a flexible flat foot refers to a structural deformity in which the medial longitudinal foot 
 arch has become abnormally depressed under weight - bearing conditions1 , 2 . due to the flat 
 foot ,
the loading on the foot can not be properly distributed , and more activities of the 
 intrinsic and extrinsic muscles are needed1 . in a pronation deformity , where the valgus position is combined 
 with the pronation of the subtalar joint , the talus , navicular , and first cuneiform bone are 
 placed downward and medially3 , and a 
 biomechanical change occuress2 . 

furthermore , flat foot causes the hyper - adduction of the knee joint as well as the foot 
 deformity , leading to the general structural deformity of the lower extremity4 . among the lower extremity muscles , the medial gastrocnemius ( mg ) and the long head of the 
 biceps femoris muscle ( bf ) located in the posterior part on the frontal plane , and the 
 tibialis anterior muscle ( ta ) and the rectus femoris muscle ( rf )
as they have the continuity of myofascial meridian , they are mutually involved in 
 posture and balance5 and are activated 
 during walking5 , 6 . however
, due to the influence of the flat foot , the lower extremity 
 muscle activity becomes higher or lower than that of the normal foot during walking or when 
 standing on one leg6 , 7 .
most studies on the flexible flat foot have used electromyography ( emg ) to investigate the 
 structural deformities2,3,4 of feet and changes in the 
 activities of the lower extremity muscles during walking7,8,9 .
recently , studies have used myotonometry to compare muscle tone and 
 stiffness between healthy subjects and neurologically impaired patients10,11,12 .
however , no studies have investigated the differences in 
 muscle tone and stiffness of the flexible flat foot . in this study , therefore , differences in the muscle tone and stiffness in both the flexible 
 flat foot and the one - side flexible flat - foot types of both lower extremities , compared to 
 the normal foot type in adults in their twenties , were analyzed to provide basic data for 
 physical therapy and evaluation .
this study was conducted with 30 subjects in their twenties who were students of the yeoju 
 institute of technology in yeoju , gyeonggi - do .
the subjects consisted of 10 in the normal 
 foot group ( nfg ) , 10 in the group with both flexible flat feet ( bffg ) , and 10 in the 
 one - side flexible flat - foot group ( offg ) .
those who had no problem in the vestibular system 
 and the neurological and musculoskeletal system and who had not done any regular exercise 
 for the last six months were selected .
we explained the purpose and method of this study to 
 the subjects , and only those who consented to participate were included in this study ( table 1table 1.general characteristics of subjectscharacteristicsnormal foot goup(n = 10)both flexible flat foot(n = 10)one side flexible flat foot(n = 10)gendermale354female756age ( years)21.2  1.121.10  1.421.1  1.5weight ( kg)60.2  10.369.0 
 10.8foot calibration(pixel / pixel)dominanant : 0.3  0.0non - dominant : 0.3  0.0dominant : 0.41  0.0non - dominant : 0.41  0.0flexible flat foot : 0.3  0.0normal foot : 0.3  0.0dominant legright ( 10)left ( 3 ) ,
right ( 7)right ( 10)flexible flat footleft ( 0 ) , right ( 0)left ( 10 ) , right ( 10)left ( 6 ) , right ( 4)meansd ) .
the study protocol was approved by the local ethics committee of the 
 namseoul university of cheonan ( nsu-150429 - 7 ) . in this study ,
the global posture system 400 ( gps 400 , redbalance , italy ) was used to 
 determine the flexible flat foot .
if the length of the narrowest region of the foot exceeded 0.37 
 ( pixel / pixel ) of the length of the widest region in the gps system , the foot was classified 
 as a flat foot13 .
based on previous 
 studies7 , 14 , the feet were also analyzed using strake s line and marie s line . 

strake s line refers to the line from the medial line of the fore - foot to the medial line of 
 the rear foot .
if the line of the medial sole falls inside marie s line ( the line from the 
 center of the third metatarsal bone to the center of the rear foot ) , it is a flat foot .
when 
 both of these two conditions are met , the foot is classified as flexible flat foot and 
 marie s line .
strake s line refers to the line from the medial line of the fore - foot to the 
 medial line of the rear foot . if the line of the medial sole falls inside marie s line ( the 
 line from the center of the third metatarsal bone to the center of the rear foot ) , it is a 
 flat foot . when both of these two conditions are met , the foot is classified as flexible 
 flat foot .
the myotonpro ( myotonas , estonia ) was used measure the muscle tone and 
 stiffness of the subjects .
this device can quickly and safely measure the muscle tone , 
 elasticity , and stiffness of various body regions with high reliability10 , 11 , 15 .
every measurement was made in a quiet , isolated room , and 
 the room temperature was 25.5  c . before measurement , the highest regions of the muscle 
 belly of the bf , mg , rf , and
the muscle tone and 
 stiffness were measured with the measurement device positioned vertically on the skin marker 
 in a relaxed condition in a prone position for the bf and mg and in a supine position for 
 the rf and ta12 .
all the data were encoded and analyzed using the statistical analysis program spss win 
 ( ver .
the average and standard deviation of the general characteristics ( age , weight , 
 and height ) were calculated using descriptive statistics for each foot type .
the homogeneity 
 was tested with one - way anova , and no significant differences were found ( p > 0.05 ) . for each foot type
, the offg was reclassified into a flexible flat - foot side and a normal 
 foot side , and the bffg was reclassified into dominant foot and non - dominant foot . in the 
 case of the offg , the wilcoxon signed - rank test was used to compare the differences between 
 the flexible flat - foot side and the normal foot side , and the differences in the muscle tone 
 and stiffness of the dominant foot and non - dominant foot , compared to the bffg and the nfg . 

in addition , the kruskal - wallis h test was used to analyze the difference in muscle tone and 
 stiffness between the dominant leg of the bffg and nfg and the flexible flat - foot side of 
 the offg .
to investigate the differences in muscle tone and stiffness by foot type , the dominant leg 
 of the nfg and bffg and the flexible flat - foot side of the offg ( one - side flexible flat 
 foot ) were compared . in the measurement results , only the ta and mg stiffness of the nfg and 
 bffg showed a significant difference ( p < 0.05 ) . although it was insignificant , the 
 muscle tone and stiffness was highest in the bffg , followed by the offg and nfg . 

furthermore , in the nfg , the non - dominant leg showed greater muscle tone and stiffness , on 
 average , than the dominant leg , although the difference was not significant .
the comparison 
 in muscle tone and stiffness between the normal foot side and the flexible flat - foot side in 
 the offg showed no significant difference ( table 
 2table 2.muscle tone and stiffness on each foot typestypemusclesidemuscle tone ( hz)stiffness ( n / m)normal footrectus femorisdominant leg14.1  0.3245.5  9.0non - dominant leg14.0  0.2240.2  7.3tibialis anteriordominant leg19.0  0.8380.6  17.5non - dominant leg19.3  0.6393.5  14.3medial gastrocnemiusdominant leg15.6  0.4253.7  10.7non - dominant leg15.7  0.5254.3  10.9biceps femoris ( long head)dominant leg14.7  0.5236.7  15.8non - dominant leg15.1  0.7244.5  19.1both flexible flat footrectus femorisdominant leg15.3  0.4269.3  10.3non - dominant leg15.2  0.5264.1  13.1tibialis anteriordominant leg20.6  0.8436.7  17.7*non - dominant leg20.5  1.0450.5  26.1medial gastrocnemiusdominant leg16.6  0.3277.0  6.7non - dominant leg16.5  0.3279.4  6.8biceps femoris ( long head)dominant leg15.9  0.4274.8  12.8non - dominant leg16.0  0.3281.7
 0.8421.9  27.9medial gastrocnemiusflexible flat foot15.8  0.5263.1  8.5normal foot15.6  0.5254.9  11.0biceps femoris ( long head)flexible flat foot15.3
* significant difference between normal foot and both flexible flat foot in tibialis 
 anterior of dominant leg .
significant difference between normal foot and both flexible flat foot in 
 medial gastrocnemius of dominant leg ) .
* significant difference between normal foot and both flexible flat foot in tibialis 
 anterior of dominant leg .
significant difference between normal foot and both flexible flat foot in 
 medial gastrocnemius of dominant leg
the flat foot causes generally negative changes to the body due to the structural 
 deformity4 of the lower extremity joint , 
 changes in muscle activity7,8,9 , biomechanical 
 changes2 , and decreased muscular 
 strength and balance16 .
caillet et al.17 claimed that the 
 stiffness of the ankle dorsiflexor was associated with difficulty during walking due to an 
 asymmetric posture and the loss of balance and motor control .
therefore , an examination of 
 the stiffness of muscles in the flexible flat foot is a critical part of treatment and 
 evaluation . in this study ,
a comparison of the muscle tone and stiffness of both lower extremity 
 muscles by foot type indicated a significant difference in the stiffness of the ta and mg 
 muscles of the dominant leg between the nfg and the bffg ( p < 0.05 ) . the muscle tone and 

stiffness was the highest in the bffg , followed by the offg and nfg , although the 
 differences were insignificant .
this means that the muscle tone and stiffness of the flat 
 foot are higher than those of the normal foot .
furthermore , a comparison of the muscle tone 
 and stiffness of the normal foot side and the flexible flat - foot side in the offg revealed 
 that the muscle tone and stiffness of the mg were higher on the flexible flat - foot side , 
 whereas the muscle tone and stiffness of the rf , ta , and bf were higher on the normal foot 
 side , although the differences were insignificant .
it seems that the differences were 
 insignificant because the muscle tone and stiffness were measured in a relaxation condition . 

the differences in muscle tone and stiffness could increase during physical activities such 
 as standing or walking . in the case of the nfg ,
the muscle tone and stiffness of the rf were 
 higher on average in the dominant leg , and those of the ta , mg , and bf were higher in the 
 non - dominant leg , however , the differences were insignificant .
the reason for this seems to 
 be that the frequency of raising the dominant leg with the support of the non - dominant leg 
 is higher . among the studies on emg ,
lee et al.4 
 compared the muscle activity between the normal foot and the flat foot when standing on one 
 leg and found that the muscle activities of the mg , ta , and vastus medialis ( vl ) of the flat 
 foot were lower , whereas the muscle activity of the rf of the flat foot was higher , however , 
 the differences were insignificant .
only the muscular activity of the abductor hallucis 
 muscle of the normal foot was significantly higher than that of the flat foot , suggesting 
 the importance of the intrinsic foot muscles .
therefore , more research on the intrinsicfoot 
 muscles is required in the future . in a study that analyzed the muscle activity of the flexible flat foot using the root mean 
 square ( rms ) of the emg , vittore et
al.18 
 mentioned that the ta needs to be strengthened during the rehabilitation of flat foot , 
 because the more severe the flexible flat foot was , the lower the muscle activity of ta in 
 the supine and orthostatic position became . in this study , however , the muscle tone and 
 stiffness of the lower extremity muscles of the flat foot were higher compared to those of 
 the normal foot .
the reason for this seems to be that the lower muscle activity increased 
 the muscle tone and stiffness . in this study , only the stiffness of the ta and mg muscles of the nfg and bffg showed 
 significant differences , but on average , the muscle tone and stiffness of both the lower 
 extremity muscles of the flexible flat foot were higher than those of the normal foot .
these 
 changes will be a negative factor in walking or other physical functions , and in cases of 
 older age or injury to the lower extremity , the function of the lower extremity will become 
 lower , and more neuromuscular control will be required .
for the limitations of this study , the findings can not be generalized to all ages , because 
 the subjects were in their 20s , and differences by gender were not examined .
furthermore , 
 this study could not be compared to many previous studies , because there were few previous 
 studies about the muscle tone and stiffness of the flexible flat foot .
however , the findings of this study reveal the need for an adjustment of the muscle tone 
 and stiffness of a flexible flat foot during physical therapy , and the results of this study 
 can be used as basic data for various physical therapies such as strengthening exercise , 
 therapeutic massage , and myofascial release .
 [ purpose ] the aim of this study was to examine differences in the muscle tone and 
 stiffness of leg muscles according to types of flexible flat foot . [ subjects and methods ] 
 for 30 subjects 10 in a normal foot group ( nfg ) , 10 in group with both flexible flat feet 
 ( bffg ) , and 10 in a group with flexible flat feet on one side ( offg ) , myotonometry was 
 used to measure the muscle tone and stiffness of the tibialis anterior muscle ( ta ) , the 
 rectus femoris muscle ( rf ) , the medial gastrocnemius ( mg ) , and the long head of the biceps 
 femoris muscle ( bf ) of both lower extremities . 
 [ results ] in the measurement results , only 
 the stiffness of ta and mg of the nfg and the bffg showed significant differences . 
 the 
 muscle tone and stiffness were highest in the bffg , followed by the offg and nfg , although 
 the difference was insignificant . in the case of the offg , there was no significant 
 difference in muscle tone and stiffness compared to that in the ngf and the bffg . 
 
 furthermore , in the nfg , the non - dominant leg showed greater muscle tone and stiffness 
 than the dominant leg , although the difference was insignificant . 
 [ conclusion ] during the 
 relax condition , the flexible flat foot generally showed a greater muscle tone and 
 stiffness of both lower extremities compared to the normal foot . 
 the stiffness was 
 particularly higher in the ta and mg muscles . 
 therefore , the muscle tone and stiffness of 
 the lower extremity muscles must be considered in the treatment of flat foot . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION