muscle morphological characteristics , such as anatomical muscle cross - sectional area ( csa ) , 
 muscle length , and muscle volume ( mv ) are important physiological variables for assessing 
 the functional capacity of a muscle .
although magnetic resonance imaging ( mri ) and computed 
 tomography scans are the gold standard for measuring anatomical csa and mv , ultrasound is a 
 utilizable technique that can be easily applied to clinical assessment and field 
 surveys1 . in a small muscle , anatomical 
 csa and mv can be measured using multiple images of individual muscles created by portable 
 ultrasound .
one study2 investigated the 
 validity and reliability of mv measurements of the medial gastrocnemius using 
 three - dimensional ultrasound , and reported that ultrasound overestimated mv by approximately 
 2  ml ( 1.1% ) and underestimated muscle length by 3  mm ( 1.3% ) across all joint angles compared 
 to mri - measured values .
in addition , the same study reported excellent reliability for 
 repeated measures of mv ( intrarater correlation coefficient ( icc ) = 0.99 ) and muscle length 
 ( icc = 0.97 ) .
these results suggest that multiple ultrasound images of a small muscle can 
 accurately measure muscle morphological variables .
the foot is the point of direct contact between the body and ground surface during standing 
 and walking .
muscle forces generated by the toes and ankles may play an important role in 
 maintaining balance because muscle strength is essential for posture and stability3 , 4 .
a 
 few studies have reported that toe flexor muscle strength is associated with postural 
 control .
for example , handa et al.5 
 reported significant positive correlations between toe flexor strength and one - leg standing 
 balance with the eyes open ( r = 0.443 , p < 0.01 ) as well as functional reach ( r = 0.620 , 
 p < 0.01 ) , in an analysis grouping men and women together .
kurihara et al.6 examined the relationships between toe 
 flexor muscle strength and intrinsic and extrinsic foot muscle sizes in young men and women 
 and found significant correlations between toe flexor muscle strength and mri - measured 
 anatomical csa in the medial parts ( r = 0.775 , p < 0.01 ) and lateral parts ( r = 0.739 , p 
 < 0.01 ) of foot intrinsic muscles . recently ,
abe et al.7 reported a significant positive association between toe flexor 
 muscle strength and accelerometer - determined light and moderate physical activities and 
 average step counts .
the results of these previous studies suggest that bigger intrinsic 
 foot muscles in active individuals may be associated with greater toe flexor strength as 
 well as good postural control . in a previous mri study6 , however , only one mri image was used to determine the anatomical 
 csa of individual intrinsic foot muscles , even though the foot has several toe flexor 
 muscles with differing distributions among the toes .
therefore , the purpose of this study 
 was to test the hypothesis that toe flexor muscle strength is related to the anatomical and 
 physiological csa of intrinsic toe flexor muscles and that these morphological and 
 functional variables associate with physical performance .
thirty - four young adults aged 20 to 35  years ( 17 men and 17 women ) were recruited through 
 printed advertisements and by word of mouth . before accepting their informed consent , a 
 written description of the purpose and safety of the study was distributed to all of the 
 potential subjects .
all subjects were healthy and free of overt chronic disease ( e.g. , 
 neuromuscular disorders , arthritic disorders , etc ) as assessed by self - report .
the rate of 
 regular sports activity ( at least twice a week ) , among the subjects was 65% ( 82% in men and 
 47% in women ) .
the main types of sports activities were judo ( 29% ) , resistance exercise 
 ( 21% ) , and soccer ( 21% ) for men , and jogging / running ( 50% ) and canoeing ( 25% ) for women .
the 
 study was conducted in accordance with the principles of the declaration of helsinki and was 
 approved by the ethics committee for human experiments of the national institute of fitness 
 and sports in kanoya , japan .
anatomical csa was measured using b - mode ultrasound ( aloka prosound 6 , tokyo , japan ) with 
 a 7.5  mhz linear array transducer ( 76  mm wide ) in two intrinsic toe flexor muscles , the 
 flexor digitorum brevis ( fdb ) and abductor hallucis ( abh ) , as described previously8 .
all subjects lay in the prone position 
 during scanning of the two muscles . using anatomic landmarks described by crofts et al.8 , a linear transducer coated with 
 water - soluble transmission gel
was placed on the skin surface of the measurement sites , and 
 cross sections of each muscle were imaged .
ultrasound images of each site were stored on a 
 personal computer , and anatomical csa was measured using image - j software .
the mean values 
 ( two images ) of each site were used for data analysis .
all ultrasound measurements were 
 performed on the left ( non - dominant ) foot , and dominance was ascertained by asking each 
 subject which foot they used to perform well - learned skills using a questionnaire9 .
the muscle volume and muscle length of the fdb were estimated using multiple ultrasound 
 images from the sole of the foot .
after foot length ( the distance between the tip of the 
 great toe and the edge of the heel ) measurements , all measurement sites were marked at 50% 
 of the foot length as well as at contiguous 1-cm intervals from the point of 50% of the foot 
 length in both the proximal and distal directions .
1.before the start of ultrasound testing , foot length ( the distance between the tip of 
 the great toe and the edge of the heel ) was measured and then all measurement sites 
 were marked at 50% of the foot length as well as at contiguous 1-cm intervals from the 
 point of 50% of the foot length in both the proximal and distal directions .
typical 
 ultrasound images ( young woman , 20 yr ) revealing transverse scans of the foot at 
 contiguous 1-cm intervals from the point of 50% of the foot lengthfdb : flexor digitorum brevis ) and the anatomical csa of the fdb was measured using the procedure described above . 

muscle volume was calculated by multiplying anatomical csa by distance interval ( 1  cm ) .
the 
 distance between the most proximal image and the most distal image in which the fdb was 
 visible was defined as the length of the fdb muscle . to calculate physiological csa ,
the 
 fiber length of the fdb was estimated using the ratio of fiber length to muscle length 
 ( average of the second , third , fourth toes ) as reported by kura et al10 .
the physiological csa of the fdb was calculated by 
 dividing muscle volume by fiber length .
the test - retest reliability ( icc , sem and minimal 
 difference ) was previously determined using the data of 7 young subjects ( 5 men and 2 women ) 
 scanned twice within 7 days ( at least one day apart ) for anatomical csa of the fdb ( 0.924 , 
 0.13  cm , 0.36  cm ) and abh ( 0.949 , 0.21  cm , 
 0.58  cm ) and muscle volume of fdb ( 0.971 , 0.57  cm , 
 1.57  cm ) . before the start of ultrasound testing , foot length ( the distance between the tip of 
 the great toe and the edge of the heel ) was measured and then all measurement sites 
 were marked at 50% of the foot length as well as at contiguous 1-cm intervals from the 
 point of 50% of the foot length in both the proximal and distal directions .
typical 
 ultrasound images ( young woman , 20 yr ) revealing transverse scans of the foot at 
 contiguous 1-cm intervals from the point of 50% of the foot length fdb : flexor digitorum brevis toe flexor muscle strength ( tfs ) was measured using a toe - grasp dynamometer ( tkk3361 , 
 takei , tokyo , japan ) , as described previously11 , 
 12 . while barefoot , subjects stood in 
 front of a wall and the left foot
then the subjects were 
 instructed to lift the right foot and maintain a one - legged upright standing position on the 
 dynamometer , with both hands on the wall in front of them , while holding the dynamometer 
 grasp bar with their toes .
the distance between the bar and the heel was adjusted to the 
 foot size of the subjects so that the distal phalanges of the great toe and fifth toe and 
 the middle phalanges of the second to fifth toes could be placed on the toe grasp bar . 

subjects were allowed to perform one test trial , followed by two maximum effort trials 
 ( tfs-5-toes ) , and the best value of the left foot was used for the data analysis .
the 
 subjects also performed two maximum effort trials to measure toe flexor muscle strength 
 without the contribution of the great toe ( tfs-4-toes ) . in these trials
, a small metal plate 
 was placed between the great toe and the toe grasp bar during these measurements for 
 preventing the great toe from grasping the toe grasp bar of the dynamometer .
maximal toe 
 flexor strength divided by body weight was calculated to evaluate the relative toe flexor 
 strength .
the test - retest reliability of toe flexor strength ( tfs-5-toes and tfs-4-toes ) 
 measurements using the icc , sem and minimal difference was previously determined using the 
 data of 7 young subjects ( 5 men and 2 women ) tested twice within 7 days ( at least one day 
 apart ) : 0.962 , 1.2  kg and 3.3  kg for tfs-5-toes and 0.883 , 1.1  kg and 3.1  kg for 
 tfs-4-toes .
maximum walking speed was measured by timing each subject as they walked along a 10-meter 
 corridor with a ceramic floor surface .
the total length of the marked corridor was 14 
 meters , allowing 2-meter acceleration and deceleration zones .
the width of the corridor was 
 constricted to 1 meter to encourage subjects to maintain a straight course .
subjects used 
 their own footwear and had to start 2 meters before the beginning of the start line , and to 
 continue until the 2 meters past the goal line . after one practice trial , subjects performed 
 two maximum speed timed trials .
subjects were asked to walk down the corridor as fast as 
 possible without running .
times were measured with an electronic timing system ( nearest 
 0.01  s , brower timing system , draper , usa ) .
the best time was converted to a maximum speed 
 measurement ( unit , m / s ) and the best value was used as the maximum walking speed .
the 
 test - retest reliability of this measurement using the icc , sem and minimal difference was 
 previously determined using the procedure described above : 0.930 , 0.14
the functional reach test was measured using the method described in a previously reported 
 study4 . before the start of the test
, 
 subjects were instructed to stand with both feet touching a marked line and to maintain that 
 foot position throughout testing .
the subject then performed shoulder forward flexion with 
 the right shoulder until an angle of 90 degrees was reached .
next , the subjects tried to extend their middle finger as 
 far forward as possible without moving their feet and keeping their arm parallel to the 
 ground . the distance moved by the end of the middle finger between the starting position and 
 maximum forward position
subjects performed two trials , and the best value was used 
 for the functional reach test .
the test - retest reliability of this measurement using the 
 icc , sem and minimal difference was previously determined fusing the procedure described 
 above : 0.889 , 2  cm and 5  cm , respectively .
before 
 comparisons were made , the distributions of the dependent variables were tested for 
 normality using the shapiro - wilk test . the difference between men and women
was tested for 
 significance using the unpaired student s t - test , and when variables were not normally 
 distributed , the mann - whitney u test was used .
pearson product correlations were performed 
 to determine the relationships between toe flexor muscle strength and intrinsic toe flexor 
 muscle size as measured by ultrasound and between the morphological and functional variables 
 of toe flexor muscle and physical performance .
men were taller and heavier than women . compared with 
 women , men had higher anatomical csa in the fdb and abh , as well as muscle volume and 
 physiological csa of the fdb .
maximum tfs-5-toes and tfs-4-toes were higher in men than in 
 women ; however , the specific strength ( tfs-4-toes per unit physiological csa ) of the fdb , 
 walking speed and functional reach were similar for both men and women ( table 1table 1.maximum toe flexor muscle strength , physical performance and ultrasound 
 measurements of intrinsic foot muscle sizes of young men and womenmenwomenoverall(n=17)(n=17)(n=34)age ( yrs)24 ( 4)24 ( 4)24 ( 4)height ( m)1.71 ( 0.05)*1.60 ( 0.05)1.66 ( 0.07)body mass ( kg)72.9 ( 11.4)*52.1 ( 5.1)62.5 ( 13.6)body mass index ( kg / m)24.8 ( 3.1)*20.4 ( 2.0)22.6 ( 3.4)foot length ( cm)25.2 ( 1.2)*22.6 ( 0.7)23.9 ( 1.6)flexor digitorum brevisacsa max ( cm)2.61 ( 0.38)*1.65 ( 0.33)2.13 ( 0.60)mv ( cm)11.95 ( 2.93)*6.66 ( 2.10)9.31 ( 3.67)pcsa ( cm)6.38 ( 1.04)*3.95 ( 0.97)5.17 ( 1.58)abductor hallucisacsa ( cm)2.89 ( 0.69)*2.02 ( 0.59)2.46 ( 0.77)tfs-5 toes ( kg)29.1 ( 5.3)*21.2 ( 4.7)25.1 ( 6.4)tfs-4 toes ( kg)10.4 ( 3.2)*6.4 ( 2.6)8.4 ( 3.5)tfs-4 toes / pcsa ( kg / cm)1.63 ( 0.40)1.62 ( 0.56)1.63 ( 0.48)walking speed ( m / s)3.19 ( 0.65)3.14 ( 0.66)3.17 ( 0.64)functional reach ( cm)38.1 ( 6.3)39.1 ( 4.0)38.6 ( 5.2)maximum toe flexor muscle strength with ( tfs-5 toes ) and without ( tfs-4 toes ) the 
 contribution of the great toe ; acsa , anatomical cross - sectional area ; mv , muscle 
 volume ; pcsa , physiological cross - sectional area . *
maximum toe flexor muscle strength with ( tfs-5 toes ) and without ( tfs-4 toes ) the 
 contribution of the great toe ; acsa , anatomical cross - sectional area ; mv , muscle 
 volume ; pcsa , physiological cross - sectional area .
* significant difference from women , p<0.001 there was a significant correlation between tfs-5-toes and tfs-4-toes of men ( r = 0.739 , p 
 < 0.001 ) and women ( r = 0.731 , p < 0.001 ) .
both tfs-5-toes and tfs-4-toes correlated 
 positively with the maximum walking speed of men ( r = 0.584 , p = 0.014 and r = 0.553 , p = 
 0.021 , respectively ) , women ( r = 0.748 , p < 0.001 and r = 0.533 , p = 0.028 , respectively ) 
 and the whole sample ( r = 0.535 , p = 0.001 and r = 0.459 , p = 0.006 , respectively ) . however , 
 the correlations between tfs-5-toes and functional reach of both men ( r = 0.399 ) and women 
 ( r = 0.166 ) were not significant ( p>0.05 ) .
anatomical csa of the abh did not significantly correlate with tfs-5-toes of either men ( r 
 = 0.034 , p = 0.896 ) or women ( r = 0.387 , p = 0.125 ) ; however , the correlation of the whole 
 sample was significant ( r = 0.454 , p = 0.006 ) . for women ,
there were significant positive 
 correlations between tfs-5-toes and anatomical csa max ( r = 0.713 , p = 0.001 ) , muscle volume 
 ( r = 0.604 , p = 0.010 ) , and physiological csa ( r = 0.687 , p = 0.002 ) of the fdb . for men , 
 however , the observed anatomical csa max ( r = 0.143 , p = 0.584 ) , muscle volume ( r = 0.332 , p 
 = 0.192 ) , and physiological csa ( r = 0.333 , p = 0.191 ) of the fdb did not correlate 
 significantly with tfs-5-toes . physiological csa of the fdb correlated positively with 
 tfs-4-toes of men ( r = 0.541 , p = 0.025 ) , women ( r = 0.573 , p = 0.016 ) and the whole sample 
 ( r = 0.720 , p < 0.001 ) ( fig . 2fig .
2.relationships between physiological cross - sectional area ( csa ) in the flexor 
 digitorum brevis ( fdb ) and toe flexor muscle strength ( tfs-4 toes ) of young men and 
 women . tfs-4
open circles are men and filled circles are women ) . relationships between physiological cross - sectional area ( csa ) in the flexor 
 digitorum brevis ( fdb ) and toe flexor muscle strength ( tfs-4 toes ) of young men and 
 women . tfs-4
the main findings of the present study were that physiological csa of the fdb was 
 significantly correlated with tfs-4-toes of both men and women ; physiological csa of the fdb 
 was significantly correlated with tfs-5-toes of women , but not that of men ; there was a 
 significant correlation between toe flexor muscle strength and maximum walking speed in both 
 genders . in the present study ,
the fdb was selected as representative of the intrinsic toe flexor 
 muscles , because a more accurate estimation of muscle volume by ultrasound and calculation 
 of physiological csa of the muscle can be achieved than of the other intrinsic toe flexor 
 muscles .
the fdb muscle volume reliability results ( icc of 0.971 and sem of 
 0.57  cm ) indicate that the ultrasound method of the present study is a good 
 repeatable technique for measuring muscle volume .
the fdb is located in the sole of the foot 
 and separates into four tendons that insert onto the middle phalanges of the four lateral 
 toes10 ( without the great toe ) . 

therefore , our results demonstrate that the physiological csa of the fdb is associated with 
 tfs-4 toes of both men and women , even though the correlation was only moderate .
toe flexor 
 muscle strength is generated from a combination of the intrinsic and extrinsic foot muscles . 
 in the hand ,
the grip strength decreases by approximately 50% after median and ulnar nerve 
 ( intrinsic muscles ) blocks compared with the pre - block measurement13 .
accordingly , the contribution of extrinsic toe flexor 
 muscles may reflect the moderate correlations between tfs-4 toes and physiological csa of 
 the fdb .
in addition , individual differences in the moment arm ( located between the center 
 of curvature of the metatarsal head and the center of the flexor tendon ) and/or differences 
 in the dominant / non - dominant sides may also be unknown factors14 .
our findings show that the anatomical and physiological csa of the fdb of women were 
 significantly correlated with tfs-5 toes . for men
only one study6 has examined the relationships between toe flexor muscle strength 
 and intrinsic and extrinsic foot muscle sizes in young sedentary adults ( 14 men and 12 
 women ) and a pooled sample was used for data analysis .
that study reported significant 
 correlations between tfs-5 toes and mri - measured anatomical csa of the medial parts 
 ( r=0.775 , p<0.01 ) and lateral parts ( r=0.739 , p<0.01 ) of the foot intrinsic muscles , 
 although only one mri image was used in that investigation . in the present study
, the reason 
 for the lack of a significant correlation between tfs-5 toes and the fdb muscle size 
 observed in men is unknown .
approximately half of our young women 
 were sedentary and the other half performed mainly jogging / running or canoeing .
therefore , 
 our women participants may have had relatively homogeneous morphological and functional 
 properties of the toe flexor muscles , which might partially explain the significant 
 correlations . on the other hand ,
most of our young men were physically active and performed 
 different sports including judo , resistance exercise and/or soccer .
a likely explanation is 
 that physical activity in different sports may elicit non - homogeneous features among toe 
 flexor muscles , especially between the great toe and the four other toes .
together , the 
 results of the present and previous studies suggest that differences in sports experience 
 may be a factor underlying the poor correlation of tfs-5-toes and fdb muscle size in young 
 men .
additional research into these issues is needed . in the present study , both tfs-4-toes and tfs-5-toes
this finding is consistent with the results of 
 previous studies5 , 15 which found significant correlations between 10-m walking 
 performance and tfs-5-toes ( r=0.459 , p<0.01)5 and improved 50-m dash time following 8 weeks of toe flexor strength 
 training15 .
thus , the results of the 
 present and previous studies demonstrate that toe flexor muscle strength is an important 
 factor determining maximum walking speed . on the other hand ,
no significant correlation of 
 tfs-5-toes and functional reach of either young men or women was found .
although a pooled 
 sample with a wide age range ( 20 to 84 yrs ) was used , one study reported a significant 
 positive correlation ( r=0.620 , p<0.01 ) between tfs-5-toes and functional reach5 .
the discrepancy in the results of the 
 present and that previous study is not known , but the difference in subject age ranges 
 between the two studies may have played a role . in conclusion , although toe flexor muscle strength correlated positively with walking speed 
 in both genders , its correlation with functional reach was not statistically significant . 

the lack of a 
 significant relationship between tfs-5-toes and physiological csa of the fdb in men may be 
 related to different experiences in sports .
 [ purpose ] to investigate the relationships between toe flexor muscle strength with 
 ( tfs-5-toes ) and without ( tfs-4-toes ) the contribution of the great toe , anatomical and 
 physiological muscle cross - sectional areas ( csa ) of intrinsic toe flexor muscle and 
 physical performance were measured . 
 [ subjects ] seventeen men ( 82% sports - active ) and 17 
 women ( 47% sports - active ) , aged 20 to 35  years , volunteered . 
 [ methods ] anatomical csa was 
 measured in two intrinsic toe flexor muscles ( flexor digitorum brevis [ fdb ] and abductor 
 hallucis ) by ultrasound . 
 muscle volume and muscle length of the fdb were also estimated , 
 and physiological csa was calculated . 
 [ results ] both tfs-5-toes and tfs-4-toes correlated 
 positively with walking speed in men ( r=0.584 and r=0.553 , respectively ) and women 
 ( r=0.748 and r=0.533 , respectively ) . 
 physiological csa of the fdb was significantly 
 correlated with tfs-5-toes ( r=0.748 ) and tfs-4-toes ( r=0.573 ) in women . in men , 
 physiological csa of the fdb correlated positively with tfs-4-toes ( r=0.536 ) , but not with 
 tfs-5-toes ( r=0.333 ) . 
 [ conclusion ] our results indicate that physiological csa of the fdb 
 is moderately associated with tfs-4-toes while toe flexor strength correlates with walking 
 performance . 