upper - limb pain and dysfunction are frequent complaints associated with manual wheelchair 
 propulsion , which is physically demanding and involves repetitive movements1 . about 73% of wheelchair users suffer from 
 chronic upper - limb pain , which is primarily attributed to two factors : wheelchair propulsion 
 and transfers2 . therefore , selecting the 
 appropriate wheelchair and seating system is one of the most important decisions for those 
 who spend long periods of time in their wheelchairs3 . active wheelchair users report shoulder , elbow , and wrist / hand 
 pain , chiefly shoulder pain4 . the chair 
 backrest is designed to decrease the stresses on the vertebral column by : allowing 
 relaxation of the erector spinae muscles , maintaining lumbar lordosis , and improving 
 comfort5 . while biomechanical variables 
 are important criteria for wheelchair users , they have not been studied sufficiently . prevention is the best way to reduce chronic pain , and researchers have suggested various 
 methods of preventing the development of problems during wheelchair propulsion . back belts 
 might have positive effects on sustaining an erect trunk and lumbar lordosis by supporting 
 the force from the knees , but they may secondarily lead to knee discomfort . it is also important to 
 investigate how the backrest thickness affects comfort , especially in a dynamic setting 
 during wheelchair propulsion . therefore , this study investigated the effects of backrest 
 thickness on the lumbar load and comfort by assessing the surface electromyography ( semg ) 
 activity of the shoulder muscles during wheelchair propulsion with lumbar support . all of the subjects were informed of 
 the study purpose and methods before participating , and provided their informed consent 
 according to the principles of the declaration of helsinki . our study consisted of three randomized backrest 
 conditions : no pad ; a 3-cm - thick lumbar pad ; and a 6-cm - thick lumbar pad . the lumbar pads of 
 the backrests used in this study had a density of 27  kg / m . the location of the 
 lumbar pad was standardized by aligning it at the mid - lumbar level ( l3 ) . they propelled the wheelchair 
 30 times with their hands , the upper body in an upright position at an average rate of once 
 per second . for stable propulsion , the semg activity of the upper extremity muscles was 
 collected , amplified , digitized , and analyzed using an me6000-biosignal monitor ( mega 
 electronics , kuopio , finland ) . the electrode locations were as follows : anterior deltoid , 
 anterior aspect of the arm , approximately 4  cm below the clavicle ; upper trapezius , slightly 
 lateral to and halfway between the cervical spine at c-7 and the acromion ; posterior 
 deltoid , 2  cm below the lateral border of the spine of the scapula and angled obliquely to 
 the arm ; and biceps brachii , attached at the middle muscle belly over the short and long 
 heads . the emg data were converted into root mean square ( rms ) values in a window comprising 
 300  ms of data . we obtained the reference voluntary contraction ( rvc ) using the submaximal 
 normalization method . spss 22.0 ( spss , chicago , il , usa ) was used to analyze the differences 
 in shoulder muscle activities . smirnov test was performed to test for a 
 normal distribution before using parametric statistics . the significance of differences 
 among wheelchair propulsion with no lumbar pad , the 3-cm lumbar pad , and the 6-cm lumbar pad 
 was tested using one - way repeated - measures analysis of variance ( anova ) . for the significant main 
 effect , the bonferroni correction was used to identify the specific mean differences . the normalized emg data obtained while performing the wheelchair propulsion task with no 
 pad , a 3-cm - thick lumbar pad , and a 6-cm - thick lumbar pad revealed significant differences 
 in the activation of the anterior deltoid , upper trapezius , posterior deltoid , and biceps 
 brachii muscles . muscle activation of the anterior deltoid , upper trapezius , and biceps 
 brachii muscles significantly decreased when the 3-cm pad was used compared to none , and 
 significantly increased in the anterior deltoid , upper trapezius , posterior deltoid , and 
 biceps brachii when the participants used the 6-cm pad compared to the 3-cm pad . there were 
 no significant differences in muscle activation between the no pad and the 6-cm lumbar pad 
 conditions ( table 1table 1.muscle activations during wheelchair propulsion with the different lumbar pad 
 conditions ( n=15)muscle%rvc ( meansd)none3-cm pad6-cm padanterior deltoid70.1322.1549.8116.8266.0221.72upper trapezius100.2218.2083.8215.33110.8220.53posterior deltoid105.3123.7195.8516.77100.1818.90biceps brachii110.8916.3590.3015.19118.7013.49p<0.05 ) . many studies have indicated that there is a relationship between wheelchair propulsion and 
 upper limb injuries1 , 8 . previous study has shown that a lumbar support increased ( or 
 preserved ) the comfort while sitting . especially , it indicated that a 23 change in lumbar 
 posture influences the compressive load at l4l5 when performing spinal loading work9 . our present results show that there was a 
 significant decrease in the activation of the anterior deltoid , upper trapezius , and biceps 
 brachii muscles when the participants used the 3-cm pad compared to no pad , while the 
 activation of the anterior deltoid , upper trapezius , posterior deltoid , and biceps brachii 
 significantly increased when the participants used the 6-cm pad compared to the 3-cm pad . generally , 
 the push phase mainly uses the anterior deltoid , pectoralis major , and biceps brachii , 
 whereas the recovery phase primarily uses the upper trapezius and middle and posterior 
 deltoid10 , 11 . one possible explanation for our result is the effect of muscle 
 activation on the propulsion efficiency when performing wheelchair propulsion with the 3-cm 
 pad compared with no pad or a 6-cm pad . it has also been reported that an appropriate lumbar 
 pad provides a biomechanical advantage to the shoulder during seated work . a decrease in 
 muscle load might elicit the optimal curvature of the lumbar spine when propelling a 
 wheelchair . this suggests that although users might not benefit from a lumbar pad to improve 
 their temporal muscle load , long - term users develop musculoskeletal shoulder pain . goosey et 
 al . indicated that a lower propelling frequency could explain the effectiveness of pushing 
 economy . a high frequency is associated with more shifts during the deceleration and 
 acceleration phases , and increased inertial limb moments . as a result , a lower frequency 
 leads to less muscle activity , possibly reducing rotator cuff fatigue , while maintaining the 
 stability of the humeral head4 , 12 , 13 . in this study , the backrest was rated as more comfortable , and it had better dampening capability , which 
 allowed leverage in the contact area between the lumbar spine and backrest during wheelchair 
 propulsion . the results of our present study confirm that an appropriate lumbar pad allows 
 more effective coordination of the shoulder muscles and increases comfort during wheelchair 
 propulsion .