Objective:To explore the repeatability of ultrasound shear wave elastography (SWE) in evaluating the Young′s modulus (E) of the lumbar multifidus (LM) and the difference in E of the bilateral LM, and to analyze the E′s change ratio of LM and its impact factors at upright and 90° forward bend positions.Methods:A total of 60 healthy volunteers, selected from interns, standardized training residents and refresher doctors in department of diagnostic ultrasound of Sir Run Run Shaw Hospital of Zhejiang University School of Medicine from July to December in 2020, were examined twice by SWE to estimate the E value of LM at prone, upright and 90° flexion positions, respectively. Firstly, the intraclass correlation coefficient (ICC) was applied to test the reliability between the two repeated measurements.Secondly, the differences in E of the bilateral LM were discussed. The discrepancies in the E value of male and female volunteers at prone, upright and 90° flexion positions and the change ratio of the E value at upright and 90° forward bend positions were also discussed, respectively. Finally, Spearman correlation coefficient was utilized to evaluate the correlations of E and its change ratio with gender, age, BMI and activity level at upright and 90° forward bend positions, respectively.Results:The ICC value in LM measured by SWE was in the range of 0.691 to 0.951. No obvious change in bilateral E values of the same posture was observed for an individual(all P>0.05). The change ratios of E at upright and 90° flexion positions were approximate to 2 and 7, respectively. Moreover, at prone position, the E value exhibited negative correlation with age and activity level of self-assessment ( rs=-0.300, P=0.020; rs=-0.383, P=0.002). The E in female was higher compared with that in male. At active states the change ratio in E was positively correlated with age and activity level ( rs=0.278, P=0.031; rs=0.495, P<0.001), and the E′s change ratio in male was higher than that in female. Conclusions:SWE possesses excellent repeatability in evaluating the E value of LM with no significant difference in E of bilateral LM. The E′s change ratio in LM varies with different contraction strategies and changes with age, sex and self-reported activity level.