Citation: Rattanakoch, J.; Samala, M.; Limroongreungrat, W.; Guerra, G.; Tharawadeepimuk, K.; Nanbancha, A.; Niamsang, W.; Kerdsomnuek, P.; Suwanmana, S. Validity and Reliability of Inertial Measurement Unit (IMU)-Derived 3D Joint Kinematics in Persons Wearing Transtibial Prosthesis. Sensors 2023, 23, 1738. https://doi.org/10.3390/ s23031738 Academic Editors: Sigal Portnoy and Jason Friedman Received: 16 January 2023 Revised: 27 January 2023 Accepted: 29 January 2023 Published: 3 February 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). sensors Article Validity and Reliability of Inertial Measurement Unit (IMU)-Derived 3D Joint Kinematics in Persons Wearing Transtibial Prosthesis Jutima Rattanakoch 1 , Manunchaya Samala 1, * , Weerawat Limroongreungrat 2 , Gary Guerra 3 , Kittichai Tharawadeepimuk 2 , Ampika Nanbancha 2 , Wisavaporn Niamsang 1 , Pichitpol Kerdsomnuek 4 and Sarit Suwanmana 2 1 Sirindhorn School of Prosthetics and Orthotics, Faculty of Medicine Siriraj Hospital, Mahidol University, Bangkok 10700, Thailand 2 College of Sports Science and Technology, Mahidol University, Nakhon Pathom 73170, Thailand 3 Exercise and Sport Science Department, St. Mary’s University, San Antonio, TX 78228, USA 4 Department of Orthopaedic Surgery, Faculty of Medicine Siriraj Hospital, Mahidol University, Bangkok 10700, Thailand * Correspondence: manunchaya.saa@mahidol.edu Abstract: Background: A validity and reliability assessment of inertial measurement unit (IMU)- derived joint angular kinematics during walking is a necessary step for motion analysis in the lower extremity prosthesis user population. This study aimed to assess the accuracy and reliability of an inertial measurement unit (IMU) system compared to an optical motion capture (OMC) system in transtibial prosthesis (TTP) users. Methods: Thirty TTP users were recruited and underwent simultaneous motion capture from IMU and OMC systems during walking. Reliability and validity were assessed using intra- and inter-subject variability with standard deviation (S.D.), average S.D., and intraclass correlation coefficient (ICC). Results: The intra-subject S.D. for all rotations of the lower limb joints were less than 1 ◦ for both systems. The IMU system had a lower mean S.D. ( o ), as seen in inter-subject variability. The ICC revealed good to excellent agreement between the two systems for all sagittal kinematic parameters. Conclusion: All joint angular kinematic comparisons supported the IMU system’s results as comparable to OMC. The IMU was capable of precise sagittal plane motion data and demonstrated validity and reliability to OMC. These findings evidence that when compared to OMC, an IMU system may serve well in evaluating the gait of lower limb prosthesis users. Keywords: inertial measurement unit (IMU); optical motion capture (OMC); intra- and inter-subject variability; transtibial prosthesis; gait parameters assessment 1. Introduction Amputation has a marked effect on an individual’s quality of life (QOL) [1], yet it is well understood that a prosthesis can facilitate a return to a higher quality of life [2]. Returning to an ambulant lifestyle and activities of daily living is the cornerstone of pros- thetic intervention [3]. Mobility is the most important factor related to QOL in lower limb prosthesis users [4]. It is the responsibility of the prosthetist and rehabilitation team to make informed and customized choices for a suitable prosthetic prescription based on patient preference and practitioner expertise. Provision of an acceptable continuum of care requires pre- and post-prosthetic outcome measurement and continuous long-term ambulatory mea- surements. Evaluating the effects of a prosthetic intervention on a prosthesis user’s quality of life is conducted using biomechanical outcome measurements. Adequately measuring the outcome of prosthetic interventions is an important and recommended procedure for ensuring a continuum of quality routine clinical care [5]. Sensors 2023, 23, 1738. https://doi.org/10.3390/s23031738 https://www.mdpi.com/journal/sensors