human feet function to directly absorb shock from the ground thereby insulating the body 
 from impact , supporting body weight , and propelling the body .
extrinsic foot flexors that cause toe flexion are the flexor hallucis longus and flexor 
 digitorum longus .
they are responsible for flexing the interphalangeal ( ip ) and 
 metacarpophalangeal ( mp ) joints of the first toe , as well as the distal interphalangeal 
 ( dip ) , proximal interphalangeal ( pip ) , and mp joints of the second through fifth toes . 

intrinsic foot flexors that cause toe flexion are the flexor hallucis brevis and the flexor 
 digitorum brevis .
they are responsible for flexing the mp joint of the first toe , as well as 
 the pip and mp joints of the second through fifth toes .
the majority of studies on the treatment and prevention of foot injuries have focused on 
 the recovery of arch function , prevention of flat foot , range of motion training for the 
 sole and lower leg , and muscle stretching and strengthening10,11,12,13,14 .
all these treatment and prevention methods focus on extrinsic foot 
 flexors , primarily because it is difficult to evaluate intrinsic foot flexors individually 
 and separately during training .
the contractile force of intrinsic foot flexors , which is exerted in ankle plantar flexion , 
 is evidenced by the anatomical structure of the muscles , observation and palpation during 
 dynamic contraction , and findings regarding loss of flexion capability in the ip joint of 
 the first toe , as well as the dip joints of the second through fifth toes .
hayashi and ukai 
 et al . established a method for measuring the strength of intrinsic foot flexor muscles by 
 considering ankle joint plantar flexion as a position that inhibits extrinsic flexion ( fig .
differences in muscle activity as a result of the ankle joint position a few studies have shown that intrinsic foot flexors are important for dynamic 
 stabilization of the arches of the feet , enabling a more efficient exertion of extrinsic 
 foot flexor muscle force and improving walking performance
. however , the effects of training 
 these muscles are still unclear18,19,20,21 . in a previous study
, we described a 
 method of intrinsic foot flexor strength training to improve the full toe flexion gripping 
 force of the mp joint of the first toe and the pip and mp joints of the second through fifth 
 toes .
the ankle joint was secured in the maximum plantar flexion position , and 200 
 repetitions were performed thrice with a load of 3  kg over a period of 8 weeks .
we found 
 improvements in the strength of intrinsic flexor muscles , arch structure ( the medial 
 longitudinal arch and the transverse metatarsal arch in a static position ) , and dynamic 
 performance ( vertical jump , one - legged long jump , 50-m dash time)22 .
however , the extent of dependence of muscle activity on 
 intrinsic foot flexors while performing toe flexion exercises with maximum plantar flexion 
 position is still unclear
. therefore , the present study is significant , in that we examined 
 the effectiveness of intrinsic foot flexor strength training performed in the plantar 
 flexion position using highly reliable needle electromyography .
this study used needle electromyography to examine how different ankle positions , 
 specifically the position between plantar flexion and dorsiflexion ( hereafter , the 
 intermediate position ) and the maximum plantar flexion position ( hereafter , the plantar 
 flexion position ) , affect the activity levels of intrinsic foot flexor muscles .
secondary 
 aim of this study was to demonstrate the effectiveness of intrinsic foot flexor strength 
 training performed in the plantar flexion position .
after receiving approval from the juntendo university graduate school of health and sports 
 science ethics committee ( nos .
22 - 31 ) , the purpose and content of this study were explained 
 to participants , and their written consent was obtained .
their mean 
 age , height , and weight ( mean  sd ) were 31.6  8.3  years , 170.9  5.1  cm , and 65.2  
 5.5  kg , respectively ( table 1table 1 .
characteristics of subjectsn=18age ( years)31.6 8.3height ( cm)170.9  5.1weight ( kg)65.2  5.5bmi ( kg / m)22.31.7mean  sd , bmi : body mass index ) .
mean  sd , bmi : body mass index needle electromyography was performed with an evoked potential using an electromyograph 
 ( neuropack meb-2200 , nihon kohden ) .
the sampling frequency was 10 khz , and emg signals were 
 converted to direct current by full wave rectification after filtering with a band - pass 
 filter ( 105,000  hz ) .
the activity levels the flexor hallucis brevis ( fhb ) and flexor digitorum brevis ( fdb ) were 
 measured .
a 10-kg tubular spring scale ( shinwa ) , which displays load values in the units of 0.1  kg , 
 was used for measurements .
the ankle joint was secured in the intermediate and maximal 
 plantar flexion positions using a 50  14  60  cm 3wooden half - box stabilizer built for this 
 study , and the tubular spring scale was secured to the box .
measurements were obtained in 
 the intermediate and maximal plantar flexion ankle joint positions with the subjects in a 
 long sitting position . to set up the initial position for the full toe flexion exercise , a 
 finger sling ( c7791 , sakai ) was perpendicularly attached to the proximal phalanx of each of 
 the first through fifth toes in the intermediate position .
it was confirmed that the load reached 3  kg when the mtp joint of the first toe 
 and the pip and mtp joints of the second through fifth toes were in the maximal flexion 
 position before securing the foot in each position . after inserting needles into the muscles 
 to be measured ,
5-s isometric full toe flexion exercises were performed with a 3-kg load 
 applied to the ankle joint in the intermediate and maximal plantar flexion positions . 

the activity levels of fhb and fdb in the two measurement positions were quantified as the 
 one - second product ( mvms ) .
wilcoxon s signed rank test was used to determine the differences 
 in mean muscle activity , rate of increase ( % ) , and muscle activities of the fhb and fdb in 
 the intermediate and maximal plantar flexion positions24 , 25 .
statistical analyses were performed with spss version 18.0 ( spss 
 japan inc . ) .
mean fhb muscle activity was 410.7  255.1  mvms ( median : 423.2 ) in the intermediate 
 position and 997.1  524.9  mvms ( median : 870.4 ) in the maximal plantar flexion position .
the rate of increase from the 
 intermediate position to the plantar flexion position was 43% ( table 2table 2 .
fhb muscle activity in the intermediate and plantar flexion positionsfhb intermediate the one - second product ( mvms)fhb maximal plantar flexion the one - second product ( mvms)410.7255.1997.1524.9***fhb : flexor hallucis brevis , mean  sd , * * * : p<0.001 ) .
fhb : flexor hallucis brevis , mean  sd , * * * : p<0.001 mean fdb muscle activity was 493.9  297  mvms ( median : 455.1 ) in the intermediate position 
 and 1,215.3  429.8  mvms ( median : 1,096.5 ) in the plantar flexion position . a significant 
 increase in activity was observed ( p < 0.001 ) .
the rate of increase from the intermediate 
 position to the plantar flexion position was 46% ( table 
 3table
3 . fdb muscle activity in the intermediate and plantar flexion positionsfdb intermediate the one - second product ( mvms)fdb maximal plantar flexion the one - second product ( mvms)493.92971,215.3429.8***fdb : flexor digitorum brevis , mean  sd , * * * : p<0.001 ) .
studies have shown that intrinsic foot flexors support the transverse metatarsal and medial 
 longitudinal arches , that they dynamically protect suspensory ligaments and joint tissue in 
 the feet , and that excessive loads in the standing position , specifically loads over 400 lbs 
 ( 1,780 n ) while standing on one foot , increase their muscle activities26,27,28,29,30,31 .
while running , 
 intrinsic foot flexors function to create a strong lever for ankle joint plantar flexion and 
 are constantly active under the load of body weight while running at maximal speed . 
 furthermore , these muscles are an important factor in improving jumping ability32,33,34
. however , detailed methods for assessing 
 intrinsic foot flexors and the positions that yield effective activation have not yet been 
 revealed . in this
study , needle electromyography was used to measure muscle activity , because 
 intrinsic foot flexors are thin and deep muscles .
full toe flexion exercise was performed , 
 but not all toes could be secured at the same angle . in addition , it was difficult to 
 standardize the angle of toe flexion between the intermediate and the plantar flexion 
 positions due to differences in the degree of extension of toe extensors . therefore , to 
 perform measurements in each position , the study conditions were the resistance position , 
 resistance angle , identical load , and maximal toe flexion .
our results show that the muscle activities of both fhb and fdb significantly increased 
 between the intermediate and plantar flexion ankle joint positions .
toe flexion in the 
 intermediate ankle joint position is within the same angular range of motor execution as 
 standing or walking , and there is little intrinsic flexor muscle activity in this position 
 due to support from ligaments and extrinsic foot flexors .
however , toe flexion during 
 plantar flexion of the ankle joint shortens the origin and insertion of extrinsic foot 
 flexors , and plantar flexion torque is minimized in this position .
this also decreases the 
 active tension of extrinsic foot flexors , including that of the flexor hallucis longus , 
 flexor digitorum longus , tibialis posterior , and peroneus longus that act during plantar 
 flexion of the ankle joint and toe flexion exercises , and puts these muscles in a 
 restrictive position34 . therefore , 
 intrinsic foot flexors either contract easily or must contract to this position , and this is 
 believed to increase muscle activity ( fig .
it is essential the ankle joint should be in a plantar flexion position in movements that 
 are faster than walking , such as running or jumping , as well as in movements used during 
 sports .
a sequence of movements associated with plantar flexion of the ankle joint secures 
 and stabilizes the foot , compensates for decreased extrinsic flexor function , and creates 
 propulsion .
in addition to the method outlined in our previous study , toe flexion exercises in a long 
 sitting or supine position , a prone position that makes it easy to achieve maximal plantar 
 flexion with body weight , a ital ( a japanese - style sitting ) position , and a position in 
 which toe flexion and ankle plantar flexion is maintained under controlled body weight with 
 support from the arms are other methods for training intrinsic foot flexors with the ankle 
 joint in a plantar flexion position .
positions for training the intrinsic foot flexor muscles this study demonstrated that it is possible to evaluate intrinsic foot flexors , in addition 
 to the numerous reports on treatment methods focusing on extrinsic foot flexors . 

furthermore , the results suggest that toe flexion exercises performed during plantar flexion 
 of the ankle joint are an effective method for intrinsic foot flexor strength training .
   [ purpose ] the effectiveness of intrinsic foot flexor strength training performed in the 
 plantar flexion position was examined using needle electromyography . [ subjects ] 
 the 
 subjects of this study were 18 healthy men . 
 [ methods ] we used needle electromyography to 
 measure the muscle activities of the flexor hallucis brevis ( fhb ) , and the flexor 
 digitorum brevis ( fdb ) in maximum plantar and an intermediate position . [ results ] 
 significant increases in muscle activities were observed for both fhb and fdb , and the 
 rates of increase from the intermediate position to the plantar flexion position were 43% 
 for fhb and 46% for fdb . 
 [ conclusion ] this study demonstrated that it is possible to 
 evaluate intrinsic foot flexors , in addition to the numerous reports on treatment methods 
 focusing on extrinsic foot flexors . 
 furthermore , the results suggest that toe flexion 
 exercises performed during plantar flexion of the ankle joint are an effective method for 
 intrinsic foot flexor strength training . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION