## 4 Discussion  
This study is the first to evaluate the postoperative in vivo kinematics, kinetics, and muscle activity of a modern implant that mimics normal knee function. The most important finding of this study was that patients with modern implants had smoother knee functions. This study revealed the following three points. First, regarding kinematics, Group I, using the modern implant, tended to show a shorter standing-up time and faster knee-extension angular velocity compared to those of Group II, who used the conventional implant. Second, regarding kinetics, Group I showed a lower hip-extension moment and vGRF, and a significant difference was observed for the two items. Third, Group I tended to exhibit lower activity in the quadriceps femoris muscle. The STS movement is a repetitive action performed in daily life, and the quality of life decreases when this motion is impaired. Reports have shown that there is still room for improvement in rising and squatting motions in patients after TKA [14; 15]. Various factors, such as pain, muscle strength, and original disease, affect the STS movement; however, knee extension muscle strength is known to be a vital factor [39]. Previous studies on patients with conventional TKA have shown a decrease in knee-extension range of motion and knee-extension angular velocity, compared with that of healthy individuals [44; 46]. In the present study, there was no significant difference in the kinematic data between the two groups; however, in Group I, the STS time was shorter and the knee-extension angular velocity was higher, indicating the possibility that the STS movement was smoother in Group I. Regarding the kinetic data, studies have shown that the maximum knee-extension moment decreases and hip-extension moment increases in patients with TKA compared to those in healthy individuals [45; 47]. Furthermore, it was found that the vGRF increases in the non-operated limb compared to the operated limb. Although no difference was observed in the knee-joint-extension moment in this study, the hip-extension moment was greater in Group II than in Group I. It has been found that patients who underwent TKA compensate by extending the hip joint during STS [45]. The STS movement was achieved by effectively using the knee joint, as the vGRF was smaller in Group I (similar to that observed in healthy individuals) compared to that in Group II. Studies have shown that the activity of the quadriceps muscle, which is involved in the knee extension mechanism, is lower on the operated side than on the non-operated side in patients with conventional TKA [45; 47; 48]. This is believed to result from STS without a load on the operating side. In contrast, a study comparing the muscle activity with different implant types showed that single-radius TKA, which uses a larger knee-extension lever arm, resulted in less quadricp muscle activity than the multi-radius type [49]. In Group I, there was a tendency for decreased quadricp muscle activity, which may be attributed to load issues similar to those reported in the study comparing different types of implants. This study demonstrated that journey II reproduces the normal anteroposterior position in terms of implant shape compared to conventional implants, implying that the lever arm in the extension mechanism is enlarged and the extensor muscle is activated effectively [50]. Overall, considering the kinematics, kinetics, and muscle-activity results, it can be suggested that Group I patients were able to perform the STS movement more smoothly and effectively using the quadriceps muscles compared to those with conventional implants. This study has several limitations. First, although there was no significant difference between the two groups, there is a possibility of selection bias, owing to the lack of uniformity in the preoperative patient's condition and the status of the opposite knee.