most of the studies about the effects of carrying bags have focused on musculoskeletal pain 
 and altered spinal curvature . moreover although a backpack is symmetrically positioned as the load is 
 carried and distributed on both shoulders , the load changes body posture and balance through 
 posterior translation of the center of pressure3 . a number of backpack carrying studies have been conducted by the 
 military which have focused on the physiological , biomechanical , and medical aspects of 
 backpack carrying3 . the epidemiological 
 and clinical literature have identified a strong association between spinal posture and the 
 use of a backpack2 , 4 . carrying a backpack also causes postural changes which appear as 
 excessive forward head angle , and forward shoulder and changed scapular positions2 , 4 , 
 and researchers have studied various backpack types and designs with the aim of preventing 
 injuries associated with prolonged load carrying5 . in the present study , we investigated the effect of the spacing of 
 backpack shoulder straps on cervical muscle activity , acromion angle , scapular distance and 
 upper trapezius pain threshold after gait with a backpack . fourteen males , aged 2032  years , with a mean height and weight of 176.1  5.0  cm and 67.1 
  4.9  kg , respectively , participated in this study . the subjects had no history of 
 musculoskeletal disorders or pain associated with the upper extremity in the past 6 months . emg data were collected using a biopac mp100wsw ( biopac system , santa barbara , ca , usa ) . the 
 midcervical paraspinal ( mps ) electrodes were placed on 2  cm lateral to the midline of the 
 spine at approximately the c4 level . paul , mn , usa ) was used to measure the distance and inclination between two 
 bony landmarks of the body . the acromion angle was determined using the palm as the 
 inclination angle between the acromion and the c7 spinous process . the scapular inferior 
 distance was determined using the palm as distance from the inferior angle to the spinal 
 process using palm . a dolorimeter ( fabrication enterprises , white plains , ny , usa ) pressure 
 algometer was used to measure the upper trapezius ( ut ) pressure pain threshold . the shoulder 
 strap spacing was 20  cm for the narrow strap spacing and 30  cm for the wide strap spacing . we created wide and narrow strap spacings using two of the same type of backpack . the 
 subjects were asked to walk for 15  min on a treadmill at a speed of 1.5 m / s while carrying a 
 backpack containing a 10  kg load . the subjects adjusted the lengths of the shoulder straps 
 so that the positional centre was placed between the t11 and t12 levels4 . the statistical package for social sciences ( spss , chicago , 
 il , usa ) was used for statistical analyses . the paired t - test were used to analyze the 
 differences in mps activity , acromion angle , scapular distance , and the ut pressure pain 
 threshold after walking with a backpack with narrow and wide shoulder strap intervals . the 
 alpha level for statistical significance the mps emg activity was significantly decreased after walking with the wide shoulder 
 straps ( 16.8  8.7% ) compared to the narrow shoulder straps ( 26.2  6.3% ) ( p<0.05 ) . the 
 acromion angle was significantly increased after walking with the wide shoulder straps ( 18.6 
  6.2 ) compared to the narrow shoulder straps ( 12.1  5.5  ) ( p<0.05 ) . the scapular 
 inferior distance was significantly decreased after walking with the wide shoulder straps 
 ( 7.0  2.6  cm ) compared to the narrow shoulder straps ( 8.6  2.0  cm ) ( p<0.05 ) . the ut 
 pressure pain threshold was significantly decreased after walking with the wide shoulder 
 straps ( 6.0  1.8 lb ) compared to the narrow shoulder straps ( 8.6  2.0 lb ) ( p<0.05 ) . this study investigated the effect of different spacings of backpack shoulder straps on 
 cervical muscle activity , acromion and scapular position , and upper trapezius pain after 
 walking with a backpack . the different shoulder strap spacings changed the position of the 
 weight support of the backpack . the mps activity was significantly decreased when using the 
 wide spacing straps compared to the narrow interval straps . in terms of the passive 
 length tension relationship , the production of muscular tension can be accomplished by 
 adopting a lengthening position above the resting length6 . similarly , posture maintenance with a low muscular requirement can 
 be adopted using a backpack with a wide shoulder strap spacing . the backpack weight with the wide interval straps was 
 positioned laterally on the shoulders . the acromion angle was significantly increased and 
 the scapular inferior distance was significantly decreased with the wide shoulder strap 
 spacing . these results indicate that scapular depression and downward rotation occurred in 
 the subjects when they carried the backpack with the wide strap spacing . the ut pressure 
 pain threshold was also significantly decreased with the wide shoulder strap spacing 
 compared to the narrow shoulder strap spacing . we consider that the pain sensitivity of the 
 ut might be increased by scapular depression and downward rotation , causing weakness due to 
 overstretching of the ut7 , 8 . scapular depression syndrome is associated with lengthening 
 or weakening and a lower pressure pain threshold in the ut muscle9 . a prolonged time in the depressed scapular position , in 
 particular , may lead to chronic ut pain7 , 8 . a backpack with wide shoulder strap spacing 
 requires a lower muscular requirement of the cervical muscles , but wearing a backpack with 
 wide interval shoulder strap spacing may produce scapular depression syndrome and chronic ut 
 pain .
[ purpose ] this study investigated the effect of the spacing of backpack shoulder straps 
 on cervical muscle activity , acromion and scapular position , and upper trapezius ( ut ) 
 pain . 
 [ subjects ] fourteen males aged 2032  years , were recruited . 
 [ methods ] we measured 
 the mps ( midcervical paraspinal ) activity , acromial angle , scapular distance , and ut pain 
 after gait carrying a backpack with different shoulder strap spacings . [ results ] the mps , 
 scapular inferior distance , and ut pressure pain threshold was significantly decreased and 
 the acromion angle was significantly increased when carrying a backpack with wide shoulder 
 straps compared to narrow shoulder straps . 
 [ conclusion ] a backpack with wide shoulder 
 straps may cause scapular depression syndrome and chronic ut pain .