Objective: Although sarcopenia and osteoporosis are inter-related conditions that are common with advancing age, few studies have explored relationships between muscle quality and bone mineral density (BMD). We investigated age- and sex-specific paraspinal muscle fat infiltration (MFI), muscle cross-sectional area (CSA), and spine volumetric BMD (vBMD) in healthy Chinese adults. Methods: 605 healthy adults aged 20-59 years (340 women, mean age 39.2 years; 265 men, mean age 38.8 years) had axial T2WI MRI imaging of the lumbar spine and CSA (cm(2)) and MFI (%) were measured in the psoas and multifidus and erector spinae (MF-ES) muscles (L3-L4). MFI measurements were calibrated against a region of interest in an adjacent area of subcutaneous pure fat. L2-L4 vBMD was measured by quantitative CT. Age- and sex-specific subgroups were compared using the Mann-Whitney test. Multiple regression was used to test independent associations of MFI and CSA with vBMD. Results: Females had lower CSA and higher MFI than males in both the psoas and MF-ES muscles (p < 0.001). In females and males, MF-ES MFI increased with age (p < 0.001) and in females age-related increases were observed for the psoas muscles (p < 0.05). Greater fat infiltration of the MS-ES muscle unit was associated with lower vBMD in both sexes (p < 0.001) but not with CSA. Following adjustment for demographic variables and CSA, MS-ES MFI remained predictive of vBMD (beta = -0.408 to -0.157, p < 0.001). Conclusion: We have demonstrated that, independent of CSA and demographic variables, MFI of the MF-ES muscles is predictive of lower lumbar spine vBMD in both sexes. Advances in knowledge: This is the first study to demonstrate that, independent of muscle size and demographic variables, MFI of the paraspinal MF-ES muscles is predictive of lower lumbar spine vBMD in both sexes.