Determining the Validity of 2D Motion Capture for Estimating Lower Extremity Joint Quasi-Stiffness During Gait in Chronic Stroke Survivors

Altered joint stiffness is common after stroke, yet clinically feasible tools to objectively quantify joint stiffness during walking are lacking. 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 (Δ: -0.008-0.007 Nm/deg/kg; ICC: 0.576-0.927 [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 (Δ: 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.