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Quasi-stiffness, defined as the slope of the joint torque-angle curve, can serve as a surrogate measure of stiffness; however, it typically requires expensive 3D motion capture systems. 2D motion capture is a potential low-cost alternative for measuring quasi-stiffness in the sagittal plane; however, it is unclear if it can accurately estimate quasi-stiffness in patient populations that often exhibit out-of-plane motions. Therefore, in this study, we aimed to identify the minimal data required to accurately estimate joint quasi-stiffness. To do so, we evaluated the agreement between quasi-stiffness measurements obtained from 3D data in fifteen individuals with chronic stroke and from a simulated set of 2D data reconstructed from the 3D coordinates. Lower-extremity kinematic and kinetic data during overground walking were collected using a 3D motion capture system and an embedded force plate. To simulate 2D data, 3D maker data were projected to a simulated camera lens positioned to view sagittal motions, and medio-lateral components of the ground reaction force data were removed. Joint angles and moments at the hip, knee, and ankle were computed for both datasets using inverse dynamics, and quasi-stiffnesses of these joints were estimated during the stance phase. A linear mixed model was used to evaluate the effects of quantification method (2D, 3D) and stroke limb (paretic, non-paretic) on quasi-stiffness. Bland-Altman analyses and Intraclass correlation coefficients (ICCs) were used to evaluate the agreement between 2D and 3D measurements. The results indicated that 2D quasi-stiffness measurements were generally in agreement with the 3D quasi-stiffness measurements (\u0394: \u22120.008\u20130.007 Nm\/deg\/kg; ICC: 0.576\u20130.927 [range]), although the 2D measurements slightly overestimated quasi-stiffness for some joints. Additionally, we found that quasi-stiffness was significantly higher in the paretic limb when the ankle was plantarflexing (\u0394: 0.024 Nm\/deg\/kg) compared to the non-paretic limb. The results of this study suggest that quasi-stiffness can be validly estimated using 2D data, supporting the development of low-cost 2D systems for clinical settings to measure and monitor joint stiffness after stroke.<\/jats:p>","DOI":"10.1177\/09226028251384400","type":"journal-article","created":{"date-parts":[[2025,10,3]],"date-time":"2025-10-03T18:56:38Z","timestamp":1759517798000},"page":"170-185","update-policy":"https:\/\/doi.org\/10.1177\/sage-journals-update-policy","source":"Crossref","is-referenced-by-count":0,"title":["Determining the Validity of 2D Motion Capture for Estimating Lower Extremity Joint Quasi-Stiffness During Gait in Chronic Stroke Survivors"],"prefix":"10.1177","volume":"44","author":[{"ORCID":"https:\/\/orcid.org\/0009-0008-7969-5998","authenticated-orcid":false,"given":"Sierra A","family":"Foley","sequence":"first","affiliation":[{"name":"Wayne State University"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Andrew F","family":"Moul","sequence":"additional","affiliation":[{"name":"Wayne State University"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-7278-7389","authenticated-orcid":false,"given":"Chandramouli","family":"Krishnan","sequence":"additional","affiliation":[{"name":"University of Michigan"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-1038-0954","authenticated-orcid":false,"given":"Edward P","family":"Washabaugh","sequence":"additional","affiliation":[{"name":"Wayne State University"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"179","published-online":{"date-parts":[[2025,10,3]]},"reference":[{"key":"e_1_3_3_2_1","doi-asserted-by":"publisher","DOI":"10.3233\/NRE-2008-23304"},{"key":"e_1_3_3_3_1","doi-asserted-by":"publisher","DOI":"10.1111\/j.1747-4949.2012.00796.x"},{"key":"e_1_3_3_4_1","doi-asserted-by":"publisher","DOI":"10.1093\/ptj\/82.1.25"},{"key":"e_1_3_3_5_1","doi-asserted-by":"publisher","DOI":"10.1093\/ptj\/67.2.206"},{"key":"e_1_3_3_6_1","doi-asserted-by":"publisher","DOI":"10.1093\/ptj\/82.9.880"},{"key":"e_1_3_3_7_1","doi-asserted-by":"publisher","DOI":"10.1016\/j.gaitpost.2004.06.009"},{"key":"e_1_3_3_8_1","doi-asserted-by":"publisher","DOI":"10.1016\/j.humov.2011.02.009"},{"key":"e_1_3_3_9_1","doi-asserted-by":"publisher","DOI":"10.1016\/j.gaitpost.2023.11.008"},{"key":"e_1_3_3_10_1","doi-asserted-by":"publisher","DOI":"10.1016\/0966-6362(95)01045-9"},{"key":"e_1_3_3_11_1","doi-asserted-by":"publisher","DOI":"10.1109\/TBME.2007.901024"},{"key":"e_1_3_3_12_1","doi-asserted-by":"publisher","DOI":"10.3390\/s23094517"},{"key":"e_1_3_3_13_1","doi-asserted-by":"publisher","DOI":"10.1016\/j.jbmt.2017.11.009"},{"key":"e_1_3_3_14_1","doi-asserted-by":"publisher","DOI":"10.1016\/j.gaitpost.2008.03.001"},{"key":"e_1_3_3_15_1","doi-asserted-by":"publisher","DOI":"10.1016\/j.jbiomech.2022.111400"},{"key":"e_1_3_3_16_1","volume-title":"StatPearls","author":"Harb A.","year":"2025","unstructured":"Harb A., Margetis K., Kishner S. 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