because the gluteus maximus ( gm ) muscle fibers are aligned perpendicular to the sacroiliac 
 joint , gm contractions compress the sacroiliac joint , and contribute to pelvic 
 stability1 .
gm contraction also 
 contributes to the transmission of force from the lower extremity to the pelvis through the 
 sacroiliac joint during functional activities , such as ambulation2,3,4 .
bruno et al.5 
 reported that patients with low back pain have a significantly delayed onset of gm activity 
 when performing prone hip extension movements compared to healthy subjects .
hungerford et 
 al.6 reported that when the 
 contralateral hip is flexed , delayed gm activity and early hamstring ( ham ) activity in the 
 supporting leg occur in patients with sacroiliac joint pain .
inappropriate timing of gm 
 activation during gait is believed to be one of the causes of low back pain , resulting from 
 a deficient shock absorption mechanism in the sacroiliac joint3 . from these observations , improving the activation pattern of the gm 
 is important for the treatment and prevention of low back and sacroiliac joint pain7 .
a systematic review that investigated exercise therapy for improving muscle activity onset 
 times concluded that training involving selective muscle activation improves muscle activity 
 onset times8 , suggesting that training 
 involving selective gm activation might improve gm activity patterns in people with low back 
 pain .
one exercise for suppressing ham activity and activating gm that is often used 
 clinically is prone hip extension with knee flexion ( phekf)9 .
al.10 
 and sakamoto et al.9 investigated the 
 impact of muscle activity during phekf exercise using hip joint abduction and external 
 rotation to discover a method for activating the gm . the impact of a compound movement 
 involving hip joint abduction and external rotation on muscle activity during phekf 
 exercise , however , has not been investigated .
therefore , this study investigated the muscle 
 activity of selective gm activation during phekf exercise in different hip joint 
 positions .
the subjects were 21 healthy , male volunteers ( average age , 20.2  0.4  years ; average 
 height , 171.1  5.0  cm ; average weight , 64.3  10.5  kg ) .
individuals were excluded if they 
 had experienced musculoskeletal pain within the past 12 months ; had a history of surgery 
 involving the lower limbs , spine , or pelvis ; demonstrated hip flexor shortness , as assessed 
 by the thomas test ; or had tensor fasciae latae shortness , as assessed by ober s test .
this 
 study was performed with approval from the kawasaki university of medical welfare ethics 
 committee ( approval number , 418 ) .
we obtained written informed consent from each participant 
 after giving them a complete explanation of the study .
muscle activities were measured using a surface electromyograph ( vital recorder 2 , kissei 
 comtec , nagano , japan ) with a 1,000-hz sampling frequency .
we measured the right gm , halfway 
 between the greater trochanter and the second sacral vertebra ; the right ham , approximately 
 halfway between the gluteal fold and the popliteal fold10 ; the bilateral lumbar erector spinae at the l1 level , 23  cm 
 external to the spinous process ; and the bilateral lumbar multifidus ( mf ) at the l5/s1 
 level , immediately lateral to the spinous process .
disposable electrodes ( blue sensor 
 m-00-s , ambu , ballerup , denmark ) applied after appropriate skin preparation .
inter - electrode 
 gaps were set to 2.5  cm , and the reference electrode was attached to the second sacral 
 vertebra .
measurements were made with the hip joint in each of 3 positions : ( 1 ) the neutral hip joint 
 position ( position n ) , ( 2 ) the abduction hip joint position ( position ab ) , and ( 3 ) the 
 abduction with external rotation hip joint position ( position aber ) .
each participant was 
 positioned prone , with 90 of right knee flexion . in position
n , the right hip joint had 0 
 of abduction and 0 of external rotation . in position
ab , the joint had 15 of abduction and 
 0 of external rotation ; and position aber involved 15 of hip joint abduction and 20 of 
 external rotation .
the measurement of the hip joint angle was conducted using a goniometer 
 ( og giken , okayama , japan ) .
the hip joint s abduction angle was determined by the angle 
 formed by the thigh center line and a line perpendicular to a line connecting both posterior 
 superior iliac spines .
the angle of external rotation was determined by the angle formed by 
 the lower leg center line and a plumb line passing through the patella .
bars were installed 
 vertically inside and outside the right thigh to avoid changing the abduction angle during 
 the measurements .
each subject was instructed to extend his right hip joint until the 
 patella was raised 5  cm above the bed , while actively maintaining the knee flexion angle , 
 hip abduction angle , and hip external rotation angles ; extension of the hip joint was 
 maintained for 5 s10 . before data 

acquisition , all subjects practiced the phekf exercise for 5  min to familiarize themselves 
 with the testing procedure .
all electromyograph waveforms were processed through a band - pass filtered ( 20500  hz ) , then 
 full - wave rectified was subsequently performed , and the average amplitudes were determined 
 over the 5  s of hip extension . to reduce the variability for each measured muscle
, we 
 averaged 3 measurements of each position as a representative value and normalized the 
 average value using the average amplitude during maximum voluntary contraction ( mvc ) as 
 % mvc .
the gm / ham ratio was calculated by dividing the gm activity by the ham activity to 
 determine whether gm could be selectively activated .
. 21 software for windows ( ibm , armonk , ny , usa ) was used for all statistical 
 analyses .
we confirmed muscle activity tested the normality of the data using the 
 shapiro - wilk test .
the intrarater reliability of muscle activity for each of the 3 hip joint 
 positions was analyzed using an intraclass correlation coefficients [ icc ( 1 , 3 ) ] .
since the 
 muscle activity data were not normally distributed , non - parametric methods were used .
the 
 friedman repeated measures analysis of variance was used to detect differences in muscle 
 activities among the three positions ( n , ab , and aber ) . post hoc analyses using the wilcoxon 
 signed - rank tests were conducted with a holm s correction ( or adjustment ) applied .
the icc ( 1 , 3 ) of muscle activities of each of the 3 hip joint positions were 0.910.98 , 
 indicating that the intrarater reliabilities were excellent .
the electromyography activities 
 ( median , interquartile range ) of each muscle in each position are shown in table 1table 1 .
electoromyographic activity of each muscle in each position 
 ( % ) positionnpositionabpositionabercomparisongm14.1 ( 9.4)22.5 ( 13.6)41 ( 23.6)n - ab*n - aber*ab - aber*ham9.7 ( 9.3)9.2 ( 11.8)7.2 ( 10.4)n - aberab - aber*rt es20.8 ( 9.4)20.1 ( 12.0)13.8 ( 12.5)n - abn - aber*ab - aber*rt lm30.9 ( 8.0)33.4 ( 10.4)32.5 ( 9.5)n - ab*lt es23.3 ( 11.6)20.1 ( 10.6)21.6 ( 7.4)n - ab*lt lm28.3 ( 11.0)27.9 ( 9.1)22.4 ( 9.8)n - aber*ab - aber*gm / ham1.3 ( 2.2)2.5 ( 3.1)4.9 ( 4.4)n - ab*n - aber*ab - aber*median ( interquartile range ) * p<0.017 , p<0.025 , p<0.05 n : position n , ab : 
 position ab , aber : position aber rt : right , lt : left , gm : gluteus maximus , ham : 
 hamstrings , es : lumbar erector spinae , lm : lumbar multifidus . the gm activity and gm / ham ratio were highest in the aber position , followed 
 by positions ab and n. we observed significantly increased gm activity and gm / ham ratio in 
 position aber compared to those in the other positions . the gm activity and gm / ham ratio 
 were also significantly higher in position ab than in position n. the ham , right lumbar 
 erector spinae , and left lumbar multifidus showed the lowest emg activity levels in the aber 
 position , followed by position ab and position n. the ham , right lumbar erector spinae , and 
 left lumbar multifidus showed significantly lower activity in position aber than in the 
 other positions .
the bilateral lumbar erector spinae showed significantly lower activity in 
 position ab than in position n , and the right lumbar multifidus showed significantly higher 
 activity in position ab than in position n. median ( interquartile range ) * p<0.017 , p<0.025 , p<0.05 n : position n , ab : 
 position ab , aber : position aber rt : right , lt : left , gm : gluteus maximus , ham : 
 hamstrings , es : lumbar erector spinae , lm : lumbar multifidus
in patients with low back and sacroiliac joint pain , gm activity delays and early ham 
 activity have been reported5 , 6 .
these muscular activity patterns cause sacroiliac 
 instability and increase the strain on the soft tissue3 .
this study , therefore , investigated ham suppression and the 
 selective activation of the gm during the phekf exercise , with the hip joint in different 
 positions .
we observed that gm activity was significantly increased in positions ab and aber compared 
 to in position n. this result is consistent with the report by kang et al.10 , who studied the effects of phekf exercise 
 during hip joint abduction . by performing hip abduction during phekf exercise ,
the direction 
 of muscle pull runs parallel to the fibers of the muscle , leading to increased emg 
 amplitudes .
gm activity was also significantly higher in the aber position than in position 
 ab .
this is likely because , in position aber , the gm is in a shortened state , making muscle 
 contraction inefficient , possibly necessitating increased gm muscle activity to extend the 
 hip joint .
ham activity was significantly lower in position aber than in positions n and ab . the 
 height to which the legs were elevated during the phekf exercise was constant , and the gm 
 activity increased in position aber .
ham is a synergist of hip extension , and the factors 
 noted above may have reduced the ham work load during hip extension , thus reducing ham 
 activity .
in addition , externally rotating the hip joint and , thereby , reducing the hip 
 extension moment that the ham exerts , may also have reduced ham activity .
the right lumbar erector spinae activity was also significantly lower in position aber than 
 in positions n or ab . the bilateral lumbar erector spinae activity was significantly lower 
 in position ab than in position n. on the other hand , the gm / ham ratio was significantly 
 higher in position aber than in positions n or ab and was significantly higher in position 
 ab than in position n. tateuchi et al.12 
 reported a negative correlation between the gm / ham ratio and the lumbar erector spinae 
 muscle activity during hip extension exercises in the prone position . similarly , we propose 
 that lumbar erector spinae muscle activity was decreased since the gm / ham values were high 
 in positions ab and aber .
one conceivable factor causing the increase in the gm / ham ratio 
 and the decreased in lumbar erector spinae activity during hip extension is that the moment 
 arm of hip extension , involving the gm in hip extension , is higher than that of the ham13 , 14 .
therefore , the increased gm activity improved the power production 
 efficiency of hip extension and decreased the compensatory lumbar erector spinae activity 
 during hip extension .
n or ab . additionally , the right lumbar multifidus activity was significantly 
 higher in position ab than in position n. the multifidus muscle stabilizes the spine ; when 
 it acts bilaterally , it has a spine extension function , and when it acts unilaterally , it 
 rotates the spine15 , 16 .
kim et al.17 
 reported that when lumbar rotation moment is applied , the multifidus muscle is activated 
 more to overcome the load of rotation . in positions ab and aber
therefore , to counteract this , the right 
 multifidus muscle activity has to increase in position ab and the left multifidus muscle 
 activity is necessarily decreased in position aber .
second , surface emg was used to investigate muscle activity , suggesting 
 the possibility of crosstalk from adjacent muscles .
third , we did not consider lumbopelvic 
 motion when measuring the effect of muscle activity in each position during phekf . 

therefore , we were not able to describe lumbopelvic motion and its relationship with muscle 
 activity .
this relationship , during phekf exercise in different hip joint positions , should 
 be confirmed in future investigations of patients with low back pain . in the present study , the gm / ham ratio was highest in the aber position , followed by 
 positions
ab and n , indicating that abduction and external rotation of the hip joint 
 selectively activates the gm . furthermore , lumbar erector spinae activity decreased in 
 positions ab and aber .
excessive activity of the erector spinae , during prone hip extension , 
 has been reported in patients with low back pain18 .
increased erector spinae activity may cause muscle pain and 
 contribute to a vicious pain - spasm - pain cycle and increased compression of the spine19 .
this suggests the possibility that 
 abduction and external rotation of the hip joint during phekf exercise may be beneficial for 
 selective gm strength training in people with low back pain .
 [ purpose ] this study investigated the selective activation of the gluteus maximus during 
 a prone hip extension with knee flexion exercise , with the hip joint in different 
 positions . [ subjects ] the subjects were 21 healthy , male volunteers . [ methods ] activities 
 of the right gluteus maximus , right hamstrings , bilateral lumbar erector spinae , and 
 bilateral lumbar multifidus were measured using surface electromyography during a prone 
 hip extension with knee flexion exercise . 
 measurements were made with the hip joint in 
 each of 3 positions : ( 1 ) a neutral hip joint position , ( 2 ) an abduction hip joint 
 position , and ( 3 ) an abduction with external rotation hip joint position . 
 [ results ] 
 gluteus maximus activity was significantly higher when the hip was in the abduction with 
 external rotation hip joint position than when it was in the neutral hip joint and 
 abduction hip joint positions . 
 gluteus maximus activity was also significantly higher in 
 the abduction hip joint position than in the neutral hip joint position . 
 hamstring 
 activity was significantly lower when the hip was in the abduction with external rotation 
 hip joint position than when it was in the neutral hip joint and abduction hip joint 
 positions . [ conclusion ] 
 abduction and external rotation of the hip during prone hip 
 extension with knee flexion exercise selectively activates the gluteus maximus . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION