when performing exercise , humans generally assume a basic , erect , bipedal posture . in this 
 posture
, we engage in different movements at varying speeds , such as walking , running , 
 kicking , and jumping .
the feet are an important body component , as they constantly function 
 as shock eliminators , weight support structures , and locomotive organs at any given 
 time1,2,3,4,5,6,7,8,9 . in recent years , many reports have 
 indicated that toe flexor strength and grip force training leads to significant changes in 
 walking speed or pace and improved dynamic balance and prevents falls10 , 11 .
however , the 
 common training methods for foot - related research , treatment , and prevention have typically 
 included flat - footed exercises such as towel gathering or toe exercises .
these exercises use 
 extrinsic and intrinsic muscles without distinction , and many points regarding their 
 frequency , load , implementation time , implementation period , and effects remain unclear . 

furthermore , previous studies , including one conducted by our research team , have reported 
 the importance of the intrinsic foot muscles , but the effects of such training have not been 
 clarified12 . if a training effect is 
 acceptable , improvements in athletic performance , treatment of foot disease , long - term foot 
 function maintenance , obstacle prevention , enhancements in the standing and walking 
 positions , and further exercise of a flat feet loads are expected .
the purpose of the present study was to focus on the intrinsic foot flexor muscles , and to 
 verify the effects of a strength training method with fixed repetitions and loads on ( 1 ) 
 muscle strength , ( 2 ) foot arch formation , and ( 3 ) dynamic test items ( 1-legged long jumping , 
 vertical jumping , and 50-m dash time ) .
the subjects were 12 healthy males ( 12 left and 12 right feet ) without motor system 
 diseases or history of foot - related conditions such as flat feet ( low arches ) .
the mean (  
 standard deviation ) age , height , and weight were 29  5  years , 172.5  7.3  cm , and 64.9  
 12.8  kg , respectively .
characteristics of the subjectssubject being testedage ( years)height ( cm)height ( cm)height ( cm)a3218210030.2b271646524.2c291806419.8d2617059.820.7e291746120.1f301685619.8g311707325.3h36174.56320.7i221655118.7j221746220.5k321867220.8l361635319.9bmi : body mass index .
there is no sport history which continued the subject being tested in the 
 period before and behind training , and it is not carrying out except this training .
the test 
 items were measured before ( pre - training ) and after 8 weeks of training ( post - training ) , and 
 the values were compared using statistical analysis .
a digital grip dynamometer ( model # 5101 , takei scientific instruments co. , ltd . ,
niigata , 
 japan ) was used to measure intrinsic foot flexor strength and could display values  5  kg in 
 increments of 0.1  kg .
a wooden box was constructed for the dynamometer , measuring 50  14  
 60  cm ( length  width  height ) , which stabilized the device and lower limbs during the 
 measurements .
the limbs were fixed with the subject in the supine position , with their knees in extension 
 and the ankles in maximum plantar flexion .
next , after correcting any distal interphalangeal 
 ( dip ) joint curvature caused by extrinsic muscle contraction resulting from passively 
 flexing the toe , a finger sling ( c7791 , sakai medical co. , ltd . ,
tokyo , japan ) was used to 
 flex each toe , with the dynamometer fixed at a 5-kg load .
the intrinsic foot flexor strength was quantified for all toes by measuring the 
 flexion - gripping force in the interphalangeal ( ip ) and metatarsophalangeal ( mp ) joints . 

these measurements were performed with the subjects barefoot , and 3 measurements were 
 performed for the left and right feet , which were then averaged for each foot .
the muscle 
 contraction time for each measurement was 3 s. the measurements were collected on a total of 
 3 days with the same testers , and were performed at a 23-day interval .
the interobserver 
 correlation ( interclass correlation coefficient , 1.3 ) indicated a high reliability of 0.98 
 between testers for muscle strength measurement values .
therefore , these measurements were 
 collected three times and the mean values of these 3 measurements were used . for arch measurement ,
the longitudinal and horizontal arch lengths were recorded for the 
 left and right feet to create a static standing position using a berkemann footprint .
the 
 longitudinal arch length was defined as the distance from the head of the first metatarsal 
 bone to the calcaneal tuberosity .
the horizontal arch length was defined as the distance 
 encompassing the head of the first metatarsal bone to the head of the fifth metatarsal bone . 

the dynamic test items were 1-legged long jumping , vertical jumping , and 50-m dash time . 
 one - legged long
jumping was evaluated on the basis of the measurement method used for the 
 standing broad jump in the new physical strength test used by the ministry of education , 
 culture , sports , science and technology , japan . in this test
, a tape measure was used to 
 measure the distance from the toes to the heel of the measured lower limb .
the measurements 
 were collected twice for the left and right lower limbs , and the mean values for these 
 measures were used .
vertical jumping measurement values were displayed numerically on a jump md measurement 
 device ( takei scientific instruments co. , ltd . , niigata , japan ) , which can display 
 measurements in 1-  cm increments .
vertical jump measurements were collected twice , and their 
 mean value was used . for measuring the 50-m dash time ,
a start signal was used by the tester who announced the 
 start time and bent their arm at the start line .
subjects started the 50-m dash from a 
 standing position , and the tester used a stopwatch to time how long it took for the 
 subject s torso to cross the finish line .
subjects were positioned so that their hip joints were flexed at 90 angles . then , similar 
 to the other methods for measuring muscle strength ,
a barefoot toe ip and mp joint - flexing 
 exercise was performed simultaneously with the left and right feet .
this exercise was 
 performed for 200 repetitions , once per day , three times per week with a 3-kg load that was 
 set using scales with a maximum capacity of 10  kg ( sanko seikohjyo co. , ltd . ,
a metronome indicated the desired exercise 
 rhythm until the subjects could independently maintain the rhythm .
the subjects flexed their 
 toes for 1  s before extending them to their resting position for 1  s , and this movement was 
 performed repeatedly , thereby creating a rhythm of 1 repetition per 2 s. the pre- and post - training measurement values are presented as mean  standard deviation 
 values ( mean  sd ) .
a wilcoxon signed - rank test was used to examine differences in the test 
 results .
the ethics committee of juntendo university graduate school of health and sports science 
 approved this study ( no . 21 - 34 ) .
all subjects were given an explanation of the objectives 
 and content of the study before they provided written consent .
the left and right intrinsic foot flexor strengths significantly increased ( p < 0.01 ) , 
 and the left and right longitudinal arch lengths ( p < 0.01 ) and horizontal arch lengths , 
 measured from a static standing position ( p < 0.01 ) , significantly decreased after 
 training compared with those before training ( table 
 2table 2 .
test item resultsinspectedpre - trainingpost - trainingtoe grip ( kg)l9.3 ( 5.611)14.4 ( 11.915.5)**r9.4 ( 6.212.7)14.2 ( 12.115.7)**arch length ( cm)l18.6 ( 17.320.1)17.7 ( 17.219.4)**r18.7 ( 17.919.6 ) 18.1 ( 16.719)**transverse arch length ( cm)l10.4 ( 9.711.3)10.1 ( 9.510.7)**r10.3 ( 1011.1 ) 10.1 ( 9.811)**long jump with one leg standing ( cm)l178.8 ( 163.5217)198.3 ( 177226.5)**r179 ( 151.5206)189.5 ( 166.5227)**vertical jump ( cm)54 ( 43.571.5)55.5 ( 49.573)*50-meter dash time ( s)7.34 ( 6.798.58)7.05 ( 6.58.22)****p<0.01 ; * p<0.05 . ) . additionally , the left and right 1-legged long jumping distances ( p < 0.01 ) and the 
 vertical jumping height ( p < 0.05 ) significantly increased , and the 50-m dash time 
 significantly decreased ( p < 0.01 ) after training compared with those before training 
 ( table 2 ) .
in contrast to previous studies , we implemented a strength training program that focused on 
 intrinsic foot muscle strength and excluded extrinsic foot muscle function as much as 
 possible . with the ankle in plantar flexion ,
the extrinsic foot muscle origin and insertion 
 points draw closer to each other , thereby lowering the flexion action and becoming more 
 dependent on the intrinsic foot muscles .
this led hayashi et al.12 to establish a dip joint flexion measurement method , so 
 that the intrinsic foot muscle strength contributed to almost 100% of the proximal ip ( pip ) 
 and mp joint flexion .
the present study implemented training and muscle strength 
 measurements specialized for the intrinsic foot flexor muscles with the gripping force of 
 all of the toes . in order to measure intrinsic foot flexor strength
, we created an original strength 
 measurement device because no commercially available device exists . as the test 
retest 
 method indicated reproducibility without observable differences and high interobserver 
 correlation , the measurement device used in this study is sufficiently reliable .
statistically significant differences were found for each measurement test item when the 
 pre- and post - training scores were compared .
that is , we observed an increased intrinsic 
 foot flexor strength , shorter longitudinal and horizontal foot arches , greater 1-legged long 
 jumping distances , greater vertical jumping heights , and shorter 50-m dash times . increased intrinsic foot flexor strength values were observed after training .
toe gripping 
 force training has been reported to elicit positive results that can be observed after 3 
 weeks according to fukuda et al.10 or 4 
 weeks according to usaba and ihara13 , 14 .
because the results of the present study 
 indicated that the training load was approximately 30% of the mean maximum strength for all 
 subjects and increased strength scores were recorded for all subjects , it appears that these 
 results were due to neurological rather than muscle adaptations ( i.e. , hypertrophy ) , as was 
 indicated by previous studies .
foot - arch measurements revealed that the longitudinal and horizontal foot arches were 
 shortened after training , which suggests arch formation .
this finding also suggests that 
 arch maintenance function improved along with increased intrinsic foot flexor strength 
 scores .
ukai et al.15 reported that 
 forefoot arch maintenance is dependent on the intrinsic foot muscles , whereas umeki et 
 al.16 reported that the arches are 
 shortened by intrinsic and extrinsic foot muscle contraction during 1-legged standing , which 
 protects the plantar support ligaments and joint tissue .
we believe the shortened arches 
 observed in the present study were due to an increased intrinsic foot flexor strength , which 
 made arch formation possible through muscle contraction .
performance of all dynamic test items improved in the post - training period , with increased 
 1-legged long jump distances , greater vertical jumping heights , and shorter 50-m dash times . 

rabita et al.17 reported that the 
 elements that improve jumping function are muscle tendon structure and intrinsic muscle 
 hardness , although it is also important to take a neuromuscular function approach to 
 training .
this suggests that we were able to achieve increased foot hardness that was 
 accompanied by an improved intrinsic foot flexor toe gripping strength and arch formation . 

mann et al.18 reported that when running 
 at top speed , the intrinsic muscles are continuously bearing weight .
thus , we inferred that 
 the volume of intrinsic foot flexor activity would improve under high - load conditions . in 
 addition , running and jumping movements involve high loads and require plantar flexion 
 position movements .
the present training position was a toe flexion movement with the ankle 
 positioned in maximum plantar flexion , and the necessary ankle plantar flexion position 
 stability in the final exercise area , along with pip and mp joint ejection power , was 
 exhibited .
we believe that this resulted in greater anterior propulsive force and jumping 
 power , leading to improved performance .
although it can not be ignored that other factors relating to the joints and muscles , such 
 as quadriceps activity , may have affected the results of the present study , we believe that 
 reinforcement of the intrinsic foot flexor muscles should receive sufficient focus . 

furthermore , we believe that intrinsic foot flexor strength training is useful for improving 
 standing and walking performance , in addition to improving the performance of sports 
 athletes engaged in activities involving greater exercise loads .
   [ purpose ] the purpose of the present study was to verify the effects of intrinsic foot 
 flexor strength training . 
 [ subjects ] the subjects were 12 healthy males without motor 
 system disease . [ methods ] a training method that involved flexion of all toe 
 interphalangeal and metatarsophalangeal joints against a 3-kg load was implemented and was 
 performed for 200 repetitions once per day , three times per week , for a period of eight 
 weeks . [ results ] 
 significant changes were observed for intrinsic foot flexor strength 
 scores , foot arches , vertical jumping , 1-legged long jumping , and 50-m dash time . 
 
 [ conclusion ] this muscle strength training method significantly improved muscle strength 
 scores , foot arch shape , and movement performance . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION