## 4 Discussion
### Body-Slip and Buttock Pressure Changes during Wheelchair Reclining  
We observed that, as the body was displaced downward and forward during wheelchair reclining, both BS and SS (seat sliding) tended to decrease, and BS and TBS decreased significantly in double-sliding compared to non-sliding. A decrease in BS indicates a downward movement of the acromial marker attached to the body, and an increase in BS indicates an upward movement of the acromial marker. In our study, since the initial starting point of the acromion marker was located above the chair marker, the decrease in BS indicates that the upper body slides down along the backrest. As a result of the experiment, the BS variation in double-sliding was \(-3.9\) mm and slip did not occur during reclining, whereas non-sliding had a relatively large slip of \(-25.3\) mm. The significant difference in BS change between the two conditions confirmed that the double-sliding system is effective in preventing upper body-slip. On the other hand, the increase in SS indicates forward movement of the greater trochanter marker attached to the body, and it can be interpreted that the lower body, including the pelvis, slides forward along the seat during reclining. The SS change was larger in double-sliding (\(-18.8\) mm) than in non-sliding (\(-10.9\) mm). BS significantly \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Condition**} & \multirow{2}{*}{**Variable**} & \multirow{2}{*}{\begin{tabular}{c} **Double-Sliding** \\ \end{tabular} } & \multirow{2}{*}{**Non-Sliding**} & \multirow{2}{*}{**Mean**} & \multirow{2}{*}{**SE**} & \multicolumn{3}{c}{**95\% CI**} & \\ \cline{5-10}  & & & & & & & **Lower** & **Upper** & **\(t\)-Value** \\ \hline \multirow{4}{*}{Reclining} & Force (\%BW) & \(-3.30\pm 4.22\) & \(-3.60\pm 4.38\) & 0.30 & 0.93 & \(-2.41\) & \(1.81\) & \(0.322\) \\ \cline{2-10}  & Mean pressure (kPa) & \(-0.21\pm 0.28\) & \(-0.24\pm 0.28\) & 0.03 & 0.05 & \(-0.15\) & \(0.09\) & \(0.521\) \\ \cline{2-10}  & \multirow{2}{*}{\begin{tabular}{c} Peak pressure (kPa) \\ (kPa) \\ \end{tabular} } & Ischial & \(-11.02\pm 0.28\) & \(-2.28\pm 9.71\) & \(-8.74\) & 2.61 & 2.83 & \(14.65\) & \(-3.344\)\({}^{**}\) \\ \cline{2-10}  & Sacrum & \(6.69\pm 7.59\) & \(4.66\pm 6.11\) & 2.03 & 2.85 & \(-8.48\) & \(4.42\) & \(0.712\) \\ \hline \multirow{4}{*}{Repositioning} & Force (\%BW) & \(3.30\pm 4.16\) & \(5.10\pm 5.30\) & \(-1.80\) & 0.81 & \(-0.04\) & \(3.64\) & \(-2.212\) \\ \cline{2-10}  & Mean pressure (kPa) & \(0.23\pm 0.30\) & \(0.34\pm 0.33\) & \(-0.11\) & 0.06 & \(-0.02\) & \(0.24\) & \(-1.968\) \\ \cline{2-10}  & \multirow{2}{*}{\
\begin{tabular}{c} Peak pressure (kPa) \\ \end{tabular} } & Ischial & \(11.87\pm 8.60\) & \(5.50\pm 13.71\) & 6.37 & 2.66 & \(-12.38\) & \(-0.35\) & \(2.394\)\({}^{*}\) \\ \cline{2-10}  & Sacrum & \(-8.43\pm 7.61\) & \(-6.05\pm 5.73\) & \(-2.38\) & 2.81 & \(-3.98\) & \(8.74\) & \(-0.846\) \\ \hline \multirow{4}{*}{IUP-RUP} & Force (\%BW) & \(0.20\pm 1.62\) & \(1.40\pm 1.51\) & \(-0.12\) & 0.47 & 0.14 & 2.26 & \(-2.571\)\({}^{*}\) \\ \cline{2-10}  & Mean pressure (kPa) & \(0.02\pm 0.11\) & \(0.10\pm 0.09\) & \(-0.08\) & 0.03 & 0.01 & 0.16 & \(-2.576\)\({}^{*}\) \\ \cline{2-10}  & Peak pressure (kPa) &
\begin{tabular}{c} Ischial \\ tuberosity \\ \end{tabular} & \(0.85\pm 5.74\) & \(3.23\pm 4.91\) & \(-2.38\) & 2.74 & \(-3.82\) & \(8.57\) & \(-0.868\) \\ \cline{2-10}  & Sacrum & \(-1.74\pm 2.63\) & \(-1.39\pm 4.50\) & \(-0.35\) & 1.75 & \(-3.62\) & \(4.32\) & \(-0.199\) \\ \hline \multicolumn{10}{c}{IUP-RUP, difference in the values between initial upright position and return upright position; SE, standard error; CI, confidence interval; \({}^{*}\)\(p<0.05\); \({}^{**}\)\(p<0.01\).} \\ \end{tabular}
\end{table} 
Table 4: Comparison of buttock pressure variation between the two conditions (double-sliding versus non-sliding). decreased in double-sliding compared to non-sliding, and SS increased. These results are considered to be influenced by the shape of the saddle between the two wheelchairs.