Background: Osteoporosis is a serious health problem, leading to an increased risk of vertebral fractures and the muscles of the spine (paraspinal and psoas) could play an important role in this risk. However, the relationship between spinal muscle characteristics, such as size and fat infiltration, and the prevalence of vertebral fractures is currently unknown. Aims: The aim of this thesis was to explore how paraspinal and psoas muscle cross section area (CSA) size and composition change with age and how their interaction with bone density characteristics may be related to the incidence of vertebral fractures. Method: A total of 201 women underwent dual-energy X-ray absorptiometry (DXA) scans of their spine, hip and total body to determine their hip and lumbar spine bone mineral density (BMD) as well as their android and gynoid fat levels. They were classified into groups based on age and BMD status as follows: pre-50 control (n = 108), post-50 control (n = 22), pre-50 osteoporotic (n = 18), post-50 osteoporotic (n = 34), vertebral fracture (n = 19). MRI of the lumbar spine muscles (erector spinae, multifidus and psoas major) at the L3-L4 level was undertaken using a T1-weighted axial sequence. The cross-sectional area and the proportion of fat infiltration were measured manually using ImageJ software. Results and conclusion: Women with vertebral fractures showed asymmetrical differences between the left and right for the total muscle CSA size (sum of the psoas, multifidus and erector spinae muscles) and fat infiltration. Additionally, women with vertebral fractures had decreased cross-sectional muscle area and greater fat infiltration compared the those without fractures. Lumbar BMD was positively correlated with psoas muscle CSA size in the osteoporosis group, while total hip BMD was negatively correlated with psoas CSA muscle in women with osteoporosis. Further, age was negatively correlated with lumbar BMD in osteoporotic women. Thus, further research is required to understand how these factors may lead to vertebral fractures or whether the presence of vertebral fractures leads to changes in muscle composition.