# Development of New Soft Wearable Balance Exercise Device Using Pneumatic Gel Muscles
## 4 Discussion  
The purpose of the current study was to design a new wearable assistive device for improving balance function by generating small perturbations. To achieve this purpose, the proposed device was required to possess various features, such as a lightweight design and the ability to generate small perturbations with a small power source, enabling users to exercise easily in a range of settings. Furthermore, we investigated the training effects of using this device to improve balance function. We successfully designed a new soft wearable balance exercise device. The total weight of the proposed device was 0.9 kg, and it took less than 3 min for a beginner to attach the device. The proposed device was able to generate small perturbations without a large power source using PGMs and a small CO\({}_{2}\) tank. In addition, therapists and examiners are able to present balance exercise easily using the proposed device, because the direction of perturbation can be easily manipulated using the controller. The p-Ax and p-Ay varied significantly compared with b-Ax and b-Ay. In the reactive postural control assessment session, the results revealed that the proposed device generated small perturbations on the dominant leg side and in the inferior direction, because the lateral side PGMs caused bending of the participants' trunk on the dominant leg side. In addition, trunk bending caused COM movement in the mediolateral direction. Thus, r-Ax also varied significantly compared with b-A. Regarding the time of perturbation and reaction, Tr-Ax, Ay, and Az significantly increased compared with Tp-Ax, Ay, and Az, respectively. All directions of variation from Tp-A to Tr-A were similar (average time was 133.3 ms). Previous studies reported that postural control responses against unexpected perturbations occurred between +100 ms and +400 ms from the occurrence of perturbation [29; 30], and the timing range depended on the outcome. Muscle activity responses against unexpected perturbation have been reported to occur very early, at approximately 60 to 100 ms after perturbation [32]. The kinematic and kinetic changes against unexpected perturbation have been reported to be delayed from muscle activity responses at approximately 50 to 80 ms because of electromechanical delay [29; 33]. Another previous study reported that maximum angular displacement due to perturbation appeared from 100 to 400 ms after perturbation [30]. These time-windows were similar to those observed in the current results using acceleration data. Considering the factors discussed above and the findings of previous studies, the proposed wearable device appears to be able to create small perturbations that are appropriate for assessing reactive postural control. In contrast, Tp-Ax, Ay, and Az occurred more than 100 ms after the moment of pushing the PGM contraction button. The PGMs used in our study took approximately 150 ms to achieve maximum contraction force. Therefore, it appears that Tp-A was likely to have been greater than 100 ms for the reason discussed above. The Ax RMS had a higher degree of non-stationarity than Ay and Az. This result was seemingly affected by the measurement task because the ML direction base of the support decrease in a single leg stance condition. ML velocity and displacement tend to increase compared to AP in a single leg stance [34; 35]. Our results were consistent with these previous studies. The Ax RMS in the post-test session was significantly decreased compared with the Ax RMS in the pre-test session. Although the exercise duration was not long, the effect size was large. RMS of the acceleration was used as the main parameter of the balance function during the balance exercise, such as single leg stance and double leg stance. Previous studies reported that older individuals with greater instability, and peripheral neuropathy patients, exhibited increased RMS values [36; 37].