one of the most important roles of feet is weight - bearing , which requires stability and 
 resilience for absorbing impact , and the medial longitudinal arch ( mla ) and transverse arch 
 provide this stability and resilience1 . in 
 particular , the mla is the primary structure absorbing the impact of the weight - bearing that 
 occurs during the physical activities of walking or running2 .
the condition in which the mla becomes chronically or abnormally 
 low is called flatfoot3 , and it is divided 
 into spastic flatfoot and flexible flatfoot4 , 5 .
the reported causes of flexible flatfoot 
 include excessive extension of the plantar muscle , which provides stability to the mla , and 
 poor function of the tibialis posterior muscle4 .
some researchers have reported that hip external rotator weakening 
 internally rotates the hip joint while inducing foot pronation6 .
hip external rotator strengthening exercises , performed by extending the hip joint when it 
 is in external rotation and the knee joint is at 90 flexion , have been reported to have the 
 greatest effect when performed in the prone position7 .
the results of one study showed that as the activity of the abductor halluces ( abdh ) 
 increased , the angle of the mla decreased , indicating that abdh exercises are helpful for 
 the maintenance and enhancement of the mla8 . in a study of various exercises for strengthening the abdh , 
 toe - spread ( ts ) exercises and short - foot exercises performed in the sitting position did not 
 elicit any significant difference in abdh activity , but the muscle activity value was shown 
 to be higher during ts exercises9 .
although many studies have been conducted of the various exercises for mla correction10 , 11 , studies examining exercises that also strengthen the hip external 
 rotator muscle are insufficient .
therefore , the purpose of the present study was to 
 investigate the effect of hip external rotator strengthening and ts exercises in subjects 
 with pronated feet , and to evaluate their effects on lower extremity muscle activities 
 during stair - walking .
a total of 20 healthy adults were selected after excluding those with present or past 
 arthritis , those who had undergone foot or ankle surgery , and those with diabetes , cuts on 
 the lower extremities , or foot abnormalities .
the subjects were selected from among those 
 with foot pronation ( > 10  mm ) based on navicular drop ( nd ) tests9 , 12 .
the participants 
 were provided with a written informed consent form in accordance with the ethical principles 
 of the declaration of helsinki .
the mean age , height , and weight of the study subjects who 
 performed both hip external rotator and ts exercises were 21.81.62  years , 162.96.9  cm , and 
 59.314.6  kg respectively , and the mean age , height , and weight of the study subjects who 
 performed only ts exercises were 22.31.95  years , 165.08.1  cm , and 63.211.8  kg , 
 respectively . a surface emg dts system ( telemyo dts , noraxon inc .
, az , usa ) was used to record and 
 process the electromyographic ( emg ) signals from the gluteus maximus , the vastus medialis , 
 the tibialis anterior , and the abdh in order to measure the muscle activities of these 
 muscles .
the collected surface emg signals 
 were processed using the myoresearch master 1.07 xp program ( noraxon inc .
the 
 signals were digitally sampled at a rate of 1,500  hz and noise was removed using a 20500  hz 
 bandpass filter and a 60  hz notch filter .
the emg data were normalized using the maximal 
 voluntary contraction ( % mvc ) values .
the study subjects performed exercises five times per week for four weeks : two times per 
 week at home and three times per week in the laboratory .
the subjects who performed both hip 
 external rotator exercises and ts exercises performed three sets of 20 repetitions of hip 
 external rotator exercises in the prone position along with 100 repetitions of ts exercises 
 in the sitting position while maintaining both the hip joints and the knee joints at 90 
 flexion .
nd tests12 , 13 were conducted before and after the four weeks of exercises and the 
 mean values of the groups were compared using the independent t - test . before
stair - climbing 
 and -descending , foot switches were attached to the center of the heel , below the first 
 metatarsal , and below the fifth metatarsal to divide the phases of weight bearing in order 
 to measure the muscle activity of the gluteus maximus , the vastus medialis , the tibialis 
 anterior , and the abdh during the individual phases .
the paired t - test was used to analyze 
 changes in lower extremity emg during stair - walking before and after the exercise period 
 within each exercise group .
the results were statistically analyzed using spss version 18.0 
 ( spss inc . , chicago , il , usa ) and significance was accepted for values of p<0.05 .
the nd of the group that performed both hip external rotator and ts exercises was 
 5.661.58  mm and that of the group that performed only ts exercises was 7.522.16  mm after 
 the intervention .
these results indicate that the nd of the group that performed both hip 
 external rotator and ts exercises decreased significantly more than the group that performed 
 only ts exercises ( table 1table 1.comparison of navicular drop test results between the group that performed both 
 hip external rotator exercises and ts exercises and the group that performed only ts 
 exercises ( n=20)hip external rotator exercise and ts exercisemean ( sd)ts exercisemean ( sd)prend ( mm)11.8 ( 1.9)11.4 ( 1.4)4 weeksnd ( mm)5.7 ( 1.6)7.5 ( 2.2)nd : navicular drop , sd : standard deviation .
p<0.05 ) . according to emg measurements taken during stair - walking before and after 
 the intervention ,
the % mvc values of the vastus medialis ( 5.567.27% ) and the abdh 
 ( 11.0510.54% ) during stair - climbing and the % mvc values of the abdh ( 7.96.23% ) during 
 stair - descending were significantly different in the group that performed both hip external 
 rotator and ts exercises ( table 2table 2.emg activities : % mvc values pre- and post - intervention ( n=10)musclehip external rotator exercises and ts exercisests exercises onlypremean ( sd)4 weeksmean ( sd)premean ( sd)4 weeksmean ( sd)stair upgm22.1 ( 7.8)24.4 ( 9)22 ( 9.1)25.8 ( 10.4)vmo31.1 ( 12.5)36.7 ( 12.9)45.5 ( 17)48.7 ( 5.7)ta21 ( 12.8)19 ( 11.3)17.2 ( 5.5)15.7 ( 6.1)abdh27.1 ( 4.7)38.18 ( 8.8)23.9 ( 7.3)37.3 ( 11.5)stair downgm13.6 ( 9)12 ( 5.1)15.4 ( 14.2)22.3 ( 12.8)vmo35.8 ( 21.7)32.5 ( 8.9)37.1 ( 15.5)41.2 ( 9.5)ta20.1 ( 10.7)19.8 ( 16)17.2 ( 10.2)20 ( 9.1)abdh32 ( 6.8)39.9 ( 9.6)25 ( 11.1)36.9 ( 9.7)gm : gluteus maximus , vmo : vastus medialis , ta : tibialis anterior , abdh : abductor 
 hallucis , sd : standard deviation .
the % mvc values of the abdh 
 during stair - climbing ( 11.5813.46% ) and stair - descending ( 11.8710.78% ) were significantly 
 different between before and after the exercise treatment ( table 2 ) .
after the 4-week intervention , there was no significance 
 different between hip external rotator and ts exercise and only ts exercise ( table 3table 3.comparison of emg activities ( % mvc ) between the group that performed both hip 
 external rotator and ts exercises and the group that performed only ts exercises after 
 the 4-week intervention ( n=20)musclehip external rotator exercises and ts exercisesmean 
 ( sd)ts exercises onlymean ( sd)stair upgm24.4 ( 9)25.8 ( 10.4)vmo36.7 ( 12.9)48.7 ( 5.7)ta19 ( 11.3)15.7 ( 6.1)abdh38.2 ( 8.8)37.3 ( 11.5)stair downgm12 ( 5.1)22.3 ( 12.8)vmo32.5 ( 8.9)41.2 ( 9.5)ta19.8 ( 16)20 ( 9.1)abdh39.9 ( 9.6)36.9 ( 9.7)gm : gluteus maximus , vmo : vastus medialis , ta : tibialis anterior , abdh : abductor 
 hallucis , sd : standard deviation ) .
p<0.05 gm : gluteus maximus , vmo : vastus medialis , ta : tibialis anterior , abdh : abductor 
 hallucis , sd : standard deviation . p<0.05
gm : gluteus maximus , vmo : vastus medialis , ta : tibialis anterior , abdh : abductor 
 hallucis , sd : standard deviation
the present study was conducted to compare changes in lower extremity muscle activities 
 during stair - walking between subjects with flatfoot who performed only ts exercises and 
 subjects who additionally performed hip external rotator exercises , in order to present 
 basic evidence for exercise treatment . since many studies set the criterion for flatfoot as 
 10  mm or greater nd12,13,14 , the present study s 
 subjects were selected based on that criterion .   in the case of flatfoot
, valgus stress is created in the knee joints during gait due to the 
 pronation and extroversion of the hind foot , the supination of the fore foot and the mid 
 foot , and the internal rotation of the lower extremities , leading to kinematic sway of the 
 lower extremities and compensatory actions1 . under the assumption that femoral anteversion contributes to hip 
 joint internal rotation
, some researchers have reported that the muscle activity of the 
 vastus medialis and the hip adductor are decreased by hip internal rotation and 40 knee 
 flexion15 .
based on these previous 
 study results , in the present study , hip external rotator muscle strengthening is considered 
 to have increased hip joint external rotation , leading to increased muscle activity of the 
 vastus medialis during stair - climbing .
the decrease in nd height that occurred after hip 
 external rotator strengthening exercises would be explained by the internal rotation of the 
 lower extremity , as described by neumann1 .
limitations of the present study include the fact that gait perse is an activity with large 
 variations among individuals , and stair - walking also has limitations for generalization 
 because individuals stair - walking characteristics are different . in addition ,
muscle 
 activity levels and joint angles at different time - points could not be identified during 
 motion , as the changes in muscle activity and the joint angles were evaluated together .
the 
 issues discussed here should be addressed in future studies to present evidence for exercise 
 treatment through kinetic and kinematic analyses .
 [ purpose ] the purpose of the present study was to examine the effects of toe - spread ( ts ) 
 exercises and hip external rotator strengthening exercises for pronated feet on lower 
 extremity muscle activities during stair - walking . 
 [ subjects and methods ] the participants 
 were 20 healthy adults with no present or previous pain , no past history of surgery on the 
 foot or the ankle , and no foot deformities . 
 ten subjects performed hip external rotator 
 strengthening exercises and ts exercises and the remaining ten subjects performed only ts 
 exercises five times per week for four weeks . [ results ] less change in navicular drop 
 height occurred in the group that performed hip external rotator exercises than in the 
 group that performed only ts exercises . 
 the group that performed only ts exercises showed 
 increased abductor hallucis muscle activity during both stair - climbing and -descending , 
 and the group that performed hip external rotator exercises showed increased muscle 
 activities of the vastus medialis and abductor hallucis during stair - climbing and 
 increased muscle activity of only the abductor hallucis during stair - descending after 
 exercise . [ conclusion ] stair - walking can be more effectively performed if the hip external 
 rotator muscle is strengthened when ts exercises are performed for the pronated foot . 