PURPOSE: The effect of skeletal muscle mass of the trunk and extremities on sagittal imbalance of the spine before and after surgery for adult spinal deformity (ASD) has not been elucidated. The purpose of this study was to examine the correlation between reduced skeletal muscle mass of the trunk and extremities, as well as spinopelvic parameters, preoperatively, postoperatively and at least 2 years after surgery for ASD. METHODS: This retrospective observational study included 140 consecutive patients who had undergone surgery for ASD and were followed-up for at least 2 years and whose skeletal muscle mass could be measured preoperatively using whole-body dual-energy X-ray absorptiometry. Correlations between skeletal muscle mass and spinopelvic parameters were assessed preoperatively, postoperatively, and after 2 years of follow-up. RESULTS: All spinopelvic parameters were significantly improved postoperatively compared with preoperatively. Between the postoperative period and two years post-surgery, a significant loss of correction was observed in SVA and GT. Trunk muscles mass showed significant negative correlations with preoperative PT, SS, PI-LL, SVA, GT, and TPA, but there was no significant correlation with postoperative parameters. Through univariate and multivariate regression analysis, lower limb skeletal muscle mass showed a significant negative correlation with SVA at two years postoperatively and the loss of corrected SVA (ΔSVA) over two years. CONCLUSION: This study revealed that lower extremity muscle mass was implicated in the loss of corrected SVA 2 years after surgery. This study is clinically meaningful as it suggests that strength training for the lower extremities, performed preoperatively and/or postoperatively, can improve sagittal imbalances of the spine in patients with severe ASD or maintain SVA after corrective surgery.