sacral sitting is a poor seated posture with a posterior pelvic tilt and thoracic 
 kyphosis1 .
it is typically observed in 
 frail older adults using a wheelchair , and occurs when they lack the ability to retain a 
 seated posture and/or their body size and function do not match the size and structure of 
 the wheelchair2 . in some cases , the size 
 of the standard wheelchair used in hospitals and elderly care facilities is not suitable for 
 individual wheelchair users3 ; however , 
 there are also structural problems with wheelchair design that can make it difficult to 
 maintain a seated posture , such as the hammocking effect of sling seats and poor back 
 support2 , both of which are likely to 
 cause sacral sitting .
patients in care facilities often sit in wheelchairs with poor seated 
 posture , including sacral sitting , for long periods of the day4 and it has been reported that sacral sitting with a posterior pelvic 
 tilt increases the risk of pressure ulcers on the buttocks in frail older adults5 , 6 .
a 
 pressure ulcer is a localized injury caused by contact pressure or pressure in combination 
 with shear force on the skin and/or subcutaneous tissues over a bony prominence7 .
the vertical and horizontal forces that 
 load the sacrococcygeal region during sitting increase with an increase in the posterior 
 inclination angle of the pelvis8 . however , 
 the influence of sacral sitting on the contact pressure and shear force load on the buttocks 
 while seated in a wheelchair has not been clarified . in this study , the pelvic inclination angle , contact pressures on the buttocks and back , 
 and shear force of healthy subjects who adopted a sacral sitting posture in a wheelchair 
 were quantitatively measured .
the goal of this investigation was to determine the influence 
 of sacral sitting on factors likely to predispose to the development of pressure ulcers .
twenty - six healthy adults ( 5 males , 21 females ; mean age , 21.0  0.9  years ) participated in 
 this study .
their body sizes were as follows : mean height : 159.3  7.4  cm ; mean thigh length 
 ( the distance from the sacrum to the popliteal fossa ) : 44.4  2.7  cm ; mean axilla height 
 ( from the seat surface to the axilla while in a sitting position ) : 42.8  3.1  cm ; mean lower 
 leg length ( from the popliteal fossa to the heel while in a sitting position ) : 39.7  
 2.5  cm ; and mean elbow height ( from the seat surface to the elbow while in a sitting 
 position ) : 21.2  2.7  cm .
measurements were performed using a modular wheelchair ( revo next ; 
 etac co. , ltd . ,
stockholm , sweden ) , each part of which was adjustable to the body size of 
 the user .
after the initial measurements , a wheelchair cushion ( type 5 tc-045 ; takano 
 heartworks co. , ltd . , nagoya , japan ) was placed on the seat and the wheelchair was adjusted 
 to the body size of each subject as follows : seat depth was adjusted to thigh depth minus 
 5  cm ; back height was adjusted to axilla height minus 10  cm ; leg length was adjusted to the 
 lower leg length ; and the height of the arm supports was adjusted to the elbow height of 
 each subject .
the subjects assumed two postures : a basic sitting posture with the buttocks placed far 
 back on the wheelchair seat and sacral sitting with a posterior pelvic tilt and the ischium 
 slid forward by 5  cm relative to its position in the basic sitting posture ( figs .
the subject sits up straight with the buttocks placed far back 
 on the wheelchair seat .
the subject sits with a posterior pelvic tilt and the ischium 
 slid forward by 5  cm compared with the basic sitting posture ) .
the subjects assumed the basic sitting posture first , and then transitioned from 
 the basic to the sacral sitting posture . in each posture , the inclination angle of the 
 sagittal pelvic line , contact pressure on the ischial region , and maximum contact pressures 
 and contact areas on the buttocks and back were measured .
in addition , the shear forces 
 generated by sliding the ischial region forward during basic sitting and during the 
 transition from basic to sacral sitting were measured .
the subject sits up straight with the buttocks placed far back 
 on the wheelchair seat .
the subject sits with a posterior pelvic tilt and the ischium 
 slid forward by 5  cm compared with the basic sitting posture the forward displacement of the ischium was measured by placing the end of a 30-cm steel 
 carpenter s square ruler against the lower edge of the subject s patella while the ruler was 
 held parallel to the thigh in a horizontal position .
the distance from the patella to the 
 point where the ruler and the front of the seat met was considered to be the forward 
 displacement of the ischium9 , 10 .
the inclination angle of the sagittal pelvic line was 
 measured using an inclination angle - measuring device ( horizon ; yuki trading co. , ltd .
, 
 tokyo , japan ) , which is a seated posture - measuring device , in accordance with the iso16840 - 1 
 standard . when the sagittal pelvic line tilted backwards from the vertical with a posterior 
 pelvic tilt , the values measured with horizon became negative . in contrast , when the 
 sagittal pelvic line tilted forward from the vertical with an anterior pelvic tilt , they 
 became positive .
the contact pressures and contact areas on the buttocks and back were 
 measured using an sr soft vision sensor pad ( fukoku busssan co. , ltd . ,
sr soft vision is a seat - type sensor pad with 256 
 pressure - sensing points indicating the pressure load on each point .
the number of responding 
 sensing points was considered to be the size of the contact area . the contact pressure on 
 the ischial region and shear force generated by sliding the ischial region forward
the paired t - test was used to analyze between - posture differences in the mean values of 
 each measurement when the measured values were normally distributed , and the wilcoxon 
 signed - rank test was used when the distribution was not normal .
correlations between the 
 inclination angle of the sagittal pelvic line and the contact pressure on the ischial 
 region , maximum contact pressures and contact areas on the buttocks and back during basic 
 and sacral sitting , and shear force during basic sitting and during the transition from 
 basic to sacral sitting were investigated using spearman s correlation coefficient when the 
 measured values were normally distributed , and pearson s rank correlation coefficient when 
 their distribution was not normal .
all analyses were performed using the japanese version of 
 spss version 21 ( ibm , armonk , ny , usa ) .
the medical ethics committee of gunma university approved this study ( 26 - 43 ) , and written 
 informed consent was provided by subjects .
the inclination angles of the sagittal pelvic line were 16.4  4.0 and 30.3  4.3 
 during basic and sacral sitting , respectively , which indicates that posterior pelvic tilt 
 was significantly greater during sacral sitting than during basic sitting ( p<0.001 ; table 1table 1.measurement values during basic and sacral sitting ( n=26)basic sitting posturesacral sittingmean  standard deviationmedian ( minimum value to maximum value)mean  standard deviationmedian ( minimum value to maximum value)inclination angle of the sagittal pelvic line ( degree ) 16.4  4.0 16.5 ( 24.0 to 8.0)30.3  4.3***30.0 ( 41.0 to 20.0)contact pressure on the ischial region ( mmhg ) 
72.5  14.4 69.8 ( 42.5 to 107.5)61.1  14.3***62.0 ( 27.5 to 88.0)buttocksmaximum contact pressure ( mmhg ) 92.1  18.3 87.5 ( 66.0 to 133.0)78.6  19.475.0 ( 56.0 to 130.0)***size of contact area ( point ) 137.3  30.4 143.5 ( 86.0 to 187.0)150.1  28.1146.0 ( 102.0 to 210.0)***backmaximum contact pressure ( mmhg ) 36.1  9.4 35.5 ( 21.0 to 57.0)50.9  13.5***50.5 ( 29.0 to 79.0)size of contact area ( point ) 17.2  10.0 15.0 ( 2.0 to 47.0)29.3  10.1***29.0 ( 14.0 to 55.0)shear force generated by sliding the ischial region forward 
 ( newton ) 6.1  3.0 5.5 ( 1.6 to 12.6)7.1  3.6**6.9 ( 1.7 to 15.8)significance was tested using the paired t - test , significance was tested using the 
 wilcoxon signed - rank test . * * p<0.01 , * * * p<0.001 ) .
maximum contact pressures on the buttocks ( p<0.001 ) and ischial region 
 ( p<0.001 ) were significantly lower during sacral sitting than during basic sitting . in 
 contrast ,
maximum contact pressure on the back ( p<0.001 ) , contact area on the buttocks 
 ( p<0.001 ) , contact area on the back ( p<0.001 ) were significantly greater during sacral 
 sitting than during basic sitting ( table 1 ) .
the 
 shear force generated by sliding the ischial region forward to a sacral sitting position was 
 significantly greater than the shear force during basic sitting ( p=0.003 ) .
the inclination 
 angle of the sagittal pelvic line was significantly correlated with the back contact area 
 during the transition from a basic to a sacral sitting posture ( |r|=0.451 , p=0.021 ) .
significance was tested using the paired t - test , significance was tested using the 
 wilcoxon signed - rank test .
the major findings of this study were that , during sacral sitting , the pressures on the 
 buttocks were lower while pressures on the back were higher than during basic sitting .
in 
 addition , significant shear forces were generated by sliding the ischium forward from a 
 basic sitting posture into a sacral sitting position .
these findings are consistent with 
 those of previous studies that investigated the influence of sacral sitting on subjects 
 without using wheelchairs .
since the posterior tilt of the pelvis was greater during sacral sitting than in a basic 
 sitting posture , the combined center of gravity of the head , arms , and trunk may have 
 shifted back relative to its position during basic sitting ; this may have increased in the 
 force pressing the back against the back support during sacral sitting and the maximum 
 contact pressure on areas of the back .
the results of this study are consistent with those 
 of a previous report , which also found that the weight borne by a wheelchair seat supporting 
 the buttocks decreased as the combined center of gravity shifted back during sacral sitting 
 with a posterior pelvic tilt , while the load on the back support increased11 .
concomitantly , maximum contact pressure 
 on the buttocks decreased and maximum contact pressure on the back increased .
this is also 
 likely due to the posterior tilt of the pelvis and backwards shift in the center of gravity 
 during sacral sitting , as is shown by the positive correlation between the pelvic posterior 
 tilt angle and back contact area .
the act of pressing the trunk against the back support produces a counterforce that shifts 
 the buttocks forward12 .
the back support 
 converts backward angular momentum of the trunk produced by posterior pelvic tilt to shear 
 force in the forward direction13 , 14 . in this study ,
posterior pelvic tilt 
 markedly increased in sacral sitting compared with that in basic sitting , which may have 
 increased the force pressing the trunk against the back support and shear force generated by 
 sliding the ischial region forward , compared to basic sitting . in a previous study , the 
 shear force in the sacrococcygeal and ischial regions , measured on the seat surface of a 
 chair created specifically for the measurement , was shown to increase with an increase in 
 the posterior pelvic tilt angle in a sitting position8 . in the present study
, shear force was measured on the seat surface 
 of a wheelchair , and the forward force in the ischial region increased when adopting a 
 sacral sitting posture with the pelvis tilted backwards , confirming the previously published 
 finding . to reduce contact pressure and shear force and prevent pressure ulcers ,
it is recommended 
 that wheelchair users maintain proper posture and postural control , and that they sit on a 
 supportive surface appropriate for their individual needs7 .
the results of the current study indicate that the shear force load 
 on the ischial region increases with the increase in posterior pelvic tilt during sacral 
 sitting .
the decreases in contact pressure on the ischial region and maximum contact 
 pressure on the buttocks during sacral sitting may reduce the risk of pressure ulcers ; 
 however , the increased shear force generated by sliding the ischial region forward remains a 
 risk factor for ulcer development .
the present study used healthy young participants ; however , similar measurements should be 
 taken of frail older individuals who use a wheelchair daily , in order to verify that these 
 results are applicable to more typical wheelchair users . furthermore
, while a sacral sitting 
 posture was defined as sitting with the ischium slid 5  cm forward relative to its position 
 in the basic sitting posture , additional measurements should be performed at various 
 posterior pelvic tilt angles in future studies , in order to clarify the influences of these 
 angles on wheelchair users . in conclusion ,
sacral sitting in a wheelchair increases the maximum contact pressure on the 
 back , contact areas of the buttocks and back , and the shear force generated by sliding the 
 ischial region forward .
these factors may contribute to the development of pressure ulcers , 
 and improvements in wheelchair seating that reduce sacral sitting by frail older adults may 
 reduce the risk of ulcer development .
 [ purpose ] the purpose of this study was to investigate the influence of sacral sitting in 
 a wheelchair on contact pressure on the buttocks and back , and shear force on the ischial 
 region . 
 [ subjects and methods ] twenty - six healthy adults assumed two postures while seated 
 in a wheelchair : a basic sitting posture , and sacral sitting with the pelvis tilted 
 posteriorly and the ischium slid forward by 5  cm relative to its position in the basic 
 sitting posture . 
 the inclination angle of the sagittal pelvic line , contact pressures and 
 contact areas on the buttocks and back , and the shear force generated by sliding the 
 ischial region forward were measured . 
 [ results ] posterior pelvic tilt was significantly 
 greater during sacral sitting . 
 maximum contact pressures on the buttocks and ischial 
 region were significantly lower during sacral than during basic sitting . however , maximum 
 contact pressure on the back , the contact areas of the buttocks and back , and the shear 
 force generated by sliding the ischial region forward were significantly greater during 
 sacral sitting . [ conclusion ] 
 sacral sitting in a wheelchair increases the maximum contact 
 pressure on the back , contact areas of the buttocks and back , and the shear force 
 generated by sliding the ischial region forward . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION