{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,9,22]],"date-time":"2026-09-22T10:01:51Z","timestamp":1790071311109,"version":"4.0.1"},"reference-count":33,"publisher":"MDPI AG","issue":"18","license":[{"start":{"date-parts":[[2026,9,17]],"date-time":"2026-09-17T00:00:00Z","timestamp":1789603200000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"DOI":"10.13039\/501100002241","name":"JST SPRING","doi-asserted-by":"publisher","award":["JPMJSP2119"],"award-info":[{"award-number":["JPMJSP2119"]}],"id":[{"id":"10.13039\/501100002241","id-type":"DOI","asserted-by":"publisher"}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Sensors"],"abstract":"<jats:p>Muscle flexibility and joint range of motion (ROM) evaluation are essential for injury prevention and rehabilitation; however, conventional manual methods are limited in large-scale screening and self-monitoring because of examiner dependency and time constraints. This study evaluated the reliability and validity of AI-based pose estimation for lower-limb flexibility and ROM tests compared with conventional manual measurements. Twenty healthy volunteers (40 lower limbs; age 24.4 \u00b1 2.6 years) underwent three flexibility tests\u2014the Active Knee Extension Test (AKET), Modified Thomas Test (MTT), and Weight-Bearing Lunge Test (WBLT). The tests were assessed by a physiotherapist and three pose-estimation models (MediaPipe, OpenPose, and HRNet). Intra-rater reliability was evaluated with ICC(3,1). Concurrent validity and agreement were assessed using linear regression and Bland\u2013Altman analysis. Intra-rater reliability was good to excellent for all methods (ICC \u2265 0.87), and concurrent validity was high (r2 \u2265 0.80), except for MTT using the HRNet (r2 = 0.73). Fixed biases were observed during the flexibility test. The limits of agreement ranged from \u00b18.04\u201310.87 cm for the AKET and MTT, and \u00b13.13\u20134.42 cm for WBLT. The WBLT showed a significant systematic bias, with an underestimation of \u22126.25\u00b0 to \u22127.17\u00b0 across all models. In young, healthy volunteers under standardized laboratory conditions, pose estimation models showed robust reliability and concurrent validity as objective tools for lower-limb flexibility assessment. However, systematic biases warrant caution regarding the interchangeability between AI-based measurements and conventional methods.<\/jats:p>","DOI":"10.3390\/s26185890","type":"journal-article","created":{"date-parts":[[2026,9,22]],"date-time":"2026-09-22T09:04:17Z","timestamp":1790067857000},"page":"5890","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["Reliability and Validity of AI-Based Pose Estimation Algorithms for Assessing Lower-Limb Flexibility and Joint Range of Motion"],"prefix":"10.3390","volume":"26","author":[{"ORCID":"https:\/\/orcid.org\/0009-0006-9120-3314","authenticated-orcid":false,"given":"Hinata","family":"Okuno","sequence":"first","affiliation":[{"name":"Graduate School of Health Sciences, Hokkaido University, Kita-12 Nishi-5, Kita-ku, Sapporo 060-0812, Hokkaido, Japan"}],"role":[{"vocabulary":"crossref","role":"author"},{"role":"conceptualization","vocabulary":"credit"},{"role":"methodology","vocabulary":"credit"},{"role":"software","vocabulary":"credit"},{"role":"validation","vocabulary":"credit"},{"role":"formal-analysis","vocabulary":"credit"},{"role":"investigation","vocabulary":"credit"},{"role":"resources","vocabulary":"credit"},{"role":"data-curation","vocabulary":"credit"},{"role":"writing-original-draft","vocabulary":"credit"},{"role":"visualization","vocabulary":"credit"},{"role":"project-administration","vocabulary":"credit"},{"role":"funding-acquisition","vocabulary":"credit"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-0174-7416","authenticated-orcid":false,"given":"Tomoya","family":"Ishida","sequence":"additional","affiliation":[{"name":"Department of Rehabilitation Science, Faculty of Health Sciences, Hokkaido University, Sapporo 060-0812, Hokkaido, Japan"},{"name":"Department of Orthopaedic Sports Science, Faculty of Health Sciences, Hokkaido University, Sapporo 060-0812, Hokkaido, Japan"}],"role":[{"vocabulary":"crossref","role":"author"},{"role":"conceptualization","vocabulary":"credit"},{"role":"methodology","vocabulary":"credit"},{"role":"software","vocabulary":"credit"},{"role":"writing-original-draft","vocabulary":"credit"},{"role":"writing-review-editing","vocabulary":"credit"},{"role":"supervision","vocabulary":"credit"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-1942-7798","authenticated-orcid":false,"given":"Yuta","family":"Koshino","sequence":"additional","affiliation":[{"name":"Department of Rehabilitation Science, Faculty of Health Sciences, Hokkaido University, Sapporo 060-0812, Hokkaido, Japan"},{"name":"Department of Orthopaedic Sports Science, Faculty of Health Sciences, Hokkaido University, Sapporo 060-0812, Hokkaido, Japan"}],"role":[{"vocabulary":"crossref","role":"author"},{"role":"conceptualization","vocabulary":"credit"},{"role":"writing-review-editing","vocabulary":"credit"}]},{"given":"Satoshi","family":"Kasahara","sequence":"additional","affiliation":[{"name":"Department of Rehabilitation Science, Faculty of Health Sciences, Hokkaido University, Sapporo 060-0812, Hokkaido, Japan"},{"name":"Department of Orthopaedic Sports Science, Faculty of Health Sciences, Hokkaido University, Sapporo 060-0812, Hokkaido, Japan"}],"role":[{"vocabulary":"crossref","role":"author"},{"role":"conceptualization","vocabulary":"credit"},{"role":"writing-review-editing","vocabulary":"credit"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-0317-8088","authenticated-orcid":false,"given":"Harukazu","family":"Tohyama","sequence":"additional","affiliation":[{"name":"Department of Orthopaedic Sports Science, Faculty of Health Sciences, Hokkaido University, Sapporo 060-0812, Hokkaido, Japan"}],"role":[{"vocabulary":"crossref","role":"author"},{"role":"conceptualization","vocabulary":"credit"},{"role":"writing-review-editing","vocabulary":"credit"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-4663-598X","authenticated-orcid":false,"given":"Mina","family":"Samukawa","sequence":"additional","affiliation":[{"name":"Department of Rehabilitation Science, Faculty of Health Sciences, Hokkaido University, Sapporo 060-0812, Hokkaido, Japan"},{"name":"Department of Orthopaedic Sports Science, Faculty of Health Sciences, Hokkaido University, Sapporo 060-0812, Hokkaido, Japan"}],"role":[{"vocabulary":"crossref","role":"author"},{"role":"conceptualization","vocabulary":"credit"},{"role":"methodology","vocabulary":"credit"},{"role":"writing-original-draft","vocabulary":"credit"},{"role":"writing-review-editing","vocabulary":"credit"},{"role":"supervision","vocabulary":"credit"}]}],"member":"1968","published-online":{"date-parts":[[2026,9,17]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"41","DOI":"10.1177\/03635465030310011801","article-title":"Muscle Flexibility as a Risk Factor for Developing Muscle Injuries in Male Professional Soccer Players: A Prospective Study","volume":"31","author":"Witvrouw","year":"2003","journal-title":"Am. J. Sports Med."},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"1377","DOI":"10.1136\/bjsports-2014-093737","article-title":"Clinical Findings Just after Return to Play Predict Hamstring Re-Injury, but Baseline MRI Findings Do Not","volume":"48","author":"Reurink","year":"2014","journal-title":"Br. J. Sports Med."},{"key":"ref_3","doi-asserted-by":"crossref","first-page":"833","DOI":"10.4085\/1062-6050-50.5.07","article-title":"Weight-Bearing Dorsiflexion Range of Motion and Landing Biomechanics in Individuals with Chronic Ankle Instability","volume":"50","author":"Hoch","year":"2015","journal-title":"J. Athl. Train."},{"key":"ref_4","doi-asserted-by":"crossref","first-page":"3330","DOI":"10.1007\/s00167-023-07378-z","article-title":"Developmental Stage and Lower Quadriceps Flexibilities and Decreased Gastrocnemius Flexibilities Are Predictive Risk Factors for Developing Osgood\u2013Schlatter Disease in Adolescent Male Soccer Players","volume":"31","author":"Takei","year":"2023","journal-title":"Knee Surg. Sports Traumatol. Arthrosc."},{"key":"ref_5","doi-asserted-by":"crossref","first-page":"766","DOI":"10.1136\/bjsports-2023-107878","article-title":"Baseline Clinical and MRI Risk Factors for Hamstring Reinjury Showing the Value of Performing Baseline MRI and Delaying Return to Play: A Multicentre, Prospective Cohort of 330 Acute Hamstring Injuries","volume":"58","author":"Zein","year":"2024","journal-title":"Br. J. Sports Med."},{"key":"ref_6","doi-asserted-by":"crossref","unstructured":"Chung, J.-L., Ong, L.-Y., and Leow, M.-C. (2022). Comparative Analysis of Skeleton-Based Human Pose Estimation. Future Internet, 14.","DOI":"10.3390\/fi14120380"},{"key":"ref_7","doi-asserted-by":"crossref","first-page":"112826","DOI":"10.1016\/j.measurement.2023.112826","article-title":"Reliability and Validity Analysis of MediaPipe-Based Measurement System for Some Human Rehabilitation Motions","volume":"214","author":"Latreche","year":"2023","journal-title":"Measurement"},{"key":"ref_8","doi-asserted-by":"crossref","first-page":"104297","DOI":"10.1016\/j.cviu.2025.104297","article-title":"Comparing Human Pose Estimation through Deep Learning Approaches: An Overview","volume":"252","author":"Dibenedetto","year":"2025","journal-title":"Comput. Vis. Image Underst."},{"key":"ref_9","doi-asserted-by":"crossref","unstructured":"Ino, T., Samukawa, M., Ishida, T., Wada, N., Koshino, Y., Kasahara, S., and Tohyama, H. (2023). Validity of AI-Based Gait Analysis for Simultaneous Measurement of Bilateral Lower Limb Kinematics Using a Single Video Camera. Sensors, 23.","DOI":"10.3390\/s23249799"},{"key":"ref_10","doi-asserted-by":"crossref","first-page":"188","DOI":"10.1016\/j.gaitpost.2022.08.008","article-title":"Comparing the Accuracy of Open-Source Pose Estimation Methods for Measuring Gait Kinematics","volume":"97","author":"Washabaugh","year":"2022","journal-title":"Gait Posture"},{"key":"ref_11","doi-asserted-by":"crossref","first-page":"1703","DOI":"10.1080\/02640414.2025.2518359","article-title":"Reliability and Validity of Lower Extremity and Trunk Kinematics Measured with Markerless Motion Capture during Sports-Related and Functional Tasks: A Systematic Review","volume":"43","author":"Yoma","year":"2025","journal-title":"J. Sports Sci."},{"key":"ref_12","doi-asserted-by":"crossref","first-page":"62","DOI":"10.1016\/j.gaitpost.2020.05.027","article-title":"Verification of Reliability and Validity of Motion Analysis Systems during Bilateral Squat Using Human Pose Tracking Algorithm","volume":"80","author":"Ota","year":"2020","journal-title":"Gait Posture"},{"key":"ref_13","doi-asserted-by":"crossref","first-page":"957","DOI":"10.1589\/jpts.25.957","article-title":"Interrater and Intrarater Reliability of the Active Knee Extension (AKE) Test among Healthy Adults","volume":"25","author":"Hamid","year":"2013","journal-title":"J. Phys. Ther. Sci."},{"key":"ref_14","doi-asserted-by":"crossref","first-page":"470","DOI":"10.4085\/1062-6050-43.5.470","article-title":"Reliability Limits Of The Modified Thomas Test For Assessing Rectus Femoris Muscle Flexibility About The Knee Joint","volume":"43","author":"Peeler","year":"2008","journal-title":"J. Athl. Train."},{"key":"ref_15","doi-asserted-by":"crossref","first-page":"319","DOI":"10.3109\/09593985.2012.731140","article-title":"Using Digital Photography to Document Rectus Femoris Flexibility: A Reliability Study of the Modified Thomas Test","volume":"29","author":"Peeler","year":"2013","journal-title":"Physiother. Theory Pract."},{"key":"ref_16","doi-asserted-by":"crossref","first-page":"37","DOI":"10.1016\/j.ptsp.2016.06.010","article-title":"Intra-Rater Reliability and Agreement of Various Methods of Measurement to Assess Dorsiflexion in the Weight Bearing Dorsiflexion Lunge Test (WBLT) among Female Athletes","volume":"23","author":"Emparanza","year":"2017","journal-title":"Phys. Ther. Sport"},{"key":"ref_17","unstructured":"Bazarevsky, V., Grishchenko, I., Raveendran, K., Zhu, T., Zhang, F., and Grundmann, M. (2020). BlazePose: On-Device Real-Time Body Pose Tracking. arXiv."},{"key":"ref_18","doi-asserted-by":"crossref","first-page":"172","DOI":"10.1109\/TPAMI.2019.2929257","article-title":"OpenPose: Realtime Multi-Person 2D Pose Estimation Using Part Affinity Fields","volume":"43","author":"Cao","year":"2021","journal-title":"IEEE Trans. Pattern Anal. Mach. Intell."},{"key":"ref_19","doi-asserted-by":"crossref","first-page":"3349","DOI":"10.1109\/TPAMI.2020.2983686","article-title":"Deep High-Resolution Representation Learning for Visual Recognition","volume":"43","author":"Wang","year":"2021","journal-title":"IEEE Trans. Pattern Anal. Mach. Intell."},{"key":"ref_20","doi-asserted-by":"crossref","first-page":"175","DOI":"10.1016\/S0004-9514(14)60377-9","article-title":"Intra-Rater and Inter-Rater Reliability of a Weight-Bearing Lunge Measure of Ankle Dorsiflexion","volume":"44","author":"Bennell","year":"1998","journal-title":"Aust. J. Physiother."},{"key":"ref_21","doi-asserted-by":"crossref","first-page":"101","DOI":"10.1002\/(SICI)1097-0258(19980115)17:1<101::AID-SIM727>3.0.CO;2-E","article-title":"Sample Size and Optimal Designs for Reliability Studies","volume":"17","author":"Walter","year":"1998","journal-title":"Stat. Med."},{"key":"ref_22","doi-asserted-by":"crossref","first-page":"542","DOI":"10.1111\/vcp.12810","article-title":"ASVCP Guidelines: Principles of Quality Assurance and Standards for Veterinary Clinical Pathology (Version 3.0): Developed by the American Society for Veterinary Clinical Pathology\u2019s (ASVCP) Quality Assurance and Laboratory Standards (QALS) Committee","volume":"48","author":"Arnold","year":"2019","journal-title":"Vet. Clin. Pathol."},{"key":"ref_23","doi-asserted-by":"crossref","first-page":"618","DOI":"10.1016\/j.jsams.2016.11.001","article-title":"Validity of Clinical Outcome Measures to Evaluate Ankle Range of Motion during the Weight-Bearing Lunge Test","volume":"20","author":"Hall","year":"2017","journal-title":"J. Sci. Med. Sport"},{"key":"ref_24","doi-asserted-by":"crossref","first-page":"64","DOI":"10.26603\/ijspt20200064","article-title":"Within-Session Reliability for Inter-Limb Asymmetries in Ankle Dorsiflexion Range of Motion Measured During the Weight-Bearing Lunge Test","volume":"15","author":"Howe","year":"2020","journal-title":"Int. J. Sports Phys. Ther."},{"key":"ref_25","doi-asserted-by":"crossref","first-page":"155","DOI":"10.1016\/j.jcm.2016.02.012","article-title":"A Guideline of Selecting and Reporting Intraclass Correlation Coefficients for Reliability Research","volume":"15","author":"Koo","year":"2016","journal-title":"J. Chiropr. Med."},{"key":"ref_26","first-page":"231","article-title":"Quantifying test-retest reliability using the intraclass correlation coefficient and the SEM","volume":"19","author":"Weir","year":"2005","journal-title":"J. Strength Cond. Res."},{"key":"ref_27","doi-asserted-by":"crossref","first-page":"141","DOI":"10.11613\/BM.2015.015","article-title":"Understanding Bland Altman Analysis","volume":"25","author":"Giavarina","year":"2015","journal-title":"Biochem. Med."},{"key":"ref_28","doi-asserted-by":"crossref","first-page":"280","DOI":"10.3758\/s13428-025-02816-x","article-title":"Automatic Infant 2D Pose Estimation from Videos: Comparing Seven Deep Neural Network Methods","volume":"57","author":"Gama","year":"2025","journal-title":"Behav. Res."},{"key":"ref_29","doi-asserted-by":"crossref","unstructured":"Yang, S., Quan, Z., Nie, M., and Yang, W. (2021). TransPose: Keypoint Localization via Transformer. Proceedings of the 2021 IEEE\/CVF International Conference on Computer Vision (ICCV), IEEE.","DOI":"10.1109\/ICCV48922.2021.01159"},{"key":"ref_30","doi-asserted-by":"crossref","unstructured":"Xu, Y., Zhang, J., Zhang, Q., and Tao, D. (2022). ViTPose: Simple Vision Transformer Baselines for Human Pose Estimation. arXiv.","DOI":"10.52202\/068431-2795"},{"key":"ref_31","doi-asserted-by":"crossref","unstructured":"Li, Y., Zhang, S., Wang, Z., Yang, S., Yang, W., Xia, S.-T., and Zhou, E. (2021). TokenPose: Learning Keypoint Tokens for Human Pose Estimation. Proceedings of the 2021 IEEE\/CVF International Conference on Computer Vision (ICCV), IEEE.","DOI":"10.1109\/ICCV48922.2021.01112"},{"key":"ref_32","doi-asserted-by":"crossref","first-page":"776","DOI":"10.26603\/ijspt20200776","article-title":"The reliability and minimal detectable change of the ely and active knee extension tests","volume":"15","author":"Olivencia","year":"2020","journal-title":"Int. J. Sports Phys. Ther."},{"key":"ref_33","doi-asserted-by":"crossref","first-page":"20150039","DOI":"10.1515\/ijb-2015-0039","article-title":"Sample Size for Assessing Agreement between Two Methods of Measurement by Bland\u2212Altman Method","volume":"12","author":"Lu","year":"2016","journal-title":"Int. J. Biostat."}],"container-title":["Sensors"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/1424-8220\/26\/18\/5890\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2026,9,22]],"date-time":"2026-09-22T09:07:36Z","timestamp":1790068056000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/1424-8220\/26\/18\/5890"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2026,9,17]]},"references-count":33,"journal-issue":{"issue":"18","published-online":{"date-parts":[[2026,9]]}},"alternative-id":["s26185890"],"URL":"https:\/\/doi.org\/10.3390\/s26185890","relation":{},"ISSN":["1424-8220"],"issn-type":[{"value":"1424-8220","type":"electronic"}],"subject":[],"published":{"date-parts":[[2026,9,17]]}}}