Association of sagittal spinal alignment with thickness and echo intensity of lumbar back muscles in middle-aged and elderly women

Quantitative changes, such as a decrease in muscle mass, and qualitative changes, such as an increase in the amount of intramuscular non-contractile tissue, occur with aging. However, it is unclear whether quantitative or qualitative changes in back muscles are associated with spinal alignment in the standing position. We investigated the association of sagittal spinal alignment with muscle thickness as an index of the mass of lumbar back muscles and muscle echo intensity as an index of the amount of non-contractile tissue within these muscles.Study participants comprised 36 middle-aged and elderly women. Thickness and echo intensity of erector spinae, psoas major, and lumbar multifidus muscles were measured using an ultrasound imaging device. Standing sagittal spinal alignment, determined from thoracic kyphosis and lumbar lordosis angles, and the sacral anterior inclination angle was measured using the Spinal Mouse.Stepwise regression analysis performed using muscle thickness, echo intensity, and age as independent variables showed that erector spinae muscle thickness was a significant determinant of the thoracic kyphosis angle. Psoas major muscle thickness and echo intensity of the lumbar multifidus muscle were significant determinants of the sacral anterior inclination angle.Our results suggest that an increase in thoracic kyphosis is associated with a decrease in the mass of the erector spinae muscle, and that a decrease in pelvic anterior inclination is associated with a decrease in the mass of the psoas major muscle and an increase in the amount of non-contractile tissue within the lumbar multifidus muscle.