the gluteus maximus ( gm ) plays a major role in functional activity as a trunk and hip 
 extensor .
weakness of the gm could cause dysfunction in the pelvic and hip joints and back 
 pain1 .
prone hip extension ( phe ) is an 
 exercise that can activate the gm effectively and is popularly used in clinics2 .
however , unwanted substitution motion , such 
 as pelvic anterior tilt or excessive lumbar extension , could happen during phe due to 
 stiffness of the hip flexor , dominance of the erector spinae ( es ) , weakness of the gm , and 
 so forth3 .
therefore , recent studies have 
 focused on controlling unwanted substitution motion of the pelvis and lower back during 
 phe4,5,6 .
many previous studies about phe have been performed at the 
 0 hip flexed position8,9,10 .
some other studies have 
 used starting positions varying from 20 to 90 hip flexion for people with hip flexor 
 contracture6 , 7 , 11 .
. however , little information 
 exists concerning whether hip joint angles during phe selectively change the emg activity of 
 the gm .
therefore , the purpose of this study was to examine the effects of initial hip joint 
 position on the muscle activity of the es , gm , and biceps femoris ( bf ) as measured by 
 electromyography ( emg ) of the hip extensor as well as the effect of the angle of pelvic 
 anterior tilt . through this process
, we sought the most effective position for selective gm 
 muscle activation during phe .
fifteen healthy adults were enrolled in this study ( age , 26.73.7  years [ meansd ] ; height , 
 167.19.2  cm ; mass , 58.111.7  kg ) .
exclusion criteria were neuromuscular or musculoskeletal dysfunction of the lower back and 
 both legs , lower back or hip pain , contracture of the hip flexor , or a hip extensor strength 
 below fair .
all subjects agreed to participate and signed an informed consent form approved 
 by the inje university ethics committee for human investigations . prior to testing ,
kinematic data of 
 the pelvis were recorded at 100  hz using an eight - camera vicon mx - t20 motion capture system 
 ( vicon motion systems ltd . ,
the respective muscle activities of the bilateral 
 es , unilateral gm , and biceps femoris bf were collected using a delsys trigno wireless emg 
 system ( delsys inc . , natick , ma , usa ) , which was synchronized with a vicon motion system . 

the emg signals were sampled at 2,000  hz and filtered at 20 to 450  hz , and the root mean 
 square values of the signals were calculated .
the emg data were expressed as a percentage 
 relative to the maximal voluntary contraction for normalization .
a newly developed 
 two - segment model consisting of the lumbar spine and pelvis was used to measure the angle of 
 pelvic anterior tilt .
the lumbar segment was defined with four markers located on the 
 subject s l1 and l2 spinous processes , and 1  cm lateral from the l2 spinous process on both 
 sides .
the pelvic segment was defined with three markers located on the mid - point of the 
 posterior superior iliac spine and the bilateral anterior and superior iliac spines .
the subjects 
 lifted their right legs during phe at three starting positions arranged in random order ( 0 , 
 20 , and 45 hip flexed position ) .
with phe at 0 hip flexion , each subject was positioned 
 prone on a table consisting of 2 segments that were jointed in a way that it was possible to 
 adjust the table s position so the subject s hip could be positioned at an initial hip 
 flexion of 20 or 45. a bar placed over the table would come in contact with the achilles 
 tendon when the subject s hip was extended to 30 from the initial position .
emg activities 
 and pelvic kinematics were measured when subjects extended from their initial position .
all 
 subjects performed 3 repetitions with a 10-second rest between trials and rested for 2 
 minutes between each testing position .
pasw statistics for window ( ver . 18.0 ; spss inc . , 
 chicago , il , usa ) was used to analyze differences in muscle activities and pelvic 
 kinematics .
repeated measures anova and bonferroni - adjusted post hoc comparisons were used 
 to compare the respective muscle activities of the es , gm , and bf and the amount of pelvic 
 anterior tilt among the three initial positions .
there were significant differences in es , gm and bf muscle activity on the left side among 
 the three positions during phe .
muscle activity for the es on the right side did not show 
 any difference in emg among the three positions .
the amount of pelvic anterior tilt showed a 
 significant decrease at the 20 hip flexion position compared with that at the 0 angle 
 ( table 1table 1.normalized emg data and amount of pelvic anterior tilt ( n=15)variableship joint position ( meansd)02045lt . es ( % mvic)49.515.344.112.431.713.2rt .
bf ( % mvic)36.59.433.39.638.711.1pelvic anterior tilt ( )7.42.05.92.47.32.8bf : biceps femoris ; es : erector spinae ; gm : gluteus maximus ; lt .
bf : biceps femoris ; es : erector spinae ; gm : gluteus maximus ; lt . : left ; mvic : muscle 
 voluntary isometric contraction ; rt . :
of the three positions , active phe at 45 resulted in a significant decrease in 
 muscle activity of the contralateral es compared with the other positions .
the role of the es as a global stabilizer was reduced at 45 
 because alteration of this position from the initial position to 30 hip extension induced 
 pelvic anterior tilt .
therefore , we consider that the 45 flexed position would be able to 
 effectively prevent excessive lumbar motion and maintain an upright pelvis alignment
we think that the 45 hip flexed 
 position decreased internal momentum due to the short lever arm in comparison with other 
 positions12 .
although the muscle activity of the contralateral es tended to decrease at 20 , this 
 decrease was not significant .
this investigation revealed that the 20 hip flexed position 
 needs an additional 10 hip extension for a total of 30 extension during phe , which may 
 result in contraction of the es .
however , this position increased gm activity compared with 
 the other positions and decreased bf activity compared with that occurring with the 45 hip 
 flexion angle . additionally , the amount of pelvic anterior tilt at 20 was lower than those 
 at 0 and 45. we consider that the pelvic anterior tilt provided by 20 hip flexion led to 
 the optimal length by elongation of the gm , which may contribute to increases in gm 
 activity12 .
worrell et al.7 demonstrated that gm 
 activity was greatest at the 0 angle and that hs muscle activity did not differ among all 
 the positions they studied , which was not in line with the results of our study .
they 
 examined gm and bf activity with changes in hip and knee joint angles during maximal 
 voluntary isometric contraction , whereas we investigated muscle activities with initial 
 positions to 30 extensions of the hip joint during active phe with knee extension . 

chance - larsen et al.9 reported that 
 restraint of pelvic anterior tilting during phe decreased the length of the bf and 
 influenced the synergistic relationship between the gm and bf .
of three positions , 0 could 
 negatively impact the lumbar segment by increasing the es activity and 45 did not activate 
 the gm in spite of the reduction of es activity .
hence , the 45 position could be 
 recommended for persons who need to minimize the weight - bearing load on the lumbar spine . in 
 conclusion , we suggest that 20 is the optimal position to activate the gm selectively by 
 minimizing additional pelvic anterior tilt .
first , our sample size was small , so it is 
 difficult to generalize the findings to all subjects .
second , we examined the changes in 
 emg - measured muscle activity at only three joint angles ; it is possible that the results do 
 not indicate the optimal position for phe .
therefore , further studies are needed to 
 investigate the changes at the various phe joint angles .
 [ purpose ] the aim of this study was to identify the effects of initial position of the 
 hip joint with changes in the hip joint angle on the respective muscle activities of the 
 bilateral erector spinae ( es ) , unilateral gluteus maximus ( gm ) , and biceps femoris ( bf ) 
 and the amount of pelvic anterior tilt during prone hip extension ( phe ) . [ subjects ] 
 fifteen healthy volunteers were enrolled in this study . 
 [ methods ] the subjects performed 
 phe in three positions : neutral , 20 , and 45 flexed hip joint . 
 the activities of the es , 
 gm , and bf were measured using surface electromyography , and kinematic values for pelvic 
 anterior tilt were calculated using a motion capture system . 
 [ results ] there was a 
 significant decrease in muscle activity of the contralateral es at 45 , and an increase in 
 the gm muscle activity and decrease in the bf muscle activity at 20. the amount of pelvic 
 anterior tilt was lower at 20. [ conclusion ] these results suggest that a hip flexion 
 position of 20 would have an advantage over the other measured positions . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION