shock absorption , stability and 
 propulsion are the main biomechanical functions of the foot .
measurement of foot pressure 
 distribution ( fpd ) is clinically useful for evaluation of foot and gait pathologies1 , 2 .
individuals with flatfoot may be at increased risk for the development of many lower 
 extremity overuse injuries including metatarsal stress fractures , iliotibial band syndrome , 
 and patellofemoral pain syndrome3 . 

mechanical overloading in flatfoot has been attributed primarily to excessive motion and 
 muscle activity .
runners with low arches are at increased risk for developing second and 
 third metatarsal stress fractures , and have an increase in rearfoot eversion velocity and 
 eversion excursion4 , 5 .
backpack load carriage also leads to higher trunk forward lean 
 compared with the normal gait .
pascoe et al.9 reported that the most common symptoms 
 associated with overweight backpacks were muscle soreness , back pain , numbness , and shoulder 
 pain .
it is widely believed that a person with a postural type of flatfoot has increased 
 susceptibility to mechanical overloading of their foot structure10 .
study of the ground reaction forces ( grf ) during load 
 carriage can provide relevant information about the mechanisms of gait , and provide a 
 measure of the impact forces acting on the foot .
it is therefore essential in the 
 understanding and prevention of lower extremity injuries in flatfoot .
however , most of the 
 research about the effect of wearing backpacks in adults focuses on use of backpacks for 
 recreation or how carrying heavy loads affects military personnel7 , 11 , 12 .
there are no previous studies that have examined changes 
 in flatfoot subjects during gait with different backpack weights .
the purpose of this study was to investigate the changes in muscle activation of the trunk 
 and lower extremities and plantar foot pressure with backpack loads of 0 , 10 , 15 , and 20% of 
 body weight while level walking in flatfoot individuals and to recommend suitable backpack 
 weight limitations for flatfoot subjects .
it was hypothesized that changes in backpack 
 weight would lead to different muscular and foot reactions in flatfoot .
twelve young flatfoot subjects ( as determined with rs scan system as described below ) and 
 14 normal foot subjects participated in this study .
an explanation of the study was given , 
 and all of the subjects consented to voluntary participation in the experiment .
this study 
 proceeded after review and approval by the clinical research and ethics review committee of 
 the national evidence - based healthcare collaborating agency .
the exclusion criteria included 
 recent injury , postural deformities , spine surgery , history of low back pain and major 
 surgery during the last 6 months . in each session , the subjects were assigned to carry four different backpack loads : ( 1 ) 
 unloaded walking ( 0% ) , ( 2 ) 10% body weight ( bw ) load , ( 3 ) 15% bw load , and ( 4 ) 20% bw load . 

trunk and lower 
 extremity muscle activities were recorded by surface emg , and contact area and plantar foot 
 pressure were determined using a rs scan system .
first , the rectus abdominis ( ra ) , erector spinae ( es ) , vastus medialis ( vm ) , biceps femoris 
 ( bf ) , tibialis anterior ( ta ) , and gastrocnemius medialis ( gm ) muscle activities were 
 recorded using surface emg ( delsys inc . ,
after standard preparation , 
 disposable silver / silver - chloride bipolar surface electrodes were attached to the muscle 
 bellies . before electrode attachment
, the skin surface was slightly abraded with sand paper 
 and wiped with rubbing alcohol to facilitate better attachment with reduced skin - electrode 
 impedance .
( data gain 1,000 , band - pass filter 20450  hz , cmrr > 100 db at 60  hz , input 
 impedance > 100 mx ) .
plantar foot pressure was measured by using an rsscan system ( rsscan deutchland , leipzig , 
 germany ) .
the participants were asked to walk barefoot at a comfortable speed on a 2-m - long 
 plate .
data regarding the pressure distribution while walking were collected at a rate of 
 126 frames / sec using the footscan 7 gait 2nd generation software , which is a commercial 
 program for the rsscan system . for each trial ,
two - way anova was performed on the parameters of muscle activities and plantar foot 
 pressure for walking with all backpack weights . to identify the main significant effect ,
all statistical tests were 
 performed with the spss 21.0 statistical analysis software , and results were considerd 
 significant at p<0.05 .
there were 26 subjects ( 14 with flatfoot/12 with normal foot ) ; and their general 
 characteristics are shown in table 1table 1 .
general characteristics of subjectsflatfoot group ( n=14)control group ( n=12)age ( yrs)22.0  0.723.1  0.7height ( cm)163.8  1.7168.8  2.2weight ( kg)57.1  3.364.2  2.1gender ( m / f)8/67/5values are mean  se . there were no statistically significant differences between the two groups 
 ( p>0.05 ) .
comparison of emg activities of trunk and lower extremity muscles during the 
 different gait conditions in both groupsmusclegroup0%10%15%20%vmflatfoot*35.2  4.644.8  5.746.2  5.747.1  6.7normal*33.6  3.435.2  6.738.7  3.945.7  2.8gmflatfoot*29.0  4.633.8  5.336.1  5.337.8  5.9normal*27.6  2.731.7  3.534.5  2.636.7  4.5taflatfoot*37.9  5.544.8  6.540.3  4.630.3  7.1normal*39.4  4.237.7  2.640.8  6.841.0  3.6bfflatfoot23.3  2.824.4  3.729.0  4.527.2  3.4normal25.6  3.824.8  2.826.7  7.226.2  2.3esflatfoot*36.3  3.541.5  4.245.3  4.946.3  5.6normal38.6  6.337.5  5.139.6  4.041.2  3.4raflatfoot18.7  1.323.0  2.822.2  3.225.1  2.7normal20.1  3.823.1 
* p<0.05 shows the emg activities of trunk and lower extremity muscles during the 
 different gait conditions . in the flatfoot group
, there were no significant changes during 
 the different walking modes in the bf and ra ( p>0.05 ) .
however , the es , vm , ta , and gm 
 muscle activities were significantly and progressively as the load increased ( p<0.05 ) . in 
 the normal foot group , there were no significant changes in the bf , ra , and es ( p>0.05 ) , 
 but the vm , ta , and gm muscle activities were significantly increased ( p<0.05 ) . unit : % mvic .
p<0.05 the contact area of forefoot , midfoot , and hindfoot were significantly and progressively 
 different with load in both groups ( p<0.05 ) ( table 
 3table 3
. comparison of contact area during the different gait conditions in both 
 groupsregiongroup0%10%15%20%forefootflatfoot*51.6  0.750.6  0.850.6  0.649.8  0.7normal*50.7  1.850.4  0.749.7  1.648.7  0.5midfootflatfoot*28.2  0.728.5  0.929.6  1.030.3  1.0normal*27.4  2.126.7  1.327.8  1.028.4  2.1hindfootflatfoot*20.7  0.520.6  0.519.8  0.419.2  0.4normal*21.9  1.124.1  3.122.5  1.822.9  2.1values are means  se .
regarding the peak pressure , only the pressures of lateral and medial heel 
 zones of flatfoot subjects were significantly and progressively increased as the load 
 increased ( p<0.05 ) ( table 4table 4 .
comparison of peak pressure during the different gait conditions in both 
 groupsregiongroup0%10%15%20%toe1flatfoot31.5  4.236.9  6.543.6  4.839.4  5.3normal35.4  5.434.7  6.240.5  5.442.7  6.0toe25flatfoot12.4  4.610.9  1.710.7  1.613.7  2.9normal10.3  3.413.1  3.411.2  0.511.8  3.6meta1flatfoot26.1  4.529.2  5.730.1  4.235.6  4.7normal28.0  2.123.2  4.826.1  3.421.2  4.9meta2flatfoot*38.0  4.658.2  9.054.7  6.154.7  5.9normal43.1  2.845.8  3.146.7  4.849.1  2.1meta3flatfoot43.4  6.761.1  10.348.1  6.254.2  6.4normal45.8  6.750.7  6.856.0  5.151.2  4.1meta4flatfoot19.1  4.641.0  11.127.5  6.832.6  6.6normal20.1  5.126.1  4.431.7  9.734.1  5.0meta5flatfoot10.9  4.820.8  5.718.2  6.115.4  6.3normal13.2  5.117.2  3.719.2  6.718.4  5.5midfootflatfoot4.3  0.96.0  1.46.5  1.78.3  2.8normal5.7  5.15.4  6.77.5 
2.08.6  3.1medial heelflatfoot*44.2  5.153.8  8.255.4  6.465.0  6.6normal46.4  6.749.8  4.848.7  6.042.5  7.1lateral heelflatfoot*36.6  5.945.5  6.349.2  5.951.7  6.6normal40.2  3.843.4  5.844.7
the objective of this study was to measure the changes in muscular activity of the trunk 
 and lower extremities and in sole pressure by applying walking loads as a proportion of body 
 weight to patients with the flatfoot condition .
flatfoot can cause pain , muscular fatigue , sprains at the articular capsule and ligament , 
 and foot imbalance during daily living , including sports activities13 .
it can also cause changes in the overall body position 
 and increase the risk of back pain due to straining of muscles and effects on the pelvis , 
 lower extremities , and even the spine in closed - chain exercise14,15,16 . however , there is little research focused on 
 load - dependent muscular activity during walking with flatfoot subjects . in a study on normal foot subjects ,
the muscular activity and trunk position were shown to 
 depend on the weight of a backpack6 . 

investigation of the muscular activity of the rectus , erector , biceps femoris , and vastus 
 medialis muscles showed that the muscular activity was significantly increased only in the 
 rectus abdominis muscle when loads of 10% , 15% , and 20% of body weight were applied ; no 
 change was observed in other muscles .
this finding was attributed to the biomechanical 
 response whereby the rectus abdominis muscle activity increases to compensate for the center 
 of gravity ( cog ) movement as the body s cog moves backward due to the backpack , . in 
 contrast , others compared muscular activity and sole pressure measurements of flatfoot and 
 normal foot subjects and reported that the muscular activities of the tibialis anterior , 
 gastrocnemius , peroneus brevis , and peroneus longus muscles were significantly higher in the 
 flatfoot subjects than in the normal subjects during the swing phase and stance phase17 . similarly , hunt et al.10 compared flatfoot and normal foot subjects 
 in the stance phase and reported that muscular activity was significantly higher in the 
 tibialis anterior , gastrocnemius , and soleus musclesof the flatfoot subjects . in this 
 present study , the activities of the muscles used for walking increased as the load , as a 
 proportion of body weight , increased .
the increased activity was particularly significant at 
 the vastus medialis , gastrocnemius , tibialis anterior , and erector spinae muscles .
however , 
 even though the rectus abdominis muscle activity was increased in response to the weight 
 load in the flatfoot persons , this increase was not statistically significant when compared 
 with the results of another study6 .
the 
 reason for this difference may be because we measured muscular activity during walking , 
 whereas the other researchers measured muscular activity in the upright position .
additional 
 studies should be carried out in the future to resolve this discrepancy . the contact area in flatfoot individuals during walking increased at the midfoot in 
 response to increasing weight loads , whereas it was significantly reduced at the forefoot 
 and hindfoot .
this result was similar to the results of a study that measured plantar 
 pressure during walking and running depending on foot types18 .
that study showed that the contacting area and the maximum force 
 were greater at the midfoot but smaller at the forefoot , in flatfeet compared with normal 
 feet .
however , the results of this study indicate a difference in loading in the forefoot , 
 with a significant decrease in load in the lateral forefoot in individuals with flatfoot , 
 which is in contrast to the results presented by sneyers et al.19 that indicated no significant medial shift in forefoot 
 loading . the maximum pressure at each region during walking tended to be greater as the load 
 increased , but a significant difference was found only for the medial and lateral heel 
 regions
this may be because the maximum pressure at the heel was increased due to the 
 greater impact during the initial contact phase .
this finding is similar to the result 
 reported by de cock et al.20 , who showed 
 that the maximum pressure at the heel became at least two times higher in response to 
 increases in walking speed . while plantar pressure and plantar loading are useful tools to investigate biomechanical 
 factors influencing foot and ankle pathology , it is important to consider the limitations of 
 such data17 , 19 , 21 . in the current study , 

the variations within the subject sample were minimized by including only young adult 
 subjects with no history of foot and ankle injury or pain .
the changes in muscular activity 
 in the trunk and lower extremities and in the sole pressure were investigated in flatfoot 
 with respect to increases in weight load .
muscular activity , contact area , and plantar 
 pressure were significantly increased as the weight load was increased as a proportion of 
 the body weight .
excessive weight load increased the muscular tension in the trunk and lower extremities , 
 resulting in a concentration of pressure on the midfoot in the flatfoot condition .
these 
 biomechanical changes may lead to musculoskeletal symptoms in lower extremities and the 
 feet .
therefore , this research data will be useful in producing overload risk preventative 
 measures .
   [ purpose ] the purpose of this study was to investigate the changes in muscle activation 
 of the trunk and lower extremities and plantar foot pressure due to backpack loads of 0 , 
 10 , 15 , and 20% of body weight during level walking in individuals with flatfoot . 
 [ methods ] fourteen young flatfoot subjects and 12 normal foot subjects participated in 
 this study . in each session , the subjects were assigned to carry a backpack load , and 
 there were four level walking modes : ( 1 ) unloaded walking ( 0% ) , ( 2 ) 10% body weight ( bw ) 
 load , ( 3 ) 15% bw load , and ( 4 ) 20% bw load . trunk and lower 
 extremity muscle activities 
 were recorded by surface emg , and contact area and plantar foot pressure were determined 
 using a rs scan system . 
 [ results ] the erector spinae , vastus medialis , tibialis anterior 
 and gastrocnemius muscle activities , but not the rectus femoris and rectus abdominis 
 muscle activities of flatfoot subjects significantly and progressively increased as load 
 increased in flatfoot subjects . 
 contact area and pressure of the lateral and medial heel 
 zones were significantly increased too . 
 [ conclusion ] based on this data , the weight of a 
 backpack could influence muscle activation and plantar foot pressure in flatfoot . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION