## 4 Discussion  
With respect to body pressure distribution, the results showed that the mean forces and mean pressures were all increased significantly on the DS than the NDS both in static and dynamic sitting conditions. It is consistent with the previous study which proposed an asymmetrical pattern of glutel pressure of patients with low back pain may be associated with poor sitting posture caused by prolonged sitting [9]. It has been also reported that improper sitting posture over long periods is one of the major causes of low back pain [23; 24]. Accordingly, it is necessary to develop a system that can help prevent habitual pad postures and posture-related musculoskeletal disease by improving postural balance caused by sitting for a long time. Previous research confirmed that exercise programs performed while sitting are effective therapeutic methods for decreasing the prevalence of LBP. In addition, it was confirmed that dynamic sitting exercises could improve lumbar mobility and stability [25]. Therefore, we hypothesized that combined stretching and strengthening exercise may have a positive influence on postural balance during sitting. In this study, the developed system was designed to provide trunk elongation, pelvic tilt exercise, trunk elongation with pelvic tilt exercise, and trunk elongation and rotation with pelvic tilt exercise by using linear actuator and MR damper. Especially, stronger stimulation than previous self-elongation can be provided to the spine by elevating and rotating the structure of the system. After performing the exercise, the differences in body pressure distribution between the DS and NDS were all reduced both in static and dynamic sitting conditions. This suggests that stretching exercise using a linear actuator may be more effective than previous exercise methods in relieving muscle tension and increasing trunk flexibility to improve postural balance [26]. Because continuous stimulation can be pro  
Figure 4: Differences in electromyographic (EMG) activities on the dominant side (DS) and non-dominant side (NDS) between before (BF) and after (AF) performing the exercise under four conditions: no pelvic tilt (NPT), lateral pelvic tilt (LPT), anterior pelvic tilt (APT), posterior pelvic tilt (PPT). vided through a linear actuator while it is difficult to maintain relaxation with conventional self-movement. Additionally, it has been also founded that there are some limitations with the accuracy and durability of resistance when using typical strengthening devices such as elastic bands [27]. The MR fluid dampers are widely applied for resistance training for patients. Therefore, we utilized a MR damper to provide accurate and constant resistance to the trunk muscles for correcting postural deviations caused by prolonged sitting [19]. From the results that asymmetrical differences on both sides were reduced as body pressure distribution on the NDS were increased significantly, we confirmed progressive resistance exercise using MR damper could help to correct body alignment and enhance postural stability by strengthening gradually the weak side. Maintaining adequate balance is fundamental for performing various activities of daily living. The trunk, which is the central part of the human body, plays a critical role in controlling body posture and providing musculoskeletal stability for functional movements [28]. It was proposed that the trunk muscle can be classified into global (thoracic and lumbar erector spinae) and local (multifidus) muscles. The thoracic erector spinae muscle, which is also called mobilizer muscle, is basically activated to perform movements. The lumbar erector spinae muscle contributes to maintain the erect posture by stabilizing the joint position. The multifidus muscle is activated to stabilize the trunk in the anterior and posterior side [29]. The active muscle contraction is one of the major factors for attributing to the stability of the trunk.