the activity of the gluteus maximus ( gm ) in standing movements is very important . in 
 particular
, it seems that the activity of the gm increases during one foot standing with the 
 trunk tilted anteriorly because the gm acts in both abduction and extension of the hip 
 joint .
however , most studies that use the moment arm vector concept have reported that the 
 abductor function of the gm is weak1 , 2 .
a straight - line model study of the gm with 
 the hip in the anatomical zero position found that the gm was unimportant for abduction of 
 the hip joint .
we question this finding based solely on anatomical neutral position , because 
 the hip joint , which is a ball and socket joint , can adopt various positions .
although gm is 
 known as an extensor of the hip joint in humans , it acts as an abductor in tetrapods3 ; therefore gm has the possibility of acting 
 as a hip abductor in flexion of the hip joint .
basmajian and deluca4 reported that the gm was active during extension of the 
 thigh at the hip joint , lateral rotation , abduction against heavy resistance with the thigh 
 flexed to 90 , and adduction against resistance that holds the thigh abducted .
although some 
 studies5 , 6 have reported results agreeing with those of basmajian and 
 deluca4 , they did not describe the 
 details of electromyography data .
several studies7,8,9 have reported that the gm is 
 functionally divided into upper ( ugm ) and lower segments ( lgm ) .
both the ugm and sartorius 
 muscles might assist hip abduction against strong muscle resistance8 . however , little attention has been given to gm activity 
 during abduction in flexion of the hip joint .
we postulated that the human gm would play the 
 same important abductor function in flexion of the hip joint as it does in tetrapods .
we , 
 therefore , hypothesized that gm activity is directly proportional to the resistance provided 
 against adduction in flexion of the hip joint .
hislop and montgomery10 reported an 
 increase in the activity of the tensor fasciae latae ( tfl ) during abduction in flexion of 
 the hip joint ; however , other studies report that the activity of the tfl decreases in the 
 same condition11 , 12
. we postulated that tfl activity would decrease with increasing hip 
 joint flexion , because the tfl force vector passes through the forward hip joint in hip 
 flexion ; therefore , the tfl would act as an abductor , or as a flexor . moreover , because the 
 gluteus medius ( gmed ) , together with the gluteus minimus , both abduct and stabilize the hip 
 joint13 , 14 , the effect of hip flexion angle on gmed activity is not known 
 well .
the purpose of this study was to determine the effect of hip flexion angle on the 
 activities of the gm , tfl , and gmed during isometric contraction in abduction .
twenty - seven healthy males ( age 21.5  1.2  years ; height 172.8  5.0  cm ; body mass 64.6  
 2.3  kg ( mean  sd ) ) with no history of hip joint problems participated in this study .
all 
 subjects provided their written informed consent prior to participation , and approval was 
 granted by the tohoku bunka gakuen university s human subjects ethics committee .
we measured the maximum abduction strength ( mas ) of the right hip joint and surface 
 electromyography ( emg ) .
emg data were collected from four segments of three muscles : the 
 right ugm , lgm , gmed , and tfl .
measurement conditions were hip flexion angles of 0 , 20 , 40 , 
 60 , and 80 degree , and strength of right hip abduction of 20 , 40 , 60 , and 80%mas . with subjects lying prone on a table , the gluteus region
was cleaned using fine sand paper 
 and alcohol before disposable electrodes were placed on the skin superficial to the belly of 
 the muscles .
ugm electrodes were placed two finger s width above the line just under the 
 spina iliaca posterior superior and the trochanter major ; lgm electrodes were set below the 
 same line ; tfl electrodes were placed between the spina iliaca anterior superior and the 
 trochanter major ; and gmed electrodes were set between the ugm and tfl .
after we had confirmed the belly of the muscles via an ultrasound 
 scan ( viewbox , tanita , tokyo , japan ) , proper electrode placement was determined .
emg were 
 amplified by bipolar leads ( mt-11 , gehc - j , tokyo , japan ) .
emg were measured during maximum 
 voluntary isometric contraction ( mvc ) in the two positions as described by hislop and 
 montgomery ( 2007 ) : the tfl and gmed were measured during side - lying abduction at 0 degrees 
 flexion of the hip joint , and the ugm and lgm were measured during prone - lying extension 
 with knee flexion .
subjects were instructed to maintain their maximum voluntary contraction 
 for 3 seconds and emg measurements were taken once .
after taking emg measurements of mvc , subjects lay supine on the table with a slippery 
 sheet spread under around the lower extremity , and the pelvis fixed to the bed with a belt . 

we measured mas at 0 degrees flexion of the hip joint with a strain gauge sensor ( -tas mt1 , 
 anima , tokyo , japan ) , which was placed between the resistance belt and the epicondylus 
 lateralis .
the knee joints were maintained in the neutral position , and the ankle joint was 
 fixed in the neutral position with a plastic shoehorn brace .
subjects were instructed to control abduction strength to the indicated value for 3 seconds , 
 and emg measurements were taken as described above .
the analog emg signal passed through a 16 bit a / d board 
 ( powerlab , adinstruments , nagoya , japan ) , and all signals were sampled at 1 khz .
emg data were filtered with a 10  hz high - pass , fir digital filter .
after the signal was 
 full - wave rectified , the integrated emg ( iemg ) was calculated using the steady data of 2 
 seconds within the 3 seconds of data .
we used the normalized iemg ( % mvc ) to express the muscle activity . a two - factor , 54 , within - subjects analysis of variance ( anova )
independent variables were the factors of hip 
 flexion angle and % mas , and the dependent variable was hip muscle activity .
differences were assessed with 
 two - sided tests , with an alpha level of 0.05 .
for the ugm , the anova showed 
 significant main effects of both hip flexion angle ( p<0.001 ) and % mas ( p<0.001 ) .
there was a significant 
 interaction effect between hip flexion angle and % mas ( p<0.001 ) . also , the change in the 
 iemg between the hip flexion angles increased with each increment of % mas ( table 1table 1 .
hip muscles activities during isometric contraction of abducation with increases 
 in hip flexion angle0 degrees20 degrees40 degrees60 degrees80 degreesgluteus maximus ( upper)20%mas4.6 3.2 7.1 6.5 7.7 7.8 8.4 9.0 9.7 8.4 40%mas7.9 7.7 11.114.1 15.418.7 19.520.8 25.524.4 60%as12.913.2 20.122.3 28.229.3 37.135.9 48.841.3 80%mas29.520.9 33.925.1 51.953.5 59.854.3 85.780.8 gluteus maximus ( lower ) 20%mas3.9 1.7 5.4 4.2 6.1 7.6 5.1 4.2 5.0 2.1 40%mas5.3 2.3 5.4 2.6 8.310.1 9.011.0 11.515.060%mas8.3 4.2 9.8 7.4 12.711.0 17.223.0 20.821.3 80%mas20.215.2 19.411.8 25.521.3 30.827.6 38.232.9 gluteus medius20%mas5.2 3.9 6.2 4.3 7.0 5.3 8.0 6.1 9.1 7.3 40%mas13.1 9.0 12.6 8.0 14.5 9.9 18.313.8 21.918.7 60%mas32.218.0 29.119.1 32.620.9 35.424.2 40.333.0 80%mas58.629.7 52.631.3 53.843.2
54.943.0 66.153.0 tensor fasciae latae 20%mas14.1 7.9 13.3 7.8 11.1 5.7 9.7 6.8 6.9 4.5 40%mas28.012.3 23.2 9.4 22.411.1 20.210.5 15.1 9.9 60%mas49.417.5 41.216.8 44.531.5 34.917.827.115.0 80%mas69.829.4 71.039.0 55.332.7 49.327.5 45.123.9 significant difference from 0 degrees at each % mas .
mas = maximum abduction strength , unit:%mvc ( maximum voluntary isometric 
 contraction ) ) .
mas = maximum abduction strength , unit:%mvc ( maximum voluntary isometric 
 contraction ) for the lgm , there were significant main effects of both hip flexion angle ( p<0.001 ) and 
 hip abduction strength ( p<0.001 ) .
although there was a significant interaction effect between the two 
 conditions ( p<0.001 ) , the simple main effect was not significant for hip flexion angles 
 at 20% mas ( table 1 ) .
although there were 
 significant main effects of both hip flexion angle ( p<0.05 ) and % mas ( p<0.001 ) , there 
 was no significant interaction effect between hip flexion angle and % mas .
the scheffe tests 
 also showed no significant the differences among the hip flexion angles ( table 1 ) .
finally , for the tfl , there were significant main effects of both hip flexion angle 
 ( p<0.001 ) and hip abduction strength ( p<0.001 ) .
tfl activity increased though with each increment of mas , just as 
 for the other muscles , but there was no significant interaction effect between hip flexion 
 angle and % mas ( table 1 ) .
this study confirmed our hypothesis that gm activity is directly proportional to resistance 
 against adduction in flexion of the hip joint .
the gm in the anatomical neutral position of 
 the hip has an action line that is nearly perpendicular to the femoral shaft , whereas its 
 action line becomes parallel to the femoral shaft in flexion of the hip joint .
more 
 specifically , mean ugm activity reaches 80% mvc , because the ugm shifts to a position nearly 
 parallel to the femoral shaft .
in contrast , the moment arm of the gm in hip extension 
 decreases as the hip flexion angle increases15 .
therefore , an abductor function in flexion of the hip joint might 
 be an important function of the gm .
human walking is characterized by erect bipedalism , and it is thought that the ugm evolved 
 alongside erect bipedalism , because daily activities performed while standing require the 
 muscle forces of abduction and extension of the hip joint . during walking ,
gm activity 
 increases synchronously with the gmed from the terminal swing to mid - stance7 , 9 . 

the ugm acts as an abductor and hip abductor muscle activity increases rapidly in the 
 loading response ( 20%mvc ) ; an external force acts on flexion and adduction of the hip joint 
 when it is flexed at approximately 30 degrees in the loading response phase16 .
gmed activity showed no significant differences with change of hip flexion angle in this 
 study .
the gmed is a fan - shaped muscle composed of three distinct parts13 , 14 . in this study
, 
 electrodes were placed over the middle part , which runs forward to the femoral neck in the 
 anatomical neutral position and attaches to the ridge on the great trochanter , which is 
 located near the center of the hip joint in the sagittal plane .
although the moment arm 
 vector of abduction for the gmed anterior segment decreases in flexion of the hip joint17 , it is not obvious for the middle and 
 posterior segments .
moreover , the gmed , together with the gluteus minimus , are known 
 stabilizers of the hip joint . for example
, a single - leg bridge exercise in the supine 
 position activated the gmed to 72.5% mvc18 .
therefore , there is a possibility that the gmed middle segment 
 acts through a wide range of hip joint motion .
this 
 result was same as that reported by matsuki et al.12 , who reported that tfl activity measured by emg with fine - wire 
 electrodes decreased in flexion of the hip joint .
although the tfl is separated into two 
 parts , anteromedial and posterolateral19 , 
 it is difficult to measure each part using emg .
for this reason , we placed the electrodes on 
 the center of the belly .
the tfl attaches from the spina iliaca anterior superior to the 
 fascia lata and inserts upon the tractus iliotibialis .
the tfl shifts to a position nearly 
 perpendicular to the femoral shaft with each increment of hip flexion .
thus the tfl is a hip 
 flexor rather than an abductor when the hip is flexed , which explains why tfl activity 
 decreased with increasing hip flexion .
first , it is 
 possible that the ugm plays an important role in posture maintenance in flexion of the hip 
 joint , for instance , in walking with anterior tilt of trunk in a passage where the ceiling 
 is low , or when ascending stairs .
the strength training of the ugm , therefore , is important 
 for improvement of the function .
second , there is suitable posture for each muscle in the 
 strength training of the hip abductors . for strengthening the tfl ,
in contrast , abduction movement in hip flexion is desirable for 
 strengthening the ugm .
limitations on this study were cross - talk from nearby muscles and changes in electrode 
 location with hip joint flexion .
in addition , we should pay attention to the spatial relation between 
 the innervation zone and the electrodes because it affects emg amplitude . in conclusion ,
ugm and lgm activities increased significantly as a function of incremental 
 increases in hip flexion angle . in comparison ,
tfl activity decreased significantly with 
 incremental increases in hip flexion angle . on the other hand ,
   [ purpose ] this study aimed to determine the effect of varying hip flexion angle on hip 
 muscle activity during isometric contraction in abduction . [ subjects ] twenty - seven healthy 
 men ( mean age=21.5  years 
 , sd=1.2 ) participated in this study . 
 [ methods ] surface 
 electromyography ( emg ) was recorded of the upper portion of the gluteus maximus ( ugm ) , 
 lower portion of the gluteus maximus ( lgm ) , tensor fasciae latae ( tfl ) , and gluteus medius 
 ( gmed ) during isometric contraction under two measurement conditions : hip flexion angle 
 ( 0 , 20 , 40 , 60 , and 80 degrees ) and abduction of the hip joint at 20 , 40 , 60 , and 80% 
 maximum strength . 
 integrated emg ( iemg ) were calculated and normalized to the value of 
 maximum voluntary contraction ( mvc ) . 
 [ results ] results indicated that the iemg of both the 
 ugm and lgm increased significantly with increases in hip flexion angle , whereas the iemg 
 of the tfl decreased significantly . 
 the maximum activities of the ugm and the lgm were 
 85.7  80.8%mvc and 38.2  32.9%mvc at 80 degrees of hip flexion , respectively , and that 
 of the tfl was 71.0  39.0%mvc at 40 degrees of hip flexion . 
 [ conclusion ] the iemg of the 
 gmed did not change with increases in hip flexion angle . 
 hip flexion angle affected the 
 activity of the gm and tfl during isometric contraction in abduction . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION