muscle contraction produces force and change in the muscle s length , i.e. , velocity .
skeletal muscles are primarily classified into parallel muscles and 
 pennate muscles , based on the muscle s form and structure . in parallel muscles , the 
 shortening velocity is faster and the range of motion is larger because the muscle fibers 
 are arranged longitudinally and directly reflect mechanical shortening .
in contrast , in 
 pennate muscles , the shortening velocity is slower and the range of motion is shorter 
 because muscle fibers are arranged diagonally with a certain pennate angle .
pennate muscles 
 exert stronger force because they comprise a greater number of muscle fibers , with a larger 
 cross - sectional area .
generally , parallel muscles are located at joints , such as the elbow 
 joint , and require exercise to change the joint angle significantly .
conversely , many 
 pennate muscles are located in the lower extremities and counteract the effects of 
 gravity2 , 3 .
the semitendinosus muscle ( st ) composes a part of the hamstrings and has a characteristic 
 form ( fig .
1.structure of the semitendinosus musclethe line of tendinous inscription ( arrow 1 ) lies at the center of the muscle belly . 

the proximal compartment has the 
 structure of a parallel muscle , whereas the distal compartment is considered a pennate 
 muscle because it has a pennate angle ( arrow 2 ; recorded at 65% from the proximal 
 tendon ) . ) .
the st has a tendinous inscription ( ti ) in the center of the muscle belly , and its 
 muscle fibers are divided into a proximal compartment ( pc ) and a distal compartment ( dc ) . 

the pc of the st is a parallel muscle , but the dc has a pennate structure with an average 
 12.9 pennate angle4 . in other words ,
the 
 st comprises both parallel and pennate muscles dependent on the existence of the ti . 

functional differences can therefore be assumed between the two compartments , because each 
 has a unique innervation5 , 6 .
structure of the semitendinosus muscle the line of tendinous inscription ( arrow 1 ) lies at the center of the muscle belly . 

the proximal compartment has the 
 structure of a parallel muscle , whereas the distal compartment is considered a pennate 
 muscle because it has a pennate angle ( arrow 2 ; recorded at 65% from the proximal 
 tendon ) .
reportedly , the st is susceptible to injury in the dc on the sports field7 .
it is believed that the muscle fibers are 
 particularly easily injured during eccentric contraction because they can not withstand 
 elongation due to the load3 , 8 .
for example , the dc is often damaged during the swing phase 
 of running .
weakness of the hamstring muscles was predicted to underlie this damage9,10,11 , and hamstring strengthening exercises 
 have been proposed as a precaution12 , 13 .
also , the increase in stiffness of the 
 hamstrings may serve as a cause of shoulder and elbow pain14 and decrease of dynamic postural control15 .
although there are many reports on the injury of the dc 
 and on the treatment of stiffness of the hamstrings16 , 17 , few studies have 
 considered the difference between the pc and dc . in the context of our clinical rehabilitation for patients who have undergone total knee 
 arthroplasty , it was observed that flexibility of the dc of the st is lost , but not that of 
 the pc .
it was also observed that muscle force or 
 range of motion in the knee joint increases with improved flexibility of the dc .
however , 
 this is difficult to prove clinically because the st has a complex form .
therefore , it is important to determine the functional differences between the pc and dc of 
 the st . in this study , using an ultrasound device , muscle fiber lengths in each compartment 
 of the st were measured during knee flexion and hip extension to determine the functional 
 differences between the dc and pc as well as changes in joint angle .
the left legs of seven men aged 2138 years , without orthopedic or neurological system 
 disorders were examined . after the purpose of this study was explained to them , all of them 
 provided written informed consent in order to participate .
all procedures in this study were 
 approved by our institutional ethics committee ( no . 39 - 2 ) .
the participants performed knee flexion ( kf ) and hip extension ( he ) movements in the prone 
 position , with 0 extension of the knee joint and 25 flexion of the hip joint .
kf was defined as a flexion movement 
 from about 0 to 80 knee flexion at 25 hip flexion .
he was defined as an extension 
 movement from about 25 to 0 hip flexion at 0 knee flexion .
these movement angles were 
 selected with reference to those that occur during normal walking and because of the ease of 
 the movements .
participants performed one movement every 4 seconds with the aid of an 
 electric metronome .
the ti of the st was recorded at least twice , and the pennate part was 
 recorded at least three times ( fig .
there were two movements and two recorded parts . in each movement 
 ( knee flexion and hip extension ) , the tendinous inscription and pennate parts were 
 recorded at different times.b .
ultrasound images were recorded at the tendinous inscription part and 
 distal pennate part , during two movements , for each subject ( a ) .
the knee and hip joints 
 angles were recorded using a video camera , in synchrony with the ultrasound images 
 ( c ) .
the ultrasound ( a ) and joint angle ( b ) images were analyzed using dartfish 
 animation software ( c ) . ; see below for further details ) .
, the participants practiced the movements in order to maintain 
 a fairly constant velocity and to avoid compensation caused by extension of the lumbar 
 vertebrae during hip extension .
experimental set - up and methods of measurement a. there were two movements and two recorded parts . in each movement 
 ( knee flexion and hip extension ) , the tendinous inscription and pennate parts were 
 recorded at different times .
b. ultrasound images were recorded at the tendinous inscription part and 
 distal pennate part , during two movements , for each subject ( a ) .
the knee and hip joints 
 angles were recorded using a video camera , in synchrony with the ultrasound images 
 ( c ) .
the ultrasound ( a ) and joint angle ( b ) images were analyzed using dartfish 
 animation software ( c ) .
the images of the ti and pennate parts of the st were captured using an ultrasound device 
 ( ssa-770a ; toshiba medical systems corporation , otawara , tochigi , japan ) .
shift of the ti 
 position was measured from images of the ti , while changes in distal muscle fiber length , 
 longitudinal length of the dc muscle , pennate angle , and muscle thickness were measured from 
 images of the pennate part .
ultrasound images were converted to digital images at 60 frames 
 per second by a personal computer . a probe with a linear form
was used , taking care not to 
 squeeze the muscle shape while lightly positioning the probe on the back of the thigh .
one 
 experimenter recorded all images , and two experimenters confirmed all images . an ultrasound image of the ti is shown in fig . 
 3afig .
3.ultrasound images(a ) an example of the tendinous inscription part ( see arrow 1 in fig .
point ( a ) is 
 the intersection of the tendinous inscription and superficial fascia .
the muscle fiber 
 on the left side of point ( a ) represents the proximal compartment .
movement of point 
 ( a ) is considered as a change in the muscle fiber length of the proximal compartment , 
 as shown in c.(b ) an example of the pennate part of the distal compartment is depicted ( see arrow 2 
 in fig .
we recorded the same muscle fiber and thickness values 
 in a single movement ( see fig .
4).(c , d ) expressed models of the tendinous inscription part ( c ) and the pennate part 
 ( d ) during movement of the knee and hip joints , respectively.sf : superficial fascia ; ti : tendinous inscription ; fas : fascia between the 
 semitendinosus and semimembranosus muscles ; st : semitendinosus muscle ; sm : 
 semimembranosus muscle ; mf : muscle fiber . muscle length in the pc was appreciated from the shift of the intersection of the ti 
 and superficial fascia ( point a in fig .
it was inferred that the muscle length of the pc shortened when point a moved in 
 a proximal direction and elongated when point a moved in a distal direction .
this was 
 because the part proximal to point a comprised muscle fibers of the pc .
sometimes , the shift 
 in ti position was longer than the width of the probe , and it was unable to be measured . 
 therefore , sonic wave opacity tapes were affixed on the skin , and these tape points were 
 considered a criteria point during repeated recordings .
( a ) an example of the tendinous inscription part ( see arrow 1 in fig
point ( a ) is 
 the intersection of the tendinous inscription and superficial fascia .
the muscle fiber 
 on the left side of point ( a ) represents the proximal compartment .
movement of point 
 ( a ) is considered as a change in the muscle fiber length of the proximal compartment , 
 as shown in c. ( b ) an example of the pennate part of the distal compartment is depicted ( see arrow 2 
 in fig .
we recorded the same muscle fiber and thickness values 
 in a single movement ( see fig .
( c , d ) expressed models of the tendinous inscription part ( c ) and the pennate part 
 ( d ) during movement of the knee and hip joints , respectively . sf : superficial fascia ; ti : tendinous inscription ; fas : fascia between the 
 semitendinosus and semimembranosus muscles ; st : semitendinosus muscle ; sm : 
 semimembranosus muscle ; mf : muscle fiber an ultrasound image of the pennate part is shown in fig . 

the pennate part was captured at the clearest point ( which is usually located 
 65% toward the tibial tuberosity from the ischial tuberosity ; fig .
it was supposed that superficial fascia and muscle fibers 
 that did not appear were in a straight line when the pennate part was measured ( dotted line 
 in fig . 4fig .
4.model of the pennate part of the semitendinosus musclethe superficial fascia and muscle fiber are composed of two planes with pennate 
 angle , .sf : superficial fascia ; mf : muscle fiber ;  : pennate angle ; ll : longitudinal length 
 of the distal compartment muscles ; mfl : muscle fiber length ; t : muscle thickness ) .
the length of muscle fiber in the dc and the longitudinal length of the dc muscle 
 were calculated from the pennate angle and muscle thickness , because the muscle fiber was 
 longer than that shown by the ultrasound image ( fig . 

shortening and elongation of the muscle fiber length in the dc were determined 
 relative to those during rest .
model of the pennate part of the semitendinosus muscle the superficial fascia and muscle fiber are composed of two planes with pennate 
 angle , . sf : superficial fascia ; mf : muscle fiber ;  : pennate angle ; ll : longitudinal length 
 of the distal compartment muscles ; mfl : muscle fiber length ; t : muscle thickness joint movement was recorded by a video camera ( gz - hd300 ; victor company of japan , ltd . , 
 yokohama , kanagawa , japan ) during all experiments , simultaneously with ultrasound images . 

joint angles were analyzed from video images and shift of the ti position , while muscle 
 length of the dc , longitudinal length of the dc muscle , pennate angle of the dc , and muscle 
 thickness of the dc were analyzed from ultrasound images .
the joint angle was measured using the automatic tracking 
 mode of dartfish ( dartfish japan co. , ltd .
each time the joint 
 angles changed by 1 , the other data were measured manually because ultrasound images were 
 too unclear to use an automatic mode .
these 
 were measured in both the ti and pennate parts during kf and he ( one point for each part 
 during each movement ) .
in addition , the sum of the decreases in muscle length of the pc 
 muscles and the longitudinal length of the dc muscles were obtained during kf and he .
the 
 measurement errors , errors arising owing to unclear images , and analysis errors for the dc 
 were greater than those for the pc , because the length of the dc muscle fibers was 
 calculated using two measurements ( i.e. , pennate angle and muscle thickness ) .
thus , three 
 data points were obtained for the pennate part in order to calculate the average of every 
 three measurements and to decrease the effects of these errors .
statistical computing r ( version 3.0.1 ; the r foundation for statistical computing , vienna , 
 austria ) was used for statistical analysis .
decreases in muscle fiber length and the joint 
 angle in both compartments were correlated using the pearson s correlation coefficient . 

during kf , there were significant correlations in both compartments , with decreases in 
 muscle fiber length and joint angle in both compartments showing correlations ( pc , r=0.75 , 
 p<0.00001 ; dc , r=0.54 , p<0.00001 ) . during he
, there were no significant correlations 
 in either compartment ( pc ; r=0.12 , p=0.134 , dc ; r=0.13 , p=0.111 ) .
there were significant 
 correlations in longitudinal length of the whole st and joint angle during kf and he ( kf , 
 r=0.86 , p<0.00001 ; he , r=0.19 , p=0.037 ) .
the decreases in muscle fiber length and joint angle are presented in fig . 5fig .
5.decreases in muscle fiber length during joint movementthe graphs depict decreases in muscle fiber length in ( a ) the proximal compartment 
 ( pc ) during knee flexion ; ( b ) the proximal compartment during hip extension ; ( c ) the 
 distal compartment ( dc ) during knee flexion ; ( d ) the distal compartment during hip 
 extension ; and decreases in the sum of muscle fiber length of the proximal compartment 
 and longitudinal length of the distal compartment ( e ) during knee flexion and ( f ) 
 during hip extension .
the gray lines 
 indicate the standard deviation of the decreases in muscle fiber length.there was a strong correlation between joint angle and muscle fiber length during 
 knee flexion ( a and b ) . during hip extension ( c and d )
, there was no correlation 
 between joint angle and muscle fiber length .. during kf in both compartments ( fig . 5a and 
 5c )
however , 
 during he in both compartments , the decreases in muscle fiber length were disconnected 
 ( fig . 5c and 5d )
. decreases in muscle fiber length during joint movement the graphs depict decreases in muscle fiber length in ( a ) the proximal compartment 
 ( pc ) during knee flexion ; ( b ) the proximal compartment during hip extension ; ( c ) the 
 distal compartment ( dc ) during knee flexion ; ( d ) the distal compartment during hip 
 extension ; and decreases in the sum of muscle fiber length of the proximal compartment 
 and longitudinal length of the distal compartment ( e ) during knee flexion and ( f ) 
 during hip extension .
the gray lines 
 indicate the standard deviation of the decreases in muscle fiber length .
there was a strong correlation between joint angle and muscle fiber length during 
 knee flexion ( a and b ) . during hip extension ( c and d )
, there was no correlation 
 between joint angle and muscle fiber length . only the decreases in muscle fiber length in the dc during he tended to be negative ; in 
 other words , the muscle fibers were extended ( fig . 

the other decreases in muscle fiber length tended to be positive ; in other 
 words , the muscle fibers shortened .
the mean and standard deviation of the changes in pennate angle and muscle thickness in the 
 dc are shown in table 1table 1.changes in pennate angle of the distal compartments and muscle thickness of the 
 distal compartments for all subjectsknee flexionhip extensionpennate angle(degrees)muscle thickness(cm)pennate angle(degrees)muscle thickness(cm)subject 14.6  3.40.7  3.12.1  2.22.2  2.3subject 23.7  2.00.9  1.34.2  4.05.1  5.2subject 36.6  4.71.3  2.02.1  1.92.9  1.9subject 44.8  3.33.1  1.51.7  2.04.4  2.1subject 59.4  4.23.2  3.14.7  3.84.0  2.6subject 65.0  2.41.1  1.24.2  2.44.5  2.2subject 75.0  3.50.1 
1.90.7  3.84.9  2.8during knee flexion , the pennate angle increased and muscle thickness tended to 
 increase . during hip extension ,
values are shown as mean  standard deviation .. overall , the means increased . during knee flexion ,
the pennate angle increased and muscle thickness tended to 
 increase . during hip extension ,
this study aimed to identify the functional differences between the pc and dc of the st 
 during kf or he .
decreases in muscle fiber length in the pc and dc were measured using 
 ultrasound exam , and the correlation between decreases in muscle fiber length and joint 
 angle were calculated .
previous studies have reported differences in the examined muscles . 
 these studies compared muscle activity in the medial and lateral sites in the biceps brachii 
 longus muscle using wire electrodes and reported a difference in the activities of these 
 muscles during elbow flexion , forearm supination , or shoulder exorotation19 , 20 .
additionally , some studies compared muscle fiber and tendon length 
 during walking using an ultrasound device and reported differences in the length of these 
 fibers during each walking phase21 , 22 .
however , the aforementioned studies did 
 not examine in - series muscle fibers as well as the pc and dc of st .
the present study 
 hypothesized that the proximal muscle fiber and distal muscle fiber might have functional 
 differences .
it was found that decreases in muscle fiber length in the pc and dc correlated with the 
 joint angle during kf .
in addition , the longitudinal length of the whole st was correlated 
 with the joint angle during kf .
the difference in correlation was too small to determine the 
 exact functional difference between the pc and dc , and it is possible that there are not 
 functional differences between the pc and dc during kf . during he , there were no significant correlations between decreases in muscle fiber length 
 in the pc and dc and joint angles .
however , there were differences in the shortening or 
 elongation of muscle fibers ; i.e. , these displayed different fascicle behavior .
these 
 results differed from other results in that the muscle fiber of the pc shortened during he . 

the pennate muscle can control the change of muscle tendon complex length by changing the 
 pennate angle23 .
this difference during 
 he could be meaningful and was one of the functional differences .
it is important to note that the measurement methods used for each compartment were 
 different .
measurement of decreases in muscle fiber length in the pc used the shift of the 
 ti , recording all muscle fibers of the pc .
conversely , measurement of the dc did not cover 
 all muscle fibers of the dc , but only those in the pennate part .
also , it is necessary to 
 obtain three - dimensional measurements because the st is staggered5 , and it is possible that this staggering differs between the 
 pc and dc .
next , it is necessary to consider that each compartment affects the tension of 
 the other .
when one compartment is undergoing contraction , afferent information from muscle 
 spindles and golgi tendon organs in the other compartment changes . the response from 
 afferent information was different even in the same muscle at every site , e.g. medial and 
 lateral head of the gastrocnemius24 . in 
 st muscle ,
the afferent information of pc and dc may produce different responses . during kf , 
 the correlation between the pc and dc did not determine those differences , which suggested 
 that the pc and dc produced similar level of tension . during he , the muscle fiber of the dc 
 might be eccentrically contracted , although it might be passively stretched by muscle 
 activity of the pc .
the muscle activity of the dc might be weaker than that 
 of the pc , but would not be inactive .
in addition , the longitudinal length of the whole st was correlated with joint angle during 
 he .
the fascicle behavior during he differed between the pc and dc , but both compartments 
 shortened as a whole because the muscle activity was coordinated .
it was unclear why the pc , 
 parallel muscle , contracted stronger than did the dc during he .
in parallel muscles , the 
 shortening velocity is faster and the range of motion is larger than those in pennate 
 muscles . during he ,
the change of muscle tendon complex length should be smaller , because 
 few joint ranges were measured . during he , it was predicted that the pc played fewer roles 
 and contracted weakly , but our results did not support this prediction .
therefore , the dc that exerted stronger force might contract more weakly than 
 does the pc .
our results can be applied in clinical contexts . in the context of sports , it is thought 
 that the st is prone to injury7 , 
 particularly during eccentric contraction3 , 8 .
our results suggest that muscle fibers tend 
 to elongate in the dc , which may be the cause of many injuries to the dc of the st . in 
 addition
, rapid hip extension may cause injury to the dc of the st , because hip extension 
 caused elongation of the muscle fibers of the dc .
first , the muscle fiber length of the pc 
 was measured movement for the skin , but movement of the skin for the bone was not measured . 
 the skin around the joint moved to a greater extent for the bone during joint movement25 , which may have caused a skin effect . 

the proportion of muscle activity that each 
 compartment contributed to a movement could not be determined and the muscle activity was 
 not measured with , for example , electromyograms .
in addition to the gluteus maximus , the 
 gluteus medius , gluteus minimus , hamstrings , and adductors muscles act as hip extensors . 

additionally , the hamstrings and gracilis , sartorius , popliteus , and gastrocnemius muscles 
 act as knee flexors .
regarding the hip joint in particular , the contribution of the st may 
 be minimal , because the correlation of the change in joint angle and decreases in muscle 
 fiber length was not significant .
finally , a single joint movement was examined by one method of force production and 
 contraction velocity through a partial range of its motion .
these components of movement 
 were important with respect to the differences between parallel and pennate muscles . in 
 addition , during walking and other activities ,
there are other effects , such as ground force 
 reaction , and the activity of other muscles .
the clinical implications of our findings may 
 be limited , and it is necessary to perform additional investigation and to consider various 
 other factors .
 [ purpose ] the tendinous inscription divides the semitendinosus muscle into the proximal 
 and distal compartments . 
 it was hypothesized that there are functional differences between 
 those compartments . 
 [ subjects and methods ] seven adult males performed knee flexion and 
 hip extension in the prone position . 
 an ultrasound device measured the decrease in the 
 length of muscle fibers in the two compartments during these movements . 
 the knee and hip 
 joint angles were concurrently measured using a video camera . 
 pearson s correlation 
 coefficients were calculated between the decrease in muscle fiber length in each 
 compartment and joint angle . 
 [ results ] during knee flexion , decreased muscle fiber length 
 was significantly correlated with increased knee flexion angle . during hip extension , 
 there were no significant correlations for either compartment . 
 only the decrease in muscle 
 fiber length in the distal compartment during hip extension tended to be negative ; the 
 other decreases in muscle fiber length tended to be positive . [ conclusion ] correlations 
 did not reveal any functional differences . 
 however , only the distal compartment elongated 
 during hip extension . 
 this result might show a functional difference and could be applied 
 in clinical contexts during hip extension . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION