Comparison of lumbar paraspinal muscle size and density between members of the US Framingham Heart Study and Tsimane forager-horticulturalists of Bolivia.

Paraspinal muscles support and move the spine, and reduced size and density of these muscles is associated with increased risk of vertebral fracture and back pain. We test the hypothesis that subsistence-oriented forager-horticulturalists (Bolivian Tsimane) have larger and denser paraspinal muscles, and smaller age-related differences in paraspinal muscle size and density, than more sedentary post-industrialized suburbanites (Framingham Heart Study [FHS]). We measured cross-sectional area (CSA) and density of trunk extensors and psoas major at L3 from computed tomography scans of 411 FHS and 330 Tsimane (50% women, aged 50-90 years). Sex-stratified linear regressions determined associations of muscle CSA and density with age and cohort (Tsimane or FHS [ref.]), with and without an age-cohort interaction term. Covariates were height and weight. After adjusting for covariates, Tsimane women had 11-14% larger and denser extensors, and 17-26% smaller and less-dense psoas major, than FHS women. Tsimane men had 8-21% smaller and less-dense psoas major than FHS men. The age-related difference in extensors density was 41% smaller for Tsimane men than for FHS men. There were no other significant age-related cohort differences in muscle CSA or density. We observed a different balance in the size and density of trunk extensors versus psoas major between Tsimane and FHS. A possible explanation for this observation is cohort differences in the amount and types of physical activity, although future work is required to determine the influence of lifestyle and environmental factors on paraspinal muscle size and density.