## 4 Discussion
### Body-Slip and Buttock Pressure Changes during Wheelchair Reclining
The non-sliding wheelchair used in this study was molded to fit the shape of the buttocks and to provide a more comfortable seated feeling than the double-sliding product. In particular, the slightly concave shape of the rear part of the seat where the sacrum is located is expected to prevent the body from slipping during reclining to some extent, and it is judged that these characteristics affected the amount of change in SS. The two types of wheelchairs used in this experiment were ready-made products, and there were differences in the shape, material, and cover of the seat. This is a factor that can affect body-slip. However, even excluding SS, the fact that TBS combined with BS and SS significantly decreased in double-sliding (25.9 mm) compared to those in non-sliding (42.2 mm) indicated that the double-sliding system is effective in reducing the overall body-slip during reclining. In our previous study analyzing the effect of a back-sliding system on healthy people, it was reported that two-sliding (29.9 mm) reduced the overall body-slip compared to the non-slide (121.4 mm) while reclining from 100\({}^{\circ}\) to 130\({}^{\circ}\)[14]. BS and SS in the non-slide were 99.8 mm and 21.6 mm, respectively, and BS and SS of the 2-slide were 12.8 mm and 17.1 mm, respectively, indicating relatively small slip. Although there was a difference in slip variation between the previous study and this study, the sliding system was more effective in reducing body-slip than the non-sliding system in both studies. Furthermore, the difference in these results is thought to be influenced by the characteristics of the study participants (healthy people versus patients with SCI) participating in the experiment and the difference in the type of wheelchair used in the experiment. Next, we will discuss the buttock pressure during reclining. In IUP, force and MP were higher in non-sliding than in double-sliding, though there was no statistically significant difference between the two conditions. Additionally, both conditions showed that IPP was higher than SPP, and SPP with non-sliding (13.39 kPa) was significantly higher than that with double-sliding (7.61 kPa) by 5.79 kPa. With the transition from IUP to FRP, force, MP, and IPP showed a tendency to gradually decrease, and SPP showed a tendency to increase as the body displaced downward. The maximum peak pressure was formed on the ischial tuberosity, and there was no significant difference in force and MP between the two conditions. During reclining, IPP significantly reduced by 8.74 kPa in double-sliding compared to non-sliding, and SPP showed no significant difference between the two conditions. The fact that SPP increased significantly in non-sliding compared to double-sliding in IUP is consistent with the discussion of the SS results mentioned above. In the non-sliding wheelchair used in this study, the sacrum (especially the cocyx) is located deeper than other parts with the back part of the seat where the sacrum is located slightly recessed. Although this attribute may reduce seat slip while reclining in a wheelchair, it may result in a relatively high SPP in the IUP. In FRP, there was no statistically significant difference between the two conditions; however, it was confirmed that both IPP and SPP were higher in non-sliding than in double-sliding, and the amount of change in IPP was significantly decreased in double-sliding (\(-11.02\) kPa) compared to that in non-sliding (\(-2.28\) kPa). This can be interpreted to mean that the double-sliding system reduces the slip of the upper body during reclining and more effectively relieves the compressive force applied to the buttocks, especially the ischial tuberosity. In [PERSON] et al. 's study of the sliding system in healthy people, the MP showed a tendency to decrease during reclining, and the MP was higher in the two-slide than in the non-slide, which is similar to the results of our previous study [14].