Sex- and Age-Specific Distribution of Skeletal Muscle Mass and Paraspinal Muscle Indices and Their Association With Spinal Sagittal Alignment: The Research on Osteoarthritis/Osteoporosis Against Disability (ROAD) Study.

Background Sagittal spinal malalignment is a significant contributor to reduced quality of life in older adults, associated with chronic back pain, impaired mobility, and psychosocial distress. While age-related loss of skeletal muscle mass (sarcopenia) is known to influence posture, the sex-specific progression of muscle degeneration and its association with sagittal balance remain unclear. Objective The objective of this study is to investigate the age- and sex-specific changes in skeletal and paraspinal muscle indices and their associations with sagittal spinal alignment in a large population-based Japanese cohort. Methods This cross-sectional study used data from the third visit of the Research on Osteoarthritis/Osteoporosis Against Disability (ROAD) study, including 748 community-dwelling adults (220 men, 528 women). Whole-spine lateral radiographs and lumbar magnetic resonance imaging were used to assess the C7 sagittal vertical axis (SVA) and paraspinal muscle composition, respectively. Skeletal muscle mass was evaluated using bioimpedance analysis. Participants were stratified into five age groups, and muscle quantity and quality were analyzed by sex. Multivariate regression analyses were performed to identify predictors of sagittal malalignment. Results As people age, both men and women show a clear increase in the forward tilt of the spine, measured by the C7 SVA, and an increase in fat within the deep back muscles, especially the multifidus and erector spinae. For example, the average SVA increased from 10.3 mm in those under 50 to 46.7 mm in those aged 80 and above (p < 0.001). The fat content in the multifidus at the L1/2 level rose from 12.4% to 29.6% in men and from 15.1% to 35.4% in women. Lower trunk muscle mass, rather than overall skeletal muscle or limb muscle index, was linked to worse spinal alignment in both sexes. Increased fat in the paraspinal muscles, especially at the L1/2 level, was more strongly related to poor posture in women. Conclusion Reduced trunk muscle mass and increased fat infiltration of paraspinal muscles, particularly at the L1/2 level, were independently associated with a forward-shifted SVA (SVA ≥50 mm), especially among women. These findings highlight the central role of trunk muscle quality and quantity in maintaining postural alignment in aging adults and suggest the importance of interventions targeting trunk musculature to prevent age-related sagittal imbalance.