wheelchairs are important chairs that provide structural support for the trunk , pelvis , and 
 extremities in wheelchair users1 . proper 
 structural support allows them to perform activities of daily living under comfortable 
 conditions2 . of the various support 
 systems , an unsuitable backrest thickness 
 increases the load on some shoulder muscles and can lead to shoulder pain during wheelchair 
 propulsion4 . previous studies have 
 indicated that a proper backrest for a wheelchair can help reduce lumbar load and activation 
 of the upper extremity muscles4 , 5 . these results have important implications for maintain 
 active lifestyles providing function , comfort , and support for manual wheelchair users6 . therefore , the therapist must consider the 
 most suitable backrest when evaluating and recommending a wheelchair for patients . previous studies have examined the suitability of a backrest by measuring shoulder or trunk 
 muscles with surface electromyography ( semg ) . however , the use of semg has the limitations 
 of only providing data for a limited number of channels and only being able to scan the 
 movements of a few muscles groups . also , only movements made in a certain direction can be 
 analyzed when using emg7 . an accelerometer 
 is a subjective assessment tool that can detect acceleration and deceleration in response to 
 movements in one , two , or three directions ( uni- , bi- , or triaxial accelerometers ) as well 
 as energy expendixure8 . also , an 
 accelerometer is a valid and reliable tool for monitoring the level of various 
 activities9 . muscle movement changes 
 dependent on backrest thickness can be detected by using accelerometers attached to the 
 trunk and arm during arm propulsion10 , 11 . thus , measurement using accelerometers 
 may allow a selection of a suitable backrest thickness for wheelchair users . therefore , the purpose of this study was to investigate the effects of backrest thickness 
 on the trunk and shoulder muscles by assessing acceleration using accelerometers during 
 wheelchair propulsion . fourteen healthy people participated in this study . participants with upper extremity pain 
 or neuromuscular disorder were excluded . all participants were informed of the study s 
 purpose and procedures , and all signed informed consent forms voluntarily . the backrests used in this study had no lumbar 
 pad , a 3-cm - thick lumbar pad , or a 6-cm - thick lumbar , and the density of the lumbar pads was 
 27  kg / m . mma7260q the sensitivity of the accelerometers ranges from 6 to + 6 g. data for 
 measurement and storage in this study were set at 100  hz . we calibrated the single vector 
 magnitude ( svm ) by summing the acceleration of the three axes . the three backrest conditions 
 were tested in random order , and the lumbar pads were positioned such that they were aligned 
 at the mid lumbar level ( l3 ) . the two accelerometers were attached along the right upper arm 
 and lateral trunk using velcro straps . participants were asked to propel their own manual 
 wheelchairs 30 times under the three backrest conditions . using one - way repeated - measures analysis of variance ( anova ) , we compared the differences 
 in the levels of activities of the upper arm and trunk muscles according to lumbar pad 
 thickness . all data were 
 analyzed with a level of statistical significance of p<0.05 using the ibm spss 
 statistics , version 22.0 , software ( ibm corp . the results showed that the svm for the muscle activities with the 3-cm - thick lumbar pad 
 was significantly lower than that with no lumbar pad ( no lumbar pad , 35,350  
 3,652  cm / s ; 3-cm - thick lumbar pad , 30,600  3,855  cm / s ) 
 ( p<0.05 ) . also , the svm for the 6-cm - thick lumbar pad was significantly higher than that 
 for the 3-cm - thick lumbar pad ( 6-cm - thick lumbar pad , 37,640  3,769  cm / s ; 
 3-cm - thick lumbar pad , 30,600  3,855  cm / s ) ( p<0.05 ) . on the other hand , 
 there was no significant differences in svm between no lumbar pad and the 6-cm - thick lumbar 
 pad ( p>0.05 ) . this study compared the differences in muscle activity of the trunk and upper arm according 
 to backrest thickness during wheelchair propulsion . for measuring muscle movements , we used 
 two accelerometers attached to the trunk and upper arm . the svm calculated from the 
 accelerometer measurements represented the actual intensity of movement of the trunk and 
 upper arm12 . the results of this study 
 indicated that among the three types of wheelchair backrest , the intensities of movement of 
 the upper arm and lateral trunk were decreased when participants used a wheelchair equipped 
 with a 3-cm - thick lumbar pad compared with no lumbar pad or a 6-cm - thick lumbar pad . it seems that the intensity of muscle 
 movement assessed by accelerometer is correlated with the emg amplitude of the proximal and 
 distal muscles7 . moreover , this 
 correlation also indicates that the timing variables that are changed are the push time and 
 recovery time , as increases in these times require more energy expenditure13 . , svm increases may be due to a less than optimal position 
 during wheelchair propulsion . consistent with this hypothesis , use of an appropriate 
 backrest enabled less expenditure of energy through effective arm and trunk movement14 . a previous study also showed that 
 suitable backward thoracic support can help an individual maintain a comfortable wheelchair 
 sitting posture , preventing or reducing the risks of back pain15 . also , shoulder muscle loads during manual propulsion by 
 wheelchair users can be decreased by using a suitable or adjustable backrest16 . these results indicate that an 
 appropriate backward support position can maintain neutral pelvic tit and lumbar 
 lordosis3 and provide a biomechanical 
 advantage to the shoulder4 . the use of 
 accelerometers in this study is very significant , especially considering that previous 
 studies have used accelerometers for monitoring physical activity or wheelchair 
 movement17 , 18 . accelerometers make it easy to measure variables such as movement 
 time and peak velocity , and it is also easy to obtain data such as emg records7 , 19 . 
 in the future , we must consider the appropriate backrest thickness for wheelchair users , and 
 assessment methods using accelerometers can provide feedback for appropriate wheelchair 
 measurement for wheelchair users .
[ purpose ] the purpose of this study was to investigate the effects of the thickness of a 
 wheelchair backrest provided for support and comfort on upper arm and trunk muscle load 
 during wheelchair propulsion by using accelerometers . 
 [ subjects and methods ] the fourteen 
 healthy participants were enrolled in this study . 
 the study compared effects of three 
 backrest conditions including no pad , a 3-cm - thick lumbar pad , and a 6-cm - thick lumbar 
 pad . 
 the instruments used for measurement were used two accelerometers . 
 the participants 
 were asked to propel their wheelchairs , which had been equipped with two accelerometers , 
 30 times . 
 [ results ] the intensity of muscle movement with the 3-cm - thick lumbar pad was 
 significantly lower than the intensities with no lumbar pad and the 6-cm - thick lumbar pad . 
 the muscle intensity did not differ significantly between the no pad and 6-cm - thick lumbar 
 pad conditions . 
 [ conclusion ] an appropriately thick backrest has good effects on upper arm 
 and trunk muscles during wheelchair propulsion . in the future , we must consider the 
 appropriate backrest thickness for providing wheelchair users with a comfortable 
 wheelchair .