Objective:To investigate the correlation of relative cross-sectional area of lumbar paravertebral muscle and psoas with lumbar curvature and its clinical significance.Methods:This study was conducted as a retrospective cross-sectional study. Among the patients treated at the Department of Orthopedics and Traumatology, Shuguang Hospital Affiliated to Shanghai University of Traditional Chinese Medicine from January to April 2023, 107 middle-aged and young patients with changes in lumbar physiological curvature or no apparent abnormalities were included in this study based on MRI findings. The relative cross-sectional area (RCSA) values of the left and right paravertebral muscles and psoas were measured at the lower margin of the L3 vertebral body and at the L3/L4, L4/L5, and L5/S1 intervertebral discs. Furthermore, the correlation between these RCSA values and lumbar curvature was analyzed.Results:The 107 patients were (30.51 ± 4.64) years old, and the proportion of females accounted for 71.96%. There were differences in RCSA between the lower margin of L3, L3/L4, L4/L5, and L5/S1 intervertebral discs (left psoas F = 30.21, P < 0.001; right psoas F = 31.63, P < 0.001; left paravertebral muscle F = 31.04, P < 0.001; right paravertebral muscle F = 26.55, P < 0.001). From L3 to S1, the RCSA of the psoas increased, while the RCSA of the paravertebral muscle decreased. The RCSA values of the total paravertebral muscle/RCSA values of the total psoas major muscle were positively correlated with lumbar curvature at the lower margin of the L3 vertebral body ( r = 0.40, P < 0.001). This correlation could be seen at both L3/L4 ( r = 0.31, P = 0.001) and L4/L5 ( r = 0.24, P = 0.012). However, after considering the influence of left and right muscle groups on lumbar curvature, there was no correlation between the muscle cross-sectional area at the L5/S1 disc position and lumbar curvature ( P > 0.05). Nevertheless, there was still a correlation between the muscle cross-sectional area at other lumbar segments ( P < 0.05). Conclusion:An increase in the RCSA of the psoas relative to the paraspinal muscle may lead to straightening of the lumbar spine curvature, whereas an increase in the RCSA of the paraspinal muscle relative to the psoas may result in an increase in lumbar spine curvature. Improving the quality of the paraspinal muscle may restore the physiological curvature of the lumbar spine, which holds clinical application value.