**5. Conclusions**

TMM–BIA is strongly correlated with the CSA of PVM, especially the fat-free CSA, as measured with MRI. Additionally, TMM–BIA is correlated with BMS. As CSA and BMS are gold standards for quantitative and functional assessments of trunk muscles, TMM–BIA can be considered a new method to measure these parameters. Our findings highlight the significance of TMM–BIA as a reliable, cost-effective, and efficient tool for the assessment

of trunk muscles. Given its simplicity and reliability, BIA may be an alternative method for evaluating trunk muscles in clinical settings.

**Author Contributions:** Conceptualization, S.O., M.H., Y.H., S.T., and H.N.; methodology, A.Y., H.H., A.K., T.T., and S.K.; formal analysis, A.Y., H.S., A.K., T.T., and S.K.; data curation, H.T.; writing— original draft preparation, H.S.; writing—review and editing, S.O. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Japanese Orthopaedic Association Research Grant, gran<sup>t</sup> number 2017-1.

**Institutional Review Board Statement:** The study was conducted according to the guidelines of the Declaration of Helsinki and the Ethical Guidelines for Medical and Health Research involving Human Subjects in Japan and was approved by the Institutional Review Board of Osaka City University Graduate School of Medicine (No. 3484). All methods were performed in accordance with the Declaration of Helsinki.

**Informed Consent Statement:** Written informed consent for publication was obtained from the two participants whose information is presented in the case presentation sections and in Figures 6 and 7. They were informed that individual details and images would be published in our manuscript.

**Data Availability Statement:** Data available on request due to restrictions e.g. privacy or ethical. The data presented in this study are available on request from the corresponding author. The data are not publicly available due to patients privacy.

**Acknowledgments:** The authors thank Satomi Kawabata for her help in the collection of data and in interviews with the participants. The authors also express sincere thanks to the staff of Shiraniwa Hospital for their help in conducting the Shiraniwa study.

**Conflicts of Interest:** The authors declare no conflict of interest.
