changes were calculated as percentage changes from baseline. Sex-stratified analyses were performed to compare aging-related changes between men and women. RESULTS: Over 4 years, appendicular skeletal muscle mass index (ASM/Ht²) remained relatively stable (-1.0%, P = .146), whereas physical performance declined significantly, with reductions in handgrip strength (-7.2%) and gait speed (-8.1%) (both P < .001). MRI demonstrated a clear dissociation between muscle quantity and quality. LPM cross-sectional area and total muscle volume showed modest decreases (-4.1% and -3.6%, respectively; both P < .001), whereas muscle SI increased markedly by 39.1% (P < .001), indicating substantial deterioration in muscle quality. Similar longitudinal patterns were observed in both men and women, with no significant between-sex differences in the magnitude of change. CONCLUSIONS AND IMPLICATIONS: Among community-dwelling older adults, MRI-derived muscle quality deteriorated substantially faster than muscle quantity over 4 years and more closely paralleled declines in physical performance. These findings suggest that age-related muscle deterioration is driven primarily by qualitative degeneration rather than simple loss of muscle mass and support the use of imaging-based muscle quality measures as sensitive biomarkers of biological muscle aging.