## 4 Discussion
### Body-Slip and Buttock Pressure Changes during Wheelchair Repositioning  
Our study showed that both BS and SS showed a tendency to increase during repositioning, and the difference between the BS with double-sliding (1.5 mm) and that with non-sliding (18.5 mm) was 17 mm, indicating a significant increase in non-sliding. The body shifted downward during reclining and moved upward again through repositioning. At this time, the acromion and greater trochanter markers shifted upward and backward, respectively, resulting in increased BS and SS. Similar to reclining, the BS of non-sliding (18.5 mm) was significantly increased when repositioning compared to that of double-sliding (1.5 mm), and it was observed that backrest slip barely occurred in double-sliding. There was no significant difference between the two conditions for SS, and the TBS values were 30 mm and 40.8 mm with double-sliding and non-sliding, respectively, confirming that the overall body-slip was significantly increased in non-sliding. In [PERSON] et al. 's study [7], BS occurred up to 74 mm during repositioning from 120\({}^{\circ}\) to 90\({}^{\circ}\), and in [PERSON] et al. 's study [14], the BS and SS with the non-slide condition were 97.9 mm and 25.1 mm, respectively, and the BS and SS with the 2-slide condition were 6.7 mm and 18.8 mm, respectively, during repositioning from 130\({}^{\circ}\) to 100\({}^{\circ}\). In a study of reclining wheelchairs with an ergonomic \"V-seat\" applied to patients with stroke, the BS range of the general wheelchair showed a maximum slip of 40 mm [23]. When repositioning of the general wheelchair was suggested in the previous study, the slip range of the backseat was observed to range from 74 mm to 97.9 mm, which was excessively increased from the 1.5-mm slip distance that occurred in the double-sliding system of our study. In particular, the significant reduction of 38.5 mm in the double-sliding system compared to the BS with the V-seat system explains the strong slip compensation effect of the double-sliding system. Based on the buttock pressure analysis, it was observed that the force, MP, and IPP tended to increase under both conditions upon repositioning, while SPP decreased. Particularly, during the transition from FRP to RUP, SPP was significantly reduced with double-sliding (5.87 kPa) than with non-sliding (12 kPa), which is thought to have a positive effect on relieving the pressure of the sacrum. In addition, SPP was significantly alleviated with double-sliding during repositioning; conversely, IPP was significantly increased in double-sliding (11.87 kPa) compared to in non-sliding (5.5 kPa). As a result of body-slip, SS values of double-sliding and non-sliding were 20.2 and 14.9 mm, respectively; therefore, the lower body in double-sliding moved more backward during repositioning than in non-sliding. As the buttocks shift the rear of the wheelchair seat, the sitting position becomes more upright, and the more upright posture, the greater the pressure on the ischial tuberosity rather than on the sacrum. Hence, the significant increase in IPP with double-sliding rather than with non-sliding during repositioning is attributed to this postural mechanism, suggesting that the double-sliding system is more effective for postural re-establishment than the non-sliding system. [PERSON] et al. [10] studied the effect of the position of the rotation axis of the backrest and the distance between the hip joint on the change in shear force applied to the buttock. When returning to the upright position of the backrest, the shear force was 15.0 \(\pm\) 3.6% body weight (%BW) at 3 cm forward from the axis of rotation, 16.7 \(\pm\) 3.6%BW at 6 cm forward, and 19.5 \(\pm\) 5.3%BW at 9 cm forward. It was suggested that as the distance between the rotation axis of the backrest and the hip joint increased, the shear force of the backrest increased.