Analysis of Body-Slip and Buttock Pressure Characteristics during Operation of a Double-Sliding Reclining Wheelchair in Patients with Spinal Cord Injury

: To minimize body collapse due to repetitive reclining and repositioning when using a reclining wheelchair, reclining wheelchairs with a sliding backrest function have been developed. This study compared the differences in body-slip and buttock pressure according to the presence or absence of the sliding function of the backrest during reclining and repositioning motions in 10 patients with spinal cord injury. When reclining from 100 ◦ to 130 ◦ , backrest sliding and total body-slip in the double-sliding wheelchair were signiﬁcantly decreased by 21.4 mm and 16.4 mm, respectively, compared to a non-sliding wheelchair, and the peak pressure on the ischial tuberosity was signiﬁcantly decreased by 8.7 kPa. Upon comparison of the buttock pressure between the initial upright position before reclining and the return upright position after repositioning, the force and mean pressure with the double-sliding wheelchair were signiﬁcantly reduced compared with those with the non-sliding wheelchair. It was conﬁrmed that the double-sliding system improved body-slip and buttock pressure more effectively than the non-sliding system. This study’s results are expected to provide the basic data necessary for the prescription and selection of wheelchairs in clinical practice and to be utilized in the development of related devices.


Introduction
In patients with spinal cord injury (SCI), prolonged sitting in a wheelchair increases the risk of pressure sores in the gluteal muscles due to loss of mobility and sensation. Sitting for long periods on a surface such as a wheelchair seat creates localized stress on the buttock, causing the compression and deformation of soft tissues. This mechanical environment reduces local blood supply and lymph circulation, and prolonged exposure to these conditions results in tissue destruction and pressure sores with cell necrosis [1,2].
To maintain gluteal tissue variability while seated in a wheelchair, wheelchair users lift their torso with their arms, tilt their torso to the side or forward [3,4], and use methods such as active cushioning or repositioning to relieve pressure on the buttock [5]. In particular, quadriplegic patients who are unable to change positions independently use the wheelchair's repositioning function to relieve buttock pressure. Repositioning can be defined as a change in the user's posture against gravity, which can be changed by adjusting the tilt angle of the wheelchair or the angle of the backrest of the wheelchair. Wheelchairs with tilt and recline functions are effective in reducing gluteal pressure, especially pressure and swelling under the ischial tuberosity, and are also beneficial for comfort, postural control, stability, sitting tolerance, mobility, respiration, and digestion [6,7].
However, despite the numerous advantages of reclining wheelchairs, the body collapses due to the repeated reclining and repositioning movements. In the body-slip phenomenon, the user slides from the backrest of the wheelchair during repositioning, which may increase the shear force on the back and the compression force on the buttock; this can lead to tissue destruction and pressure sores, and excessive body-slip increases the risk of falling in the wheelchair ( Figure 1) [8]. To solve the body-slip problem, Pfaff proposed "the shear-reducing" recline system, which reduces the shear forces through repositioning of the seat-to-back angle [9]. Thereafter, wheelchairs with a contoured seat back or a molded back (which maintain contact between the body and the seat) have been developed. Additionally, various studies have been conducted to derive optimal conditions by combining the tilt and recline angles of the wheelchair [7,[10][11][12][13].

Study Design and Participants
Ten patients with SCI participated in this study. The participants' general characteristics are shown in Table 1. The study participants were selected from people with SCI between the T7 and L1 levels due to trauma and at least 6 months after SCI. Exclusion criteria comprised patients with cardiovascular diseases and other diseases that could significantly affect cardiovascular function, those diagnosed with skeletal deformities (scoliosis, hip and knee contractures, etc.), those with a body mass index of ≥30 kg/m 2 , and those with active bedsores. This study was conducted with the approval of the Institutional Review Board of the Rehabilitation Engineering Research Institute (RERE-IRB-210604-1), and the purpose and procedure of the study were explained to all patients before the experiment. Informed consent was provided before study participation.

Measurements
In this study, a general reclining wheelchair without a back-sliding device (Saber GT; Previous studies that have analyzed the biomechanical characteristics related to reclining wheelchairs have mainly focused on body-slip and changes in buttock pressure according to changes in the recline angle [7,12,13]. For body-slip, a change in the movement distance between a wheelchair marker and a body marker was measured using a threedimensional motion analysis system [7]. Buttock pressure was measured by measuring the maximal pressure in the ischial tuberosity using a force sensor [14,15], or by examining skin perfusion using laser doppler flowmetry [2,13]. In most studies, the wheelchair angle at the point with the lowest buttock pressure was suggested as the optimal wheelchair angle, and guidelines for wheelchair use were presented based on this. In general, the maximum pressure develops on the ischial tuberosity in a wheelchair-sitting position. Hobson et al. reported a 14% reduction in peak pressure on the ischial tuberosity at a tilt angle of 20 • [4], while other studies also reported that the ischial tuberosity pressure was reduced by 27% and 47% at posterior tilt angles of 35 • and 65 • , respectively [16]. In addition, Aissaoui et al. reported that up to 40.2% of body weight shift occurred in a wheelchair tilted at 45 • and reclined at 120 • , and acromion marker slip of up to 74 mm occurred upon repositioning at 120 • [7].
As stated above, special devices for minimizing body-slip in a reclining wheelchair have been developed. In particular, a powered wheelchair with a sliding backrest has been designed such that the backrest slides at a specific reclining angle and a wire or linear actuator is applied to the backrest to minimize body-slip [17]. Our institute also developed a reclining wheelchair with a sliding backrest function. It is a double-sliding system with a pre-sliding function that changes the sliding distance appropriately according to the user's body type before sliding. A previous study has confirmed that a double-sliding system reduces body-slip and buttock pressure compared to single-or non-sliding systems [14]. However, since the study was conducted in healthy people, additional research is required on disabled people who actually use wheelchairs. Since patients with SCI who have lost muscle tone may show different characteristics from healthy people when reclining and repositioning a wheelchair, research on actual users is essential for clinical application.
Therefore, in this study, we aimed to verify the effect of the double-sliding system on patients with SCI by investigating the differences in body-slip and buttock pressure between the double-sliding system and general reclining systems during wheelchair reclining and repositioning. The results of this study are intended to provide basic data on prescriptions and guidelines for wheelchair selection in clinical practice and to aid in the development of wheelchairs.

Study Design and Participants
Ten patients with SCI participated in this study. The participants' general characteristics are shown in Table 1. The study participants were selected from people with SCI between the T7 and L1 levels due to trauma and at least 6 months after SCI. Exclusion criteria comprised patients with cardiovascular diseases and other diseases that could significantly affect cardiovascular function, those diagnosed with skeletal deformities (scoliosis, hip and knee contractures, etc.), those with a body mass index of ≥30 kg/m 2 , and those with active bedsores. This study was conducted with the approval of the Institutional Review Board of the Rehabilitation Engineering Research Institute (RERE-IRB-210604-1), and the purpose and procedure of the study were explained to all patients before the experiment. Informed consent was provided before study participation.

Measurements
In this study, a general reclining wheelchair without a back-sliding device (Saber GT; Karma, Chiayi, Taiwan) and a reclining wheelchair with a double-sliding device (Smart Wheelchair; KOREC, Incheon, Korea) were compared ( Figure 2). Figure 3 shows the system model and operating method of the proposed two-slide recline system. The fixed-slide system of the first stage has a link structure connecting the backrest and the seat that operates in relation to the recline-operation angles. The secondstage slide system was constructed by combining a linear guide and a gas spring to support and return the backrest position. This recline system allows the second slide to move in conjunction with the first slide motion, which is in accordance with the recline-operation angle to prevent body-slip between the user and the backrest. The first-and second-slide ranges are approximately 150 mm and 90 mm, respectively. As the second slide moves the required range according to the body type and sitting conditions of the user, it can accommodate most types of users and nearly eliminate body-slip in various positions.  Figure 3 shows the system model and operating method of the proposed two-slide recline system. The fixed-slide system of the first stage has a link structure connecting the backrest and the seat that operates in relation to the recline-operation angles. The secondstage slide system was constructed by combining a linear guide and a gas spring to support and return the backrest position. This recline system allows the second slide to move in conjunction with the first slide motion, which is in accordance with the recline-operation angle to prevent body-slip between the user and the backrest. The first-and secondslide ranges are approximately 150 mm and 90 mm, respectively. As the second slide moves the required range according to the body type and sitting conditions of the user, it can accommodate most types of users and nearly eliminate body-slip in various positions.  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° and 180°, respectively. Only the backrest angle range of 100°-130° was analyzed to compare the same 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 • and 180 • , respectively. Only the backrest angle range of 100-130 • 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 acromion 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 condyle, 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 Aissaiui 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).
TBS =|BS| + |SS| As shown in Figure 2, V i and V 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. H i and H 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.
To project the positions of the acromion and the greater trochanter markers perpendicular to the back and seat of the chair, respectively, we calculated m (4), l (5), and n (6) using the coordinate values of the two points shown in Figure 4. . Vi, distance between reference point B and the acromion marker along the backrest plane; Hi, distance between the reference point S and the greater trochanter marker along the seat plane; BS, backrest sliding; SS, seat sliding; m, perpendicular distance between the anatomical marker and the chair; l, straight-line distance between the anatomical marker and chair marker; n, distance between the anatomical marker projected on the chair and the chair marker.

Experimental Procedure
The neutral position of the wheelchair was set with a seat inclination angle of 0°, a backrest angle of 100°, and a leg-rest angle of 90°. The standard posture was defined as the position where the dorsal surface of the pelvis lightly touched the backrest while sitting, and the greater trochanter was used as an index for palpability of the hip joint in the seated position in a wheelchair. Before the experiment, the mat sensor was fixed to the wheelchair seat using double-sided tape to prevent the mat from slipping when operating the wheelchair.
The detailed experimental procedure is as follows: The participant sat in the standard posture on the wheelchair seat with a pressure sensor mat and maintained an upright position of 100°. The participant was positioned so that the pelvis touched the back of the wheelchair, the thighs were positioned parallel to the seat angle, and the feet were positioned perpendicular to the lower extremities [7]. After the participant was in the standard posture position, reflective markers for three-dimensional motion analysis were attached to the wheelchair and anatomical landmarks of the body, and the same investigator designated the landmark position to minimize the attachment error for the marker position. During the experiment, the participant leaned against the back of the wheelchair and sat in a comfortable position, such as placing his/her head on the headrest of the wheelchair and bringing his/her arms forward comfortably; voluntary movements, including position changes, were restricted during the experiment.
The initial upright position (IUP) of the backrest was set to 100° inclined by 10° vertically, and one trial was defined as reclining back from the initial position to the fully reclined position (FRP) of 130° and returning to the initial upright position (RUP) [10]. The reclining and repositioning motions were measured three times from wheelchair angles of 100°-130°, and motion analysis and buttock pressure were simultaneously measured for data synchronization. In addition, to consider the effect of the participant's posture collapse, the initial posture was reset for each movement, and measurement was performed 10 s later. . Vi, distance between reference point B and the acromion marker along the backrest plane; Hi, distance between the reference point S and the greater trochanter marker along the seat plane; BS, backrest sliding; SS, seat sliding; m, perpendicular distance between the anatomical marker and the chair; l, straight-line distance between the anatomical marker and chair marker; n, distance between the anatomical marker projected on the chair and the chair marker.
To measure buttock pressure, force (%BW), mean pressure (MP, kPa), the peak pressure at the ischial tuberosity (IPP, kPa), and the peak pressure at the sacrum (SPP, kPa) were collected at the seat and buttock interface using a Pliance system (Pliance xf32, Novel.de, Munich, Germany) [7,14]. The Pliance system consists of a sensor mat, multi-channel analyzer, and analysis software. The sensor mat is a capacitive sensor, which is a flexible mat consisting of 1024 sensors (32 each in width and length). Each sensor measures 14 × 14 mm 2 and has a sensing area of 448 × 448 mm 2 . In addition, the pressure measurement range was 2-60 kPa, and the measurement frequency was 20,000 Hz.

Experimental Procedure
The neutral position of the wheelchair was set with a seat inclination angle of 0 • , a backrest angle of 100 • , and a leg-rest angle of 90 • . The standard posture was defined as the position where the dorsal surface of the pelvis lightly touched the backrest while sitting, and the greater trochanter was used as an index for palpability of the hip joint in the seated position in a wheelchair. Before the experiment, the mat sensor was fixed to the wheelchair seat using double-sided tape to prevent the mat from slipping when operating the wheelchair.
The detailed experimental procedure is as follows: The participant sat in the standard posture on the wheelchair seat with a pressure sensor mat and maintained an upright position of 100 • . The participant was positioned so that the pelvis touched the back of the wheelchair, the thighs were positioned parallel to the seat angle, and the feet were positioned perpendicular to the lower extremities [7]. After the participant was in the standard posture position, reflective markers for three-dimensional motion analysis were attached to the wheelchair and anatomical landmarks of the body, and the same investigator designated the landmark position to minimize the attachment error for the marker position. During the experiment, the participant leaned against the back of the wheelchair and sat in a comfortable position, such as placing his/her head on the headrest of the wheelchair and bringing his/her arms forward comfortably; voluntary movements, including position changes, were restricted during the experiment.
The initial upright position (IUP) of the backrest was set to 100 • inclined by 10 • vertically, and one trial was defined as reclining back from the initial position to the fully reclined position (FRP) of 130 • and returning to the initial upright position (RUP) [10]. The reclining and repositioning motions were measured three times from wheelchair angles of 100-130 • , and motion analysis and buttock pressure were simultaneously measured for data synchronization. In addition, to consider the effect of the participant's posture collapse, the initial posture was reset for each movement, and measurement was performed 10 s later.
All variables were measured three times in the experiment, and the average of the measured values was used for the statistical analysis. To compare the difference between double-sliding and non-sliding conditions during reclining and repositioning operations, a normality test was performed using the Kolmogorov-Smirnov test, followed by a pairedt test. The statistical significance level was set at p < 0.05. In this study, the effects of covariates such as sex, body mass index, and height on the outcome variables were not considered, and only the differences in the outcome values according to the type of wheelchair were compared.

Body-Slip
In this study, the differences in backrest sliding (BS), seat-sliding (SS) and total bodyslip (TBS) between the double-sliding and non-sliding wheelchairs were analyzed when the wheelchairs were reclined and repositioned in the range of 100-130 • . In addition, the difference between the marker position in the initial upright position (IUP) before reclining and that in the return upright position (RUP) after repositioning was compared. Table 2 shows the results of the comparison of the amount of change in body-slip between double-sliding and non-sliding. During reclining, BS showed a tendency to decrease in both conditions, and the decrease in BS was 21.42 mm in non-sliding, which was significantly greater than that in double-sliding (p < 0.05). The SS also showed a tendency to decrease in both conditions, and the decrease of SS in double-sliding was significantly decreased by 7.86 mm compared to that in non-sliding (p < 0.05). The change in the TBS was 16.35 mm greater in non-sliding than in double-sliding (p < 0.05).
During repositioning, BS showed a tendency to increase in both conditions, and the increase of BS in non-sliding was significantly greater than that in double-sliding by 17 mm.
In addition, SS showed a tendency to increase in both conditions, and although the increase of SS in double-sliding was greater than that in non-sliding, there was no statistically significant difference.
The amount of change in TBS was 10.82 mm, which was significantly larger in nonsliding than in double-sliding (p < 0.05). Regarding the difference in the amount of change in body-slip between RUP and IUP, BS decreased in both conditions, while SS showed a tendency to increase in both conditions. However, there were no statistically significant differences between the two conditions. Additionally, the amount of change in TBS was greater in non-sliding than in double-sliding; however, there was no statistically significant difference.

Buttock Pressure
To analyze the characteristics of buttock pressure according to the types of wheelchairs, the differences in force, mean pressure (MP), peak pressure on the ischial tuberosity (IPP), and peak pressure on the sacrum (SPP) were compared between the double-sliding and non-sliding wheelchairs in IUP, fully reclined position (FRP), and RUP. Table 3 and Figure 5 show the results of the comparison in the average values of buttock pressure between double-sliding and non-sliding. In IUP, force and MP were higher in non-sliding than in double-sliding, though there was no statistically significant difference. Moreover, IPP was higher than SPP in both conditions. There was no significant difference in IPP between the two conditions; however, the SPP of non-sliding was 5.79 kPa, which was significantly higher than that of double-sliding (p < 0.05). In FRP, force, MP, IPP, and SPP did not show any significant difference between the two conditions, though the average values of IPP and SPP tended to be higher in non-sliding than in double-sliding. In RUP, there was no significant difference between the two conditions for force, MP, and IPP; however, SPP was significantly larger in non-sliding than in double-sliding by 6.13 kPa (p < 0.05). In addition, the difference in the force, MP, IPP and SPP between the d and non-sliding wheelchairs was analyzed when the wheelchairs were recli sitioned in the range of 100°−130°. Table 4 shows the results of the comparison in the amount of change pressure between double-sliding and non-sliding. In addition, the difference in the force, MP, IPP and SPP between the double-sliding and non-sliding wheelchairs was analyzed when the wheelchairs were reclined and repositioned in the range of 100-130 • . Table 4 shows the results of the comparison in the amount of change in the buttock pressure between double-sliding and non-sliding. During reclining, force, MP, and IPP tended to decrease in both conditions, though SPP showed a tendency to increase. Particularly, the decrease in IPP of double-sliding was significantly greater than that of non-sliding by 8.74 kPa (p < 0.01).
During repositioning, force, MP, and IPP tended to increase under both conditions, though SPP showed a tendency to decrease. In addition, the increase in SPP was significantly larger for double-sliding than for non-sliding by 6.37 kPa (p < 0.05).
The difference in buttock pressure between RUP and IUP showed a tendency to increase in RUP compared to IUP in both conditions. The change in force and MP was significantly greater in non-sliding than in double-sliding (both p < 0.05).

Discussion
As a major study result, it was confirmed that backrest sliding (BS), total body-slip (TBS), and peak pressure on the sacrum (SPP) with double-sliding were significantly reduced compared to those with non-sliding. We will classify each wheelchair operation and discuss it in detail as follows.

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 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. 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 • to 130 • [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 doublesliding 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 doublesliding 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 coccyx) 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 Chang 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]. Bogie and Bader showed that when the wheelchair was reclined, pelvic rotation occurred and the buttock center of pressure moved backward. The authors reported that when the wheelchair was laid down, the center of pressure shifted behind the ischial tuberosity and the weight shifted to the lumbar region [18]. This is similar to the results of our study, in which the pressure on the ischial tuberosity area decreased and the pressure in the sacral area increased when reclining was performed in our study. In general, previous studies that analyzed changes in the buttock pressure related to wheelchair reclining have mainly focused on the relief of buttock pressure according to changes in the tilt and reclining angle of the wheelchair. They attempted to determine the optimal wheelchair angle by analyzing the changes in pressure. Shields and Cook also reported a 10% decrease in pressure around the ischial tuberosity for every 10 • backrest angle tilted; however, this was not statistically significant [19]. In a similar study conducted on patients with SCI, IPP decreased by 27% and 47%, respectively, when the wheelchair was tilted posteriorly by 35 • and 65 • [16], and another study reported a 5% reduction in pressure when the wheelchair was tilted from 0 • to 25 • [20]. Aissaoui et al. reported that the maximum buttock pressure decreased by approximately 40% at a tilt angle of 45 • and a recline angle of 120 • [21], and another author reported a 28% reduction in vertical force when the backrest was reclined at 58 • [7,22]. However, Hobson [4] reported that IPP decreased by 14% at a wheelchair tilt angle of 20 • , and IPP increased by 12% at a 120 • reclining angle. The author said that adjusting the tilt angle of the wheelchair is more effective in reducing IPP than the reclining angle; this is contrary to our study's finding, which showed a tendency for IPP to decrease during reclining [4]. Considering that the experimental conditions of our study and those of Hobson [4] were different, it is difficult to directly compare or interpret the two studies' results.

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 doublesliding (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 Aissaoui et al.'s study [7], BS occurred up to 74 mm during repositioning from 120 • to 90 • , and in Chang 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 • to 100 • . 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 94 mm, and the V-seat 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 bodyslip, 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.
Kobara 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 ± 3.6% body weight (%BW) at 3 cm forward from the axis of rotation, 16.7 ± 3.6%BW at 6 cm forward, and 19.5 ± 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. The authors emphasized that the shear force applied to the buttock when the backrest is reclined can be reduced by adjusting the rotation of the axis of the wheelchair as a sitting access strategy for the prevention of pressure ulcers. Our findings are similar to those of Kobara et al., and it seems that as the SS of double-sliding increased compared to that of non-sliding during repositioning, the rotation axis of the wheelchair and the hip joint became closer, and the SPP decreased.
Huang et al. [23] also reported that as a result of measuring the buttock pressure for the reclining wheelchair to which the ergonomic V-seat was applied, the SPP of the V-shaped wheelchair was significantly reduced compared to that of the conventional wheelchair, which is similar to our study's results. Huang et al. [23] suggested that the use of a reclining wheelchair with a V-shaped seat may help reduce anterior sliding and sacral pressure in patients with flaccid hemiplegia.
Finally, we analyzed whether a body that slipped slightly downward due to reclining recovered to its original position upon repositioning through the difference in body-slip and buttock pressure change between IUP and RUP. In both conditions, the body was in a slightly lower position inIUP than in RUP; however, the difference was not statistically significant. The difference of TBS between IUP and RUP was larger in non-sliding than in double-sliding. It was also observed that the buttock pressure differential between RUP and IUP increased in force, MM, and IPP in RUP compared to those in IUP in both conditions. In particular, the force and MM of non-sliding increased significantly compared to those of double-sliding. This means that the buttock pressure increased compared to the initial sitting position due to repositioning, and the buttock pressure may increase due to the repeated reclining and repositioning motions.
Excessive normal and shear forces applied to the buttocks induce changes in soft tissue blood flow. Goossens et al. [24] observed that a shear force of 3.1 kPa significantly affected decreased sacral blood flow and pointed out that it is important to reduce the shear force to prevent pressure sores. Moreover, in a study by Peterson and Adkins [25], it was also reported that an external pressure of 20-30 mmHg or more may cause capillary occlusion and apoptosis. Patients with SCI who have lost mobility and sensation are at a high risk of developing pressure sores, especially in the buttock muscles [26][27][28]. Using a reclining wheelchair to periodically reduce the pressure on the buttocks is a very useful method. Specifically, the double-sliding system developed by our research institute is thought to be able to improve the user's body-slip and buttock pressure more effectively during reclining and repositioning operations than a non-sliding system.

Conclusions
This study showed that BS, TBS, IPP, and SPP were significantly reduced with doublesliding compared to non-sliding wheelchairs during reclining and repositioning operations. The limitations of this study were that differences in the characteristics (seat shape, material, cover, etc.) of the two types of wheelchairs used in the experiment may have affected the results, although we could not completely control them, and due to the limitation of the reclining angle supported by the wheelchair, the analysis was performed only up to 130 • . Therefore, additional research that considers the seat characteristics and expands the reclining angle range is needed in the future. Nevertheless, the results of our study will provide basic data necessary for the prescription and selection of wheelchairs in clinical practice and may be utilized in the development of devices related to reclining wheelchairs.
Author Contributions: Conceptualization, methodology, visualization, writing-reviewing, editing, and investigation, Y.C.; methodology, formal analysis, data curation, and investigation, J.K. and B.J.; methodology and investigation, Y.K.; project administration, funding acquisition, E.-P.H.; supervision, investigation, G.K. All authors have read and agreed to the published version of the manuscript. Informed Consent Statement: Informed consent was obtained from all subjects involved in the study.

Data Availability Statement:
The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

Conflicts of Interest:
The authors declare no conflict of interest.