Effectiveness of Combined Stretching and Strengthening Exercise Using Rehabilitation Exercise System with a Linear Actuator and MR Damper on Static and Dynamic Sitting Postural Balance: A Feasibility Study

: Postural imbalance induced by prolonged sitting can be improved by exercise therapy. The aim of study was to evaluate the inﬂuence of combined stretching and strengthening exercise using rehabilitation exercise system with a linear actuator and MR damper on static and dynamic sitting postural balance. Twelve subjects who sit almost 10 h a day participated in this study. The rehabilitation exercise system with a linear actuator and MR damper was manufactured to provide stretching and strengthening exercise. All subjects were asked to perform an exercise program that was designed to enhance postural balance by stretching the tight muscle and strengthening the weakened muscle. Body pressure distributions were analyzed for mean force and mean pressure using a seat sensor system. Trunk muscle activities were measured by attaching surface electrodes to the thoracic erector spinae, lumbar erector spinae, and lumbar multiﬁdus muscle. All data were divided into two regions (dominant and non-dominant side) under four conditions: no pelvic tilt, lateral pelvic tilt, anterior pelvic tilt, and posterior pelvic tilt. Body pressure distributions and trunk muscle activities were compared between before and after performing exercise under static and dynamic sitting conditions. Both in static and dynamic sitting conditions, there were signiﬁcant differences in body pressure distributions and trunk muscle activities between the DS and NDS before performing the exercise ( p < 0.01). After performing exercise, the body pressure distributions increased on the dominant side while those decreased on the non-dominant side signiﬁcantly ( p < 0.01). In addition, the activities of all trunk muscles on the non-predominant side increased signiﬁcantly ( p < 0.01 and p < 0.05). These results showed that postural balance was improved by decreasing the differences in body pressure distribution and trunk muscle activity between the dominant and non-dominant side after performing exercise. From the results of this study, we concluded that the rehabilitation exercise system with a linear actuator and MR damper is suitable for providing combined stretching and strengthening exercise, and it could be helpful to maintain correct posture by enhancing postural balance during sitting.


Introduction
The spine plays a critical role in maintaining postural stability both in static and dynamic conditions. Postural stability is correlated with the alignment of the spine [1]. Therefore, proper alignment of the spine is an important factor to perform optimal biomechanical functions for maintaining balance of body and posture with stable structural support during sitting.
In recent years, due to changes in social structure and working environment with the development of scientific technology as well as economic growth, most people spend a lot of time sitting [2]. Especially, 83% of students spend almost 10 h a day sitting [3,4]. The human body is not well adapted for prolonged periods of static sitting [5]. So, dynamic sitting is performed to improve comfort in the body by moving the trunk in the forward, backward, and lateral directions. It has been reported that prolonged periods of sitting time can lead to poor sitting postures, such as forward leaning sitting, slumped sitting, slouched sitting, and cross-leg sitting posture [6]. It was discovered that sitting with poor posture can contribute to increased muscle fatigue, stress on ligaments, and intervertebral discs in previous research [7]. It was discussed that postural imbalance caused by prolonged sitting may affect musculoskeletal discomfort of the neck, shoulder, upper and lower back [8]. The activities of neck and trunk muscle were also increased in habitual bad sitting posture [9,10]. In addition, perceived body discomfort in relation to prolonged static posture has been shown to be associated with the formation of spinal deformities in the frontal, sagittal, and transverse plane [11].
Good sitting posture is very important to keep spine health and quality of life [1]. Previous research suggests various types of devices for maintaining ideal sitting posture with slight curvature, such as kyphosis and lordosis, in the thoracic and lumbar regions that can help reduce trunk muscle fatigue. This curvature was more improved when using a developed new type of stool than when using a standard stool during dynamic sitting [12]. It was discovered that a smart chair seat cover based on pressure sensors and IoT platform could be utilized to provide sitting patterns and prevent postural asymmetry caused by prolonged sitting [2]. In previous research, a sitting posture monitoring system with a textile pressure sensor was presented to detect different sitting postures by analyzing the pressure changes in the surface of the back, hips, and thighs of a chair [13]. Additionally, it was emphasized that a sitting monitoring system with minimum number of sensors is necessary to consider high cost, commercialization, and utilization in various work environments [14]. Previous research has been reported that sitting balance is mainly maintained by the trunk muscles. Thus, in order to expect a continuous corrective effect, it is essential to consider in improving asymmetrical trunk muscles and postural balance caused by incorrect posture [15].
Spinal deformities related to poor postures have affected postural imbalance and unequal loads in the trunk muscles [16]. It was confirmed that postural asymmetry and muscle imbalance could be improved by physical exercises based on stretching and strengthening exercises [17]. Stretching exercises are an effective method to improve posture by relieving muscle tension and increasing the range of motion that can be helpful to perform performance in physical activities. It was identified that regular stretching exercises can decrease the pain related to the musculoskeletal discomfort [18]. Strengthening exercises have been applied to realign postural deviations and strengthen the weak muscles for correcting spinal deformities such as scoliosis, kyphosis, and lordosis [19]. Despite the fact that stretching or strengthening exercise can provide a positive effect on improving postural correction, it was proposed that a combination of stretching and strengthening exercises with axial elongation has been shown to be more effective for postural balance [20,21]. However, there was no rehabilitation exercise device that was manufactured simultaneously for both relaxing contracted muscles and strengthening weakened muscles until now. A linear actuator can be used to generate linear motion for stretching exercise [16]. In addition, magnetorheological (MR) fluid dampers, which is one of the variable resistance dampers, are often used to control effectively resistive force in real-time [22]. We hypothesized that combined stretching and strengthening exercises using a linear actuator and MR damper could be very effective in improving static and dynamic postural balance.
Consequently, this study aimed to develop a new type of rehabilitation exercise system using a linear actuator and MR damper for stretching and strengthening exercises. To evaluate the influence of combined stretching and strengthening exercise using the developed system on postural balance in static and dynamic sitting, the body pressure distributions and trunk muscle activities between before and after performing exercise were compared in this study.

Subjects
In this study, twelve subjects (age 24.1 ± 2.5 years, height 170.3 ± 7.3 cm, body weight 62.5 ± 10.2 kg and body max index 20.2 ± 1.9 kg/m 2 , Mean ± SD) who sit almost 10 h a day were recruited from the Jeonbuk National University in Jeonju, Republic of Korea. All subjects gave written informed consent with respect to the experimental protocol which was approved by the Institutional Review Board of Jeonbuk National University (IRB File No. JBNU 2019-04-005). Subjects presenting with back pain, degenerative disc disease, scoliosis, and injuries in the musculoskeletal system were excluded from the study, because continuous stretching and muscle strengthening exercise performed on the developed device may put strain on the trunk and pelvis.

Rehabilitation Exercise System
The rehabilitation exercise system was designed to provide stretching and strengthening exercise with linear actuator and MR damper. The rehabilitation exercise system is presented in Figure 1. It consists of seven components: upper extremity lift part, trunk lift part, trunk shift part, trunk rotation part, MR damper part, trunk support part, and foot support part. A motor drive-type linear actuator (GoMotorWorld, Jiaxing, Zhejiang, China) was utilized for stretching exercise. To facilitate dynamic trunk motions, including rotation, elongation, and flexion of the trunk segments, five electric actuators were installed on the trunk lift part, upper extremity lift part, trunk rotation part, and trunk shift part. The trunk lift part allows the elongation of the spine by lifting the frame of the device. For realignment of the trunk segments, stronger stimulation to the spine than previous self-elongation can be achieved by elevating the trunk lift part. The upper extremity lift part can provide the flexion of the spine by controlling a different height on both sides, respectively. Active rotation of the trunk segments can be controlled utilizing the trunk rotation part. The trunk shift part can adjust the biased body balance in the anterior and posterior side. The trunk support part can be used to fix the trunk (thoracic and lumbar spine region) while performing unilateral exercises for strengthening the weak side. A foot height control part was used to adjust the knee angle at a 90 • angle for preventing the effect of leg movements during exercises. The MR damper control part consists of three RD-2087-01 rotary damper (Lord Corporation, Cary, NC, USA) and an auxiliary spring for stability during sitting.
As shown in Figure 2, for stretching exercise, all actuators of the system were controlled by using a control panel. To provide strengthening exercise, the torque value of the MR damper was regulated in four current input levels (0.4 A, 0.8 A, 1.2 A and 1.5 A) by using a MR damper controller.
Exercise program focused on reducing the difference in body pressure distribution and trunk muscle activity on both sides, which causes postural imbalance. Accordingly, it was designed to enhance postural balance by stretching the tight muscle and strengthening the weakened muscle using linear actuator and MR damper. It comprised a combination of static stretching, trunk elongation, pelvic tilt exercise, trunk elongation with pelvic tilt exercise, and trunk elongation and rotation with pelvic tilt exercise as follows:

Experimental Procedure
The Pliance seat pressure mat (Novel GmbH., Munich, Germany), comprised of flexible capacitive sensors with a sensing area of 9.53 mm based on diameter, was utilized to measure body pressure distribution pattern during sitting on a stool, at sampling frequency of 100 Hz. Trunk muscle activity was also assessed using Noraxson Desktop Direct Transmission System (Noraxson Inc., Scottsdale, AZ, USA), with a sampling rate of 1500 Hz. This system consists of wireless surface EMG pinch lead, EMG sensors, EMG probes, desktop USB receiver. Surface electrodes were attached to the bilateral multifidus (MF) and erector spinae in the thoracic (EST) and lumbar spine (ESL) regions. To reduce skin impedance, the skin was shaved at the appropriate area and cleaned with alcohol.
Initially, the measurement of body pressure distribution and muscle activity was processed under two conditions (static and dynamic sitting condition) for investigating the biomechanical characteristics of subjects. As shown in Figure 3, in static sitting condition, subjects were asked to sit comfortably with their arms crossed for 30 s. In dynamic sitting condition, subjects were instructed to conduct lateral (left and right), anterior, and posterior pelvic tilt, with the same posture in static condition, using an unstable board whose shape and appearance is hemisphere. To prevent fatigue, subjects were given a 5 min break between experiments. Considering the results of body pressure distribution and trunk muscle activity, an exercise program was suggested by a physical therapist. All subjects were asked to perform 1 h exercise program per day (three days of the week) and it lasted for four weeks. The resistance of the MR damper was gradually increased each week because the effect of exercise may decrease if the same intensity of exercise continues. After four weeks of exercise, body pressure distribution and muscle activity in static and dynamic conditions were assessed as in the initial measurement methods.

Data Analysis
To evaluate the effect of the combined exercises using the developed system, the body pressure distributions and trunk muscle activities were analyzed between before (BF) and after (AF) exercise. Body pressure distributions were analyzed for mean force and mean pressure using Novel software (Novel GmbH., Munich, Germany). The MyoResearch 3.6 software (Noraxson Inc., Scottsdale, AZ, USA) was used to analyze trunk muscle activation. Electromyographic (EMG) data were rectified and normalized using the maximum voluntary contraction method. All data were divided into two regions such as dominant side (DS) and non-dominant side (NDS) under four conditions: no pelvic tilt (NPT), lateral pelvic tilt (LPT), anterior pelvic tilt (APT), and posterior pelvic tilt (PPT).
Statistical analysis was performed using SPSS software version 20.0 (IBM Corporation, Armonk, NY, USA). Shapiro-Wilk test was utilized to assess the normality of all variables. An independent sample t-test was used to analyze the differences in body pressure distribution and trunk muscle activity between DS and NDS. Changes in measured variables in experimental results between periods were assessed using paired t-test. Statistical significance was set at 0.05 and 0.01.

Body Pressure Distribution
Differences in body pressure distributions between the DS and NDS are shown in Table 1.
In NPT, there were significant differences in mean force and mean pressure between the DS and NDS before performing the exercise (p < 0.01). After performing the exercise, the mean force more decreased by 15.9% and 5.9% than before both for the DS and NDS, respectively. However, a significant difference in mean force was only shown on the DS (p < 0.01). The mean pressure on the DS also decreased by 5.4% while that increased on the NDS by 4.0%. There were no significant differences in mean pressure between before and after performing the exercise both for the DS and NDS. On the other hand, the significant differences in mean force and mean pressure between the DS and NDS before exercise were all shown in LPT, APT, and PPT (p < 0.01). The overall tendency in the mean forces and mean pressures increased on the DS while those decreased on the NDS after performing the exercise. In LPT, APT, and PPT, the mean forces on the DS all decreased significantly by 10.7%, 9.2%, and 8.0%, respectively (p < 0.01). On the contrary, the mean forces on the NDS all increased significantly by 13.7%, 18.6%, and 13.6%, respectively (p < 0.01). Similar to the results in the mean force, after performing the exercise, the mean pressures increased on the DS while that decreased on the NDS in LPT, APT, and PPT. On the DS, the mean pressures all decreased by 6.4%, 4.5%, and 3.5%, respectively. In contrast, on the NDS, the mean pressures all increased by 20.0%, 25.4%, and 19.7%, respectively. However, a significant difference in mean pressure was only shown on the NDS between before and after performing the exercise (p < 0.01). Table 1. Differences in body pressure distribution 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), and posterior pelvic tilt (PPT).

Trunk Muscle Activity
Differences in trunk muscle activities between the DS and NDS are presented in Figure 4. In NPT, there were significant differences in the activities of EST, ESL, and MF muscle between the DS and NDS before performing the exercise (p < 0.05 and p < 0.01). After performing the exercise, the activity of EST, ESL, and MF muscle on the DS all decreased by 8.7%, 11.5%, and 4.7% while that on the NDS all increased by 9.0%, 12.3%, 26.3%, respectively. However, there were no significant differences in muscle activities between before and after performing the exercise both for the DS and NDS. In dynamic sitting conditions, the overall tendency in the activity of all muscles was increased both for the DS and NDS after performing the exercise. Especially, the activities of all muscles increased more on the NDS than the DS. In LPT, there were significant differences in the activities of EST and ESL muscle except MF muscle between the DS and NDS before performing the exercise (p < 0.05 and p < 0.01). After performing the exercise, on the DS, the activity of EST, ESL, and MF muscle all increased by 32.7%, 21.6%, and 17.9%, respectively. However, a significant difference was only shown in the activity of EST muscle (p < 0.01). In contrast, on the NDS, the activities of all muscles increased significantly by 86.5%, 67.3%, 33.2%, respectively (p < 0.01 and p < 0.05). In APT, the significant differences in muscle activity between the DS and NDS were shown in all muscles before performing the exercise (p < 0.05 and p < 0.01). After performing the exercise, on the DS, the activity of EST and MF muscle increased by 12.3% and 7.7% while the activity of ESL muscle decreased by 0.5%. There were no significant differences in the muscle activity of all muscles between before and after performing the exercise. On the contrast, on the NDS, the activities of all muscles increased significantly by 54.3%, 50.6%, and 46.7%, respectively (p < 0.01 and p < 0.05). In PPT, before performing the exercise, the significant differences in muscle activity between the DS and NDS were shown in all muscles (p < 0.05 and p < 0.01). After performing the exercise, on the DS, the activity of EST, ESL, and MF muscle all increased by 20.3%, 25.0%, and 15.9%, respectively. The significant difference was shown in the activity of EST and ESL muscle except MF muscle (p < 0.05 and p < 0.01). In contrast, on the NDS, the activities of all muscles increased significantly by 62.5%, 92.1%, and 48.5%, respectively (p < 0.01).

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 gluteal 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-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. The erector spinae and multifidus muscles are related to various movements such as rotation, extension, and flexion of the trunk [30]. Commonly, the activity of the extensor muscles is more required in the upright sitting posture than the slumped posture. Previous studies founded that altered trunk muscle activations can be affected by the postural changes in sitting [31]. It was also reported that the muscle activities of the multifidus and thoracic erector spinae muscles were all reduced during slump sitting, when compared with upright sitting [32]. In the forward learning sitting posture, the multifidus muscle activity increased more than the upright and slumped sitting posture [9]. In contrast, some authors demonstrated that there are no significant differences in the activity of trunk muscles between various sitting postures while the lumbar flexion angle was affected by postural changes [33].
With respect to the activities of EST, ESL, and MF muscle, before performing the exercise, the significant differences in muscle activity between the DS and NDS in NPT, LPT, APT, and PPT were presented. Results of muscle activity in static sitting condition were similar to previous research reported asymmetrical EMG activity pattern of the paraspinal muscles in patients with spinal deformity [34]. It was revealed that imbalanced contralateral trunk muscle activity induced by bad postures may cause spinal deformity and eventual pain. Moreover, decreased flexibility, weak muscle activation, and poor muscle endurance during the extension movements has been related to low back pain [35]. Particularly, there are significant differences in the activities of all muscles in dynamic sitting conditions. Previous research reported that improper muscle activation pattern of the erector spinae and multifidus muscles during anterior and posterior pelvic tilting may be associated with sagittal misalignment. Because these muscles are involved in controlling the movement in the sagittal plane [36]. Similar to the results in LPT, the asymmetrical muscle activities were also displayed in patients with spinal deformity when conducting the lateral bending motions [34]. These alterations which may be correlated with the thoracic and lumbar spinal instability could negatively affect the muscles for generating force to properly stabilize the trunk. A shortened erector spinae and lumbar multifidus muscle leads to a reduced range of trunk motion which are associated with muscle stiffness results from an increase in tension. In addition, the loss of muscle strength in the trunk can be a major cause of musculoskeletal disorders [37].
In this study, it was hypothesized that stretching the tight muscle and strengthening the weakened muscle is essential for improving muscle imbalance and correcting postural balance. To provide efficiently the eccentric and concentric exercise for shortened and elongated muscles, respectively, we developed a new type of rehabilitation exercise system based on linear actuator and MR damper. The exercise was mainly focused on relaxing the shortened muscle and strengthening the elongated muscle using the structure of the developed system [16]. It was argued that eccentric contraction can be more helpful to increase flexibility by elongating the muscles than previous static stretching [38]. Therefore, the linear actuator was used to stretch the stiff muscles by gradually increasing the flexibility through active contraction. It seems that the eccentric contraction method using a linear actuator may be effective to elongate the contracted muscles because the muscle activity on the dominant side in the NPT decreased after performing exercise. In addition, this change in trunk muscle activity on the dominant side may be related to the result of a decrease in the differences in body pressure on both sides, which was distributed asymmetrically. Muscle strength is important in maintaining good posture and improving muscle balance during sitting. However, habitual improper postures induced by prolonged sitting leads to weakness of trunk extensor muscles. The target muscles for strengthening exercise were the erector spinae and multifidus muscles. Resistance exercise with strengthening devices including elastic bands and sand bag weights for increasing muscle strength has positively influenced the dynamic balance functions. However, there is a limitation in that it is difficult to confirm whether the gradual resistance is precisely provided when exercising with previous devices [39]. In this study, the MR damper was utilized to provide a precise and stable resistive force which can be controlled by varying the electric voltage [27]. The results show that the muscle activities were all increased both for the dominant and non-dominant side in LPT, APT, and PPT. In particular, trunk muscle imbalance was improved as the muscle activity in the non-dominant side increased significantly after performing exercise with MR damper. Significant increase in trunk muscle activity on the non-dominant side could be considered as a result of strengthening the weakened muscles. Further, altered muscle activation patterns on the dominant side may be a result of the necessity to stabilize the trunk by adopting positively muscle function alternation. Especially, the significant increase in the activities of thoracic erector spinae muscle on the dominant side can have a positive effect on improving posture by strengthening the weakened muscles caused by improper postures such as slouched thoracic posture [40]. These results were also correlated with body pressure distribution which was increased on the dominant side while that decreased on the non-dominant side. Reduced asymmetrical differences in muscle activity and body pressure distribution on both sides indicate that combined exercise using a developed system could help to enhance postural balance ability and correct sitting posture.
There are few limitations with this study. First of all, the sample size is relatively small. A future prospective study should be conducted with a larger sample size and over a longer period of time. Furthermore, no comparative analysis of effectiveness with conventional exercises was performed.

Conclusions
In this study, the rehabilitation exercise system was developed for combined stretching and strengthening exercise. After performing exercise, the differences in body pressure distribution and trunk muscle activity on both sides were all decreased as the weakened muscle was strengthened while the contracted muscle was relived. It means that this system may have a good potential to improve the sitting postural balance. In addition, it suggests that combined exercise using the developed system could provide an effective method for correcting postural asymmetry and muscular imbalance. Further research is needed to evaluate the efficacy of exercises with the developed system either over the long term or in patients with musculoskeletal problems.