the gluteus maximus ( gm ) prevents the hip , knee and ankle from collapsing in the early 
 stance phase of gait , creating most of the supporting power and providing stability to the 
 sacroiliac joint ( si joint)1 .
if the gm 
 works inappropriately during gait , it can cause back pain , resulting from a lack of 
 shock - absorption at the sacroiliac joint2 , 
 and hungerford et al.3 suggested that the 
 activity of gm in the legs of patients with sacroiliac joint pain is delayed , and that the 
 biceps femoris shows early activation to compensate for this delay .
moreover , individuals 
 with limited movement of gm show fast movement of the hamstring muscles ( ham ) and erector 
 spinae muscles for better stability of their back4 , 
 5 . in some studies of back pain , it has 
 been reported the activity pattern of gm was limited in functional activity or in exercise , 
 or that it shows overactivity6,7,8 .
gm and ham are 
 synergists of hip extension , and a properly controlled pattern is required , since they are 
 involved in stability and movement of the joint .
especially , since ham is a muscle connecting 
 two joints , its effects on hip extension at different knee joint flexion angles should be 
 considered .
commonly , the manual muscle test of hip extension is performed in prone hip extension with 
 knee extension . in particular , for the isolation test of gm , hip extension is initiated with knee flexion 
 of 90. this position is mainly used for the muscle test or muscle strengthening exercises in 
 clinical practice , because it is practical to assess the activity of gm , since is difficult 
 for the hamstring muscle to exert power due to insufficient activity9,10,11
. however , there have been few studies on changing of two muscles activities at different 
 angles of knee flexion .
therefore , the purpose of this study was to identify the effects of knee joint flexion 
 angle on hip extensor muscle activity .
twenty adults men and women were given an explanation about the purpose and procedures of 
 this study before they voluntarily agreed to participate . only those without pathological 
 issues related to the knee joint , hip joint and back were selected as subjects for the study 
 ( age : 22.92.4  years [ meansd ] ; height : 170.86.45  cm ; weight : 65.910.97  kg ) .
subjects adopted a prone position with both their arms at the sides of the trunk .
they 
 maintained the dominant knee at angles of 0 , 30 , 60 , 110 , and performed maximal 
 voluntary contraction of hip extension .
at this time , they were not allowed to twist or 
 extend their back and compensation ( such as pushing the bed with the arms ) was prohibited . 

both the knee joint flexion angle and hip extensor angle were measured with a myrin 
 goniometer ( norway ) .
maximum torque at each of the knee angles was measured by measuring maximum power using a 
 commander muscle tester ( jtech medical ind .
, salt lake city , ut , usa ) , placed 15  cm above 
 the knee joint .
the maximum torque was calculated as maximum power multiplied by the 
 distance from the greater trochanter to 15  cm above the knee joint .
each measurement lasted for 5 seconds , and was followed by 
 30 seconds of rest time .
after completion of measurements at each angle , 1 minute 's test was 
 given . a surface electromyography system ( noraxon telemyo dts telemetry )
the recorded emgs were processed using a personal 
 computer and myorearch xp 1.06 master edition software .
the activities of both gm and ham 
 were recorded by surface electrodes , following the recommendations of seniam12 and isek13 for skin preparation and electrode position . to minimize the 
 impedance of the skin , hair was shaved and the electrode site was cleaned with alcohol on a 
 cotton swab . to measure muscle activity , subjects adopted a prone position with their 
 dominant knee maintained at one of the flexion angles .
the hip joint was extended at 20 , 
 and resistance was provided for the performance of maximum voluntary contraction for 5 
 seconds .
the first and the last 1 second of each recording was discarded and the middle 3 
 seconds were converted to root mean square ( rms ) values and normalized to % mvic of each 
 muscle , before performing the comparative analysis .
data were processed using spss 18 for windows . to compare the maximum torque and muscle 
 activation of the gm , biceps femoris ( bf ) and semitendinosus ( st ) at the five different knee 
 joint flexion angles , one - way anova was used . to identify differences for each muscle , 
 bonferroni 's post - hoc test was performed .
gm : gluteus maximus , bf : biceps femoris , st : semitendinosus , superscripts 
 within the same column indicate significant differences ( p<0.05 ) .
maximum torque of the hip extensor showed significant differences among the knee flexion 
 angles ( p<0.05 ) , and tended to increase as the angle increased ( table 1 ) .
the activities of the hip extensor at each knee flexion angle showed significant 
 differences ( table 2 ) .
gm activity significantly increased only when the 
 knee flexion angle was more than 60 ( table2 ) .
in this study we gradually shortened the length of hamstring muscle by increasing the knee 
 joint flexion angle of from 0 , to 30 , 60 , 90 , 110. the length of gm is not alternated 
 at hip extension angle of 20. the hip extensors showed a significant decrease between 0 and 60 flexion of the knee . 

this corresponds with the results of a previous study14 , which investigated the torque of knee flexion at different hip 
 flexion angles .
hip extension torque decreased due to the decrease in ham length , when knee 
 flexion was greater than 60. increased torque of a lengthened muscle is the result of the 
 two causes .
first , when connective tissue ( which is a non - contractile factor ) is extended , 
 it creates passive tension and it helps to create active tension by muscle contraction
the second cause is the efficiency of muscle contraction , and it can 
 be explained huxley 's sliding filament model ( relationship between length and tension)15 .
this model , states that the highest 
 tension is created in the optimal position , where actin and myosin fibers overlap the most . 

if the muscle is shorter or longer than the optimal length , the actin - myosin combination is 
 weakened and its tension is decreased . therefore ,
in the present study , hip extension torque 
 decreased , because the ham muscle length was shortened at more than 60of knee flexion . in hip extension at 0 of knee flexion ,
ham activity was significantly greater than that of 
 gm . ham and gm showed no significant difference at 30of knee flexion . at 60of knee 
 flexion , however , gm activity was significantly higher than that of ham .
however , at 60 , 
 90and 110 of knee flexion , there were no big differences in gm activity .
many researchers 
 have studied the difference in muscle activity at different ham muscle lengths , and most of 
 them compared activities at different angles of knee flexion14 .
kennedy and cresswell studied the activity of the gastrocnemius in plantarflexion at 
 different knee flexion angles .
they reported results similar to those of the present study , 
 that the rms value of the gastrocnemius was significantly decreased when the gastrocnemius 
 length was shortened as knee flexion angles increased16 .
thus , in two joint muscles , when one joint is fixed as the muscle 
 is shortened by the other joint performing a movement , the number of muscle fibers used for 
 the movement is considered to be decreased .
this corresponds with the change of torque 
 explained previously . in the present study , as knee flexion increased ,
the activity of ham 
 muscle decreased , thus , the torque of the hip extensor likely decreased .
gm activity induced 
 by knee flexion is also likely to be increased as the knee flexion angle increases . 

especially , at knee flexion angles of more than 60 , more muscle fibers are used for hip 
 extension than for ham , because gm does not change its length at different angles , since it 
 is a one - joint muscle .
nevertheless , the use of muscle fibers in gm increased due to the 
 decreased activity of ham as a result of the increase in knee flexion .
a limitation of the present study was that we failed to control for the effect of hip 
 rotation and extension angle on the hip extensor , because the angle for hip extension was 
 initiated at the small angle of 20 in the neutral position .
therefore , further study of hip 
 extensor with hip rotation and extension is needed . in conclusion ,
maximum torque of hip extensor and ham activation were significantly higher 
 at 0of knee flexion than at knee flexion angles of more than 60. gm activation increased 
 at knee flexion angles of more than 60. therefore , to selectively test gm and ham , we 
 suggest that tests be performed at more than 60 of knee flexion .
   [ purpose ] the present study examined the effects of knee flexion angle on hip extensor 
 muscle activity . [ subjects and methods ] twenty healthy subjects maintained knee flexion 
 angles of 0 , 30 , 60 , 90 and 110 in the prone position and performed maximal voluntary 
 contraction in hip extension . 
 maximum torque in hip extension at the different angles was 
 measured , and surface electromyogram activities of the gluteus maximus ( gm ) , biceps 
 femoris ( bf ) and semitendinosus ( st ) were recorded and normalized by the maximum voluntary 
 isometric contraction ( mvic ) . [ results ] the maximum torque of the hip extensor showed 
 significant decreases between 0and 60 , 90 and 110 of knee flexion . 
 the muscle activity 
 of bf was significantly high at 0 , and gm showed a significantly higher activity than 
 both bf and st at 60 , 90and 110 of knee flexion . 
 [ conclusion ] the maximum torque in hip 
 extension and muscle activities of bf and st were significantly high at 0 but they 
 decreased at knee flexion angles of more than 60. therefore , we consider that more than 
 60 of knee joint flexion is required to increase gm activity , and to reduce the muscle 
 activities of bf and st . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION