Change in fatty infiltration of lumbar multifidus, erector spinae, and psoas muscles in asymptomatic adults of Asian or Caucasian ethnicities

Fatty infiltration (FI) is a feature of degenerating muscle that predominates in the low lumbar spine, associates with pain, and is confounded by age, spinal degeneration, and curvature. We determined rates for decline of lumbar muscle quality according to ethnicity, muscle, and spinal level in asymptomatic subjects. Cross-sectional simulation study assessing aggregated data; 650 Asians aged 20-89 years versus 80 Caucasians aged 20-62 years. Change in lumbar multifidus, erector spinae (ES), and psoas fat content were computed using synthetic data and Monte Carlo simulations. General linear regression models and multivariate adaptive regression splines enabled estimation of yearly decline rates [with 95% confidence intervals (CI)]. ES at L1-5 (total) shows steeply reduced density (rate; CI) for Asians in older (> 53.3 years) adulthood (-0.32; -0.27 to -0.36/year). For Asians, multifidus (-0.18; -0.15 to -0.20/year) and psoas (-0.04; -0.03 to -0.06/year) also decline, while ES in younger <= 53.3 years) adults does not (0.06; 0.01-0.12/year). Caucasian multifidus declines (increasing FI % rate; CI) insignificantly faster (L1-5; 0.23; 0.10-0.36%/year) than ES (0.13; 0.04-0.22%/year). Multifidus decline does not differ between ethnicities. ES in older Asians generally declines fastest across ethnicities and muscles, and particularly in the low lumbar levels. Low lumbar levels show higher rates of decline in Asians, with mixed level-dependencies apparent in Caucasians. Decline in lumbar muscle composition may differ between ethnicities and muscles. ES and low lumbar levels appear increasingly susceptible in Asians. Longitudinal studies examining rate of change to muscle composition may provide distinction between spinal conditions.