## 4 Discussion  
In post-total-hip-arthroplasty patients, the weight bearing on the lower limb during the STS motion is asymmetric because it is biased toward the healthy lower limb. In post-stroke patients, the center of foot pressure during the STS movement is asymmetric because it is biased toward the nonparalyzed side. Thus, previous studies have shown that muscle torque and joint angle changes exerted in the gait and during the STS movement are asymmetric due to disability or muscle weakness in one lower limb [7, 19, 20]. It was assumed that the rehabilitation patient group in this study would also have asymmetric lower limb joint angle changes during the STS motion due to disability and muscle weakness. In patients with post-stroke syndrome, the muscle synergy of the R-L lower limbs is reportedly related to the asymmetry of the lower limb joint angle changes during the STS motion [21]. It is already known that the muscle synergy in such patients is altered  
Figure 7: R–L vector line in the rehabilitation patient group. The lower limb movement pattern is symmetrical enough to be drawn near the central symmetry reference line. P-1 through P-4 in the figure refer to each rehabilitated patient in Table 1.  
compared with normal muscle synergy [22; 23], and it was inferred that the patients in this study also had altered motor synergy of the unilateral lower limb due to disability and muscle weakness, resulting in asymmetric R-L lower limb movement during the STS motion. Therefore, we hypothesized that it is important to bring the synergy of the impaired lower limb of the patient closer to the normal state for the patient to recover the STS ability [21] and that improving the R-L lower limb coordination during the STS movement means that the muscle or motor synergy of the impaired lower limb used to perform the STS movement is closer to the synergy expressed by the healthy lower limb. We hypothesized that this would be the case. For this reason, we thought that a method for evaluating the coordination of R-L lower limbs during the STS movement was necessary and sought a method for evaluating this coordination from the perspective of both EAs and the R-L vector line in this study. The R-L differences in shank and thigh EAs were also observed in the healthy participants. The reason for the observed differences in EAs in the healthy participants, despite the absence of a functional impairment, is presumably the habitual use of the dominant leg. Although there is no difference in muscle strength between the dominant and non-dominant leg [24; 25], the dominant leg has priority in postural control [26]. In addition, placing one leg posteriorly at the start of the STS movement can reduce the external tension moment of the hip joint [27], which may have caused the difference in EAs in the healthy group. In the rehabilitation patients, both EA values and temporal-spatial differences were observed. These temporal-spatial differences were visually complex and difficult to analyze. Therefore, we performed an analysis using R-L vector lines. The PLG is a geometric method used to identify the cooperative structure of lower limb movement patterns in the gait [15; 16]. However, the PLG represents the coordinated motion of one lower limb and it cannot represent the coordinated movement of the R-L lower limbs. Therefore, in this paper, we proposed a method that presents information on the changes in the EA of the R-L thighs and tibia as a single line (the R-L vector line). This method suggests that the closer the drawn R-L vector line is to the symmetry reference line, the more parallel it is to the target reference line, and the higher is the degree of synchronization of the EA and the change per time point between the R-L segments. In other words, the symmetry of the movement pattern can be evaluated as high. The STS movement has a muscle coordination structure that is reportedly similar to that of walking [28; 29], and the movements of the R-L lower limbs during the STS motion are considered to be the result of coordination.