## Appendix A Double-sliding  
Figure 3: Designed double-sliding recline system (**A**) and operation mechanism (**B**): Detailed schematic diagram of the operation of the 1 st slide (**a**) and the 2 nd slide (**b**). Figure 2: Two types of wheelchairs used in the experiment ((**A**). [PERSON], (**B**). Korec). The general reclining wheelchair and double-sliding reclining wheelchair will be hereafter referred to as non-sliding and double-sliding, respectively. The maximum backrest angles supported by non-sliding and double-sliding were 130\({}^{\circ}\) and 180\({}^{\circ}\), respectively. Only the backrest angle range of 100-130\({}^{\circ}\) was analyzed to compare the same range between the two wheelchairs. To determine the effect of the sliding system, body-slip and buttock pressure were measured when the wheelchair was reclined and repositioned. Body-slip was defined as the sliding state of the upper body, including the pelvis, and the distance traveled by the acrom along the backseat and the distance traveled by the greater trochanter along the seat were measured using a three-dimensional motion analysis system (Motion Analysis, Santa Rosa, CA, USA) [7]. Three anatomical landmarks on the human body were used as the left sagittal profile of the seated participant, and three landmarks attached on the wheelchair were used to indicate the orientation of the wheelchair in space. The detailed marker locations were the acromion, greater trochanter, lateral femoral condople, upper point on the side of the backseat of the wheelchair, axis of rotation between the backseat and seat of the wheelchair, and two-thirds point on the side of the wheelchair seat (Figure 2) [7]. A marker placed on the soft tissue of the human body may not correspond anatomically to the skeletal system due to the characteristics of the soft tissue. Therefore, to ensure the accuracy of the marker attachment location, the same measurer selected all marker locations and attached them, and the movement of the marker was minimized by fixing it with tape. In the global gravitational coordinate system, the \(X\)-axis represents the traveling direction of the wheelchair, the \(Y\)-axis represents the central axis passing through the center of the two rear wheels, the \(Z\)-axis represents the gravity axis, and the sagittal plane is defined as the Z-X plane. The standard measures in [PERSON] et al. 's study were used to calculate the amount of sliding along the backrest (backrest sliding, BS) and the amount of sliding along the seat (seat sliding, SS) [7]. BS (1) and SS (2) are defined by the following formula, and the total body-slip (TBS) is the sum of the absolute values of BS and SS (3). \[\text{BS} = \text{V}_{\text{i}}-\text{V}_{\text{n}} \tag{1}\] \[\text{SS} = \text{H}_{\text{i}}-\text{H}_{\text{n}}\] (2) \[\text{TBS} = |\text{BS}|+|\text{SS}| \tag{3}\]  
As shown in Figure 2, \(\text{V}_{\text{i}}\) and \(\text{V}_{\text{n}}\) denote the initial position (i) and the final position (n), respectively, and the distance between the backrest marker and the acromion marker perpendicular to the backrest support surface. \(\text{H}_{\text{i}}\) and \(\text{H}_{\text{n}}\) are the distances perpendicular to the seat support surface between the seat marker and the greater trochanter marker at the initial position (i) and final position (n), respectively.