## 2 Materials and Methods
### 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 lift part, trunk width 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 deg 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:  
* General warm up (5 min): static stretching exercises of chest and back muscles. * Trunk elongation (1 min, 5 reps): subjects were asked to stretch the trunk as much as possible by elevating the upper extremity lift part, and then keep the maximum position for 10 s.
* Pelvic tilt exercise to the lateral direction (1 min, 3 reps), pelvic tilt exercise to the lateral and anterior direction (1 min, 3 reps), and pelvic tilt exercise to the lateral and posterior direction (1 min, 3 reps): for improving strength and flexibility of the spine, subjects were asked to move slowly until they reached the point of maximum inclination and to keep maximum position for 10 s. Then, subjects moved back slowly to the start point. * Trunk elongation with pelvic tilt exercise to the lateral direction (2 min, 3 reps), trunk elongation with pelvic tilt exercise to the lateral and anterior direction (2 min, 3 reps), and trunk elongation with pelvic tilt exercise to the lateral and posterior direction (2 min, 3 reps): subjects performed lateral pelvic tilt exercise while maintaining trunk elongation.