Aim To compare lumbar musculature in lumbar degenerative kyphosis (LDK) patients and chronic low-back pain (CLBP) patients. Materials and methods The study group comprised LDK patients (54 women, aged 44–74 years, mean 60 years) and a control group with CLBP (54 women, aged 45–73 years, mean 60 years). The cross-sectional areas (CSA) of psoas, erector spinae, multifidus, and disc, were measured at the L4–L5 level using magnetic resonance imaging (MRI). Lumbar muscularity was expressed as three ratios: the ratio between CSA of psoas, erector spinae, multifidus and CSA of disc (PS:disc, ES:disc, MF:disc). Multifidus and erector spinae atrophy were evaluated at the L3–L4 level and the degree of fatty atrophy was estimated using three grades: mild, moderate, and severe. The shapes of thoracolumbar fascia were analysed at the L5–S1 level and were classified as flat or convex. Results Lumbar muscularity was found to be significantly smaller ( p < 0.001) in LDK patients (PS:disc = 0.79, SD 0.22; ES:disc = 1.36, SD 0.49; MF:disc = 0.55, SD 0.21) than the control group patients (PS:disc = 0.98, SD 0.23; ES:disc = 1.71, SD 0.46; MF:disc = 0.86, SD 0.30). Patients with LDK had a higher proportion of fat deposits in the multifidus and erector spinae muscle ( p < 0.001), and the thoracolumbar fascia at the L5–S1 level was more commonly flat ( p < 0.01). Conclusion Evaluation of paraspinal musculature should be considered when assessing MRI images of the lumbar spine. Measurement of the CSA, visual grading of fatty atrophy and the assessment of the fascia may help physician and radiologist reach a more confident diagnosis for the patients with clinically suspicious LDK.