Objective:To investigate the morphological difference and clinical significance of bilateral lumbar multifidus muscles in patients with unilateral lumbosacral radiculopathy due to lumbar disc herniation and lumbar spondylolisthesis.Methods:A retrospective analysis was conducted on patients with low back pain, lumbar disc herniation and lumbar spondylolisthesis. Patients with lumbar disc herniation or lumbar spondylolisthesis underwent single segment lesion either at L 4, 5 or L 5S 1, while those accompanied with unilateral lumbosacral radiculopathy underwent percutaneous endoscopic lumbar discectomy or conventional open surgery at Qingdao Municipal Hospital between January 2017 and January 2023. Patients with lumbar spondylolisthesis were subdivided into degenerative lumbar spondylolisthesis and isthmic spondylolisthesis. 53 patients with low back pain met the inclusion criteria. 170 patients with lumbar disc herniation met the inclusion criteria, with 101 at L 4, 5 and 69 at L 5S 1 level. 129 patients with lumbar spondylolisthesis met the inclusion criteria, including 91 of degenerative lumbar spondylolisthesis at L 4, 5 level and 9 at L 5S 1 level, and 11 of isthmic spondylolisthesis at L 4, 5 level and 18 at L 5S 1 level. Cross-sectional images at the mid-disc of L 3, 4, L 4, 5 and L 5S 1 segments in MRI were acquired. Relative total cross-sectional area (rTCSA), relative functional cross-sectional area (rFCSA), fat infiltration rate (FIR), relative fat distance (rFD) and differential value FIR (D-FIR) in bilateral lumbar multifidus muscle were measured respectively by using Image J software, and were then used to evaluate the atrophy and fat infiltration of bilateral lumbar multifidus muscles. Results:No significant difference was found between the both sides of multifidus muscle in low back pain patients. L 4, 5 lumbar disc herniation group had smaller rFCSA (0.34±0.10 and 0.35±0.10) and larger FIR [29.92(22.21, 36.46) and 26.48(17.54, 34.55)] and rFD [0.39(0.29, 0.54) and 0.32(0.21, 0.43)] on the affected side compared to the unaffected side in L 4, 5 segment, and had larger FIR (34.83±11.34 and 31.44±10.94) and rFD [0.59(0.43, 0.77) and 0.51(0.37, 0.69)] on the affected side in L 5S 1 segment. L 5S 1 lumbar disc herniation group had smaller rFCSA (0.41±0.11 and 0.42±0.12) and larger FIR [26.84(22.92, 35.29) and 24.02(20.03, 32.87)] and rFD (0.51±0.28 and 0.42±0.26) on the affected side in L 5S 1 segment. L 4, 5 degenerative lumbar spondylolisthesis group had larger FIR (36.49±9.76 and 34.72±9.86) on the affected side in L 4, 5 segment, and had larger FIR [35.03(28.64, 41.85) and 33.34(26.37, 39.76)] on the affected side in L 5S 1 segment. L 5S 1 degenerative lumbar spondylolisthesis group had larger FIR [42.53(37.94, 46.81) and 40.79(30.84, 43.53)] and rFD (1.12±0.79 and 0.94±0.79) on the affected side in L 5S 1 segment. L 4, 5 isthmic spondylolisthesis group had smaller rFCSA [0.24(0.20, 0.30) and 0.29(0.23, 0.34)]and larger FIR [34.19 31.30, 42.39) and 29.43(28.82, 36.89)] and rFD (0.39±0.15 and 0.29±0.15) on the affected side in L 4, 5 segment, and had larger FIR (43.18±12.71 and 34.12±11.63) on the affected side in L 5S 1 segment. L 5S 1 isthmic spondylolisthesis group had larger FIR (40.24±9.34 and 36.37±10.70) on the affected side in L 5S 1 segment. No significant difference was found of the multifidus muscle between the affected and unaffected sides in the proximal adjacent segment of the responsible segment in lumbar disc herniation or lumbar spondylolisthesis group patients. L 4, 5 isthmic spondylolisthesis group had larger D-FIR (6.75±8.46 and 1.78±5.77) in L 4, 5 segment, and had larger D-FIR (9.06±11.59 and 1.54±7.08) in L 5S 1 segment compared to L 4, 5 degenerative lumbar spondylolisthesis group. Grade Ⅱ L 4, 5 lumbar spondylolisthesis group had larger D-FIR (10.73±13.61 and 1.92±7.43) in L 5S 1 segment compared to grade Ⅰ L 4, 5 lumbar spondylolisthesis group. Conclusion:L 4, 5 or L 5S 1 lumbar disc herniation and lumbar spondylolisthesis patients with unilateral lumbosacral radiculopathy had asymmetric atrophy and fat infiltration of multifidus muscle. The atrophy and fat infiltration on the affected side showed greater. The asymmetry appeared in the responsible segment and its distal adjacent lumbar segment. Lumbar spondylolisthesis patients with a lager degree of slip or with isthmic type could be accompanied by more severe asymmetry of multifidus muscle.