## 4 Discussion  
We compared the gait patterns during a unperturbed and perturbed gait in individuals with OA who did or did not undergo TKR one year post a baseline evaluation. This is the first report of gait characteristics and perturbation change in the recovery strategy of subjects with OA (who did or did not undergo TKR) over one year. Our main findings show a decrease in pain levels and Oxford scores in most subjects following TKR, coupled with increased gait velocity and reduced pelvic tilt, as well as a different perturbation recovery strategy compared to the NTKR group. Although there were no between-group differences in the change in VAS and Oxford scores, when considering the MCID of the Oxford score and the VAS pain score of the OA limb, it is evident that the majority of TKR subjects showed improvement in these scores, while the majority of NTKR subjects did not show improvement. These findings imply that the surgery was successful in reducing the pain and disability levels, and thereby may improve the functionality of the subjects. As for the ABC and TUG, approximately half of the subjects in both groups had a reduced TUG score by more than the MDC and had an ABC score lower than the cut-off for risk of falling. These results may indicate that following one year, the fear and risk of falling were similar between individuals with knee OA who did or did not undergo surgery. Thus, although the gait parameters following TKR are improved, they might not predict the risk of falls, which still occur following TKR [10]. This might be attributed to worsening in knee proprioception [41], balance deficiency [31], or residual characteristics of pre-surgery gait [42]. A systemic review argued that gait velocity correlates with functional abilities in adults with mobility deficiencies [43]. In our study, most of the TKR subjects improved their gait velocity by at least 0.1 m/sec compared to less than a half in the NTKR group. It is therefore not surprising that most TKR subjects improved their Oxford scores, while only 10% of the NTKR subjects showed improvement. Another association of the gait velocity concerns joint kinematics. Several studies, e.g., [44], reported correlation between decreased velocity and a decrease in knee/hip flexion angles. Most of the TKR subjects increased the ROM of the OA knee during unperturbed gait. A pathology of one knee may affect the entire dynamics of all joints in both lower limbs [45]. For example, walking with a stiff knee results in decreased ROM in the CL limb [46]. In our study, most of the TKR subjects increased the ROM of their OA knee following the surgery, and in most subjects in this group, the ROM of the CL hip and the posterior pelvic tilt were decreased. The aforementioned effect found in the TKR group is not apparent in the NTKR group. The greater increase in ROM of the OA knee in the TKR group might be attributed to the reduction in pain following the surgery [47]. The lack of pain reduction in most NTKR subjects might also explain the greater reduction in swing duration in the CL limb one year post baseline, which might imply that individuals in the NTKR group increased their avoidance of weight-bearing on their OA limb. We previously reported [23] that the gait ROM of the OA limb was significantly lower in individuals scheduled for TKR compared to individuals not-scheduled for TKR. In addition, the ROM of the OA limb of both OA groups was lower than the ROM of the knees of healthy controls [23]. The findings of the current study therefore imply that the ROM of the OA knee of the TKR subjects was more similar to healthy controls [23] one year post-surgery. These changes seen in the TKR