## 1 Introduction  
Osteoarthritis (OA) is a degenerative disease that is comprised of biological, structural, and biomechanical components [1, 2, 3, 4]. Individuals with OA may suffer joint deformities, pain, and muscle weakness [3]. As the severity of OA increases, proprioceptive accuracy, i.e., the accuracy of the sense of position and movement of joints or extremity in space, decreases. Consequently the joint stabilization is compromised [5]. Poor proprioception is often associated with poor muscle strength and functional limitation [6, 7]. Consequently, poor proprioception, joint deformity, and muscle weakness are among the contributing factors of falling in individuals with OA [7, 8, 9, 10, 11]. Thus, eventually, OA may lead to physical impairment and functional limitations, e.g., gait abnormality [2, 3, 4]. Several gait parameters have been found to be predictive of falls [12]. For example, asymmetry of the double support duration was a strong predictor of repeated falls in elderly women [13]. Abnormality of the spatiotemporal gait parameters such as gait velocity [14, 15], swing [15], and stance durations, in addition to step length [14] and toe clearance [15; 16] were correlated with the incidence of falls. Another factor that may affect the outcome of tripping is the strategy of recovery following perturbation [17]. However, this possible factor was not thoroughly explored in individuals with OA, specifically those who underwent surgical treatment.  
Total knee replacement (TKR) is often required in advanced OA when there is a significant decrease in functionality and after conservative interventions have failed. The prevalence of TKR surgery increases every year [18]. TKR reduces pain, improves functionality and quality of life [19], increases gait velocity [20], and range of motion (ROM) [21]. While these are encouraging outcomes of the surgery, individuals that undergo TKR remain at a high risk of falls [10]. Previous literature showed differences in the gait parameters between individuals following TKR and non-OA groups [22]. In addition, gait biomechanics were compared between individuals with OA before TKR and OA groups that were not scheduled for TKR [23; 24]. Unfortunately, there are no studies that objectively quantified the post-surgery biomechanical effects over a long duration after TKR compared to individuals with OA who were not scheduled for TKR. In addition, most studies concerning fall strategies of individuals with knee OA were limited to perturbations applied during standing [25] or while stepping onto a translating platform [26]. To the best of our knowledge, no studies explored the recovery from a perturbation in individuals with knee OA (following TKR or not following TKR) while walking on a paved path. The gait stability, characterized by the ability to keep functional locomotion following a perturbation or contact with an obstacle [27], depends on the reactive recovery responses [28], e.g., compensatory stepping or counter rotation, in order to keep the projection of the center of gravity inside the base of support [29; 30]. Therefore, the response of the first step following gait perturbation is meaningful and is utilized for the destabilizing effects after loss of balance [17; 23; 28]. In individuals with OA, the response to perturbation might be abnormal due to muscle weakness [31; 32], pain [31], joint rigidity, decreased ROM [33], or decreased proprioception [34].  
As insight into the changes of biomechanics following gait perturbation of individuals following TKR over a long duration may shed light on the existing factors that contribute to the risk of falls in those populations, we aimed to compare the gait characteristics during unperturbed and perturbed gait in individuals with OA who did or did not undergo TKR one-year post a baseline evaluation.