Paraspinal muscle morphology and composition: a 15-yr longitudinal magnetic resonance imaging study.

PURPOSE: The purposes of this study were to define the natural progression of age-related changes of the lumbar paraspinal muscles during adulthood and to investigate the influence of lifestyle and individual factors (e.g., physical activity levels at work and leisure, body mass index, and low back pain [LBP]).

METHODS: This population-based longitudinal study included a sample of 99 adult male twins. Data were collected through a structured interview, physical examination, and magnetic resonance imaging. Measurements of the lumbar multifidus and erector spinae muscles were obtained from T2-weighted axial images at L3-L4 and L5-S1 at baseline and 15-yr follow-up. Muscle cross-sectional area (CSA), functional CSA (FCSA) (fat-free mass), and FCSA/CSA (composition) as well as CSA and FCSA asymmetry and FCSA/CSA side-to-side differences were measured.

RESULTS: Subjects' mean ± SD age was 47.3 ± 7.4 yr at baseline and 62.3 ± 8.0 yr at follow-up. During the 15-yr period, both muscles exhibited a decrease in CSA and FCSA and an increase in fatty infiltration and side-to-side differences in size and composition at both spinal levels. Both muscles displayed greater changes at L5-S1 than L3-L4. Age and BMI were found to be significantly associated with the degree of paraspinal muscle changes over time. However, there was no association between the change in paraspinal muscle size, composition, or asymmetry with the level of physical demands at work or leisure or LBP history.

CONCLUSIONS: The present longitudinal study suggests that over adulthood, the multifidus and erector spinae undergo similar morphological changes. Moreover, our findings suggest that the long-term progression of lumbar paraspinal muscle changes evaluated through magnetic resonance imaging are not associated with the range of physical demand levels as were typical of Finnish men or LBP history.