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Paper text:
it is an exercise that can 
 strengthen the lower limbs while minimizing the stress on the anterior cruciate ligament by 
 reducing the shearing force on the knee joint through reduction in compression force on the 
 joint and simultaneous contraction . in addition
, this exercise induces flexion of the ankle 
 joint , knee joint and hip joint1 . 

moreover , squat exercise has the advantage of allowing the intensity of training by using an 
 additional load , or cable machine , a smith machine and vibration platform , though it can be 
 performed with no additional load2 . since squat exercise
is being utilized for a diverse range of purposes and methods , 
 discerning the advantages and disadvantages of each exercise method is important . improper 
 posture or indiscriminate execution of the exercise
in 
 particular , posture during a squat exercise , which involves repeated bending and 
 straightening of the upper body , can become a major risk factor for a lumbar disorder and 
 can be associated with the occurrence of lower back pain and other disorders related to the 
 exercise3,4,5 .
the majority of the existing studies focusing on squat exercises have either examined the 
 extent of the activation of the lower limb muscles by diversely applying the basal 
 plane2 , 6 or have examined the changes in the ratio of the extent of 
 activation of the anterior and lateral muscles of the lower limb muscles in relation to the 
 location of the knee joint or ankle joint7 . in addition , these studies have presented a squat exercise method for 
 the effective training of the lower limb muscles by studying both the extent of the 
 activation of the lower limb muscles and the resistance against during adduction of the hip 
 joint or flexion of the shoulder joint when a squat exercise is executed8 , 9 . 

accordingly , the majority of the existing research has focused on the extent of the 
 activation of the leg muscles and the exercise dynamics in relation to the structure of the 
 supporting plane or the posture of the legs when the squat exercise is executed . however , in 
 spite of the fact that the location of the trunk of the body imparts an effect on the extent 
 of the activation of the muscles in the lower back and lower limbs during squat exercise , 
 there is inadequate research on this aspect of this exercise .
the research related to the 
 trunk curvature is mostly study of the kinematics of the trunk during restricted and 
 unrestricted squat10 , 11 .
therefore , this study aims to present a safe and effective squat exercise method that can 
 reduce injuries arising from the exercise by examining the effects of the posture of the 
 trunk of the body on the extent of the activation of the lower back and leg muscles in 
 normal people during squat exercise .
the purpose and methods of the study were explained to all potential subjects , and each 
 subject voluntarily agreed to participate .
this study was approved by the institutional review board of the 
 catholic university of pusan ( cupirb-2016 - 009 ) and was conducted in accordance with the 
 ethical principles of the declaration of helsinki , good clinical practices , and applicable 
 laws and regulations .
this study was conducted on ten healthy males and ten healthy females 
 ( age : 21.10  1.83  years [ mean  standard deviation ] , height : 168.70  8.26  cm , weight : 
 66.10  12.31  kg ) .
those with leg or spine injuries , those with limited range of the 
 movement of the trunk or leg joints , and those with an angle of the patellofemoral ( the q 
 angle ) that did not belong to 13.5 (  5 ) were excluded from this study . a surface electromyography ( emg )
system ( noraxon telemyo dts telemetry ) was used to record 
 the activity of the rectus femoris muscle , gluteus maximus muscle , erector spinae muscle and 
 biceps femoris muscle .
the recorded emgs were processed using a personal computer and 
 myorearch xp 1.06 master edition software .
the activities of the muscles were recorded by 
 surface electrodes , following the recommendations of seniam12 and isek13 , 14 for skin preparation and electrode 
 position . to minimize the impedance of the skin , the subjects hair was shaved and the 
 electrode sites were cleaned with alcohol on a cotton swab . prior to the participation and collection of the data in the experiment ,
all subjects were 
 provided with detailed explanations of the proper posture of squat movements , and these 
 postures were demonstrated for the subjects and practiced by them . for the squat posture , 
 each subject was instructed to spread both legs as far as the width of their shoulders , with 
 both arms interlocked in front of the chest .
lumbar spine was hold neutral position and 
 pelvic controlled excessive posterior rotation during each squat .
then , each subject was 
 told to slowly lower their upper body until the knee joint was bent to an angle of 90 and 
 to hold the posture for five seconds before slowly returning to the standing posture .
the 
 angle of the trunk of the body was measured three times at each of the angles of the bending 
 of the trunk by bending to 0 , 15 , and 30. the measurements were taken using a goniometer 
 at the center of the lateral part of each subject s trunk , with 30 seconds of rest between 
 each of the measurements .
the subjects were given three minutes of rest after each change in 
 the angle of the bending of the trunk in order to avoid muscle fatigue .
data were processed using spss 19 for windows . to compare muscle activity during the squat 
 exercises with 0 , 15 and 30 of truck flexion , a repeated measures analysis of variance 
 was used . to identify differences for each muscle , bonferroni s post hoc test was performed . 

although there were no statistically significant differences in the extent of activation of 
 the erector muscles of the spine , there was a trend of an increase in the extent of muscle 
 activation of the erector muscles of the spine with an increase in the angle of the bending 
 of the trunk of the body ( p>0.05 ) .
the muscle activation of the gluteus maximus and 
 biceps femoris significantly increased as the angle of the bending of the trunk increased 
 ( p<0.05 ) .
the muscle activation of the rectus femoris displayed a tendency of reduction 
 with an increase in the angle of the bending of the trunk , but this tendency was not 
 statistically significant ( p>0.05 ) ( table 
 1table 1.differences in muscles activation for each experimental condition ( n=20 , units : 
 % mvic)muscles01530erector spinae ( m)21.5  9.722.6  7.224.8  7.5gluteus maximus ( m)6.1  4.06.27 4.08.0  4.9*biceps femoris ( m)4.4  3.55.7  5.25.8  5.2*rectus femoris ( m)21.4 
the purpose of this study was to examine the changes in the extent of muscle activation of 
 the erector muscle of the spine , the gluteus maximus , the biceps femoris , and the rectus 
 femoris in relation to the angle of the bending of the trunk of the body during a squat 
 exercise .
the human trunk is associated with the transfer of energy and the connection of movements 
 between the lower and upper body15 , 16 .
therefore , the ability to execute 
 exercises such as jumping or squatting can differ depending on the location of the trunk , 
 vertical stiffness , and muscle activity17 , 18 . in this study ,
the muscle activation of the gluteus maximus and biceps femoris increased 
 with an increase in the angle of the bending of the trunk during squat exercise . in 
 particular , there was a significant increase when the trunk was bent at a 30 angle . 
 therefore , as the trunk is bent more toward the direction of gravity , the gluteus maximus 
 and biceps femoris , which are the extensors of the hip joint , control the bending of the hip 
 joint through eccentric contraction . although the gluteus maximus and biceps femoris act as the main movement muscles for the 
 extension of the hip joint , the action of the biceps femoris , a muscle involved with two 
 joints , on the flexion of the knee joint in the case of a closed - chain exercise , such as a 
 squat exercise , must be considered . in other words , the biceps femoris
acts not only on the 
 extension of the hip joint during a squat exercise but also at the moment of flexion of the 
 knee joint , thereby playing the antagonistic role against the moment of extension of the 
 knee joint19 .
the moment of the flexion 
 of the knee joint will become even larger with the increase in the moment of the extension 
 of the knee joint . however , based on the observation that there is greater transfer of the 
 center of gravity toward the front of the basal plane with an increase in the bending of the 
 trunk , which reduces the muscle activation of the rectus femoris , the moment of extension 
 decreases
. therefore , the more the trunk is bent , the more the action of the biceps femoris 
 on the extension of the hip joint would have increased compared with the moment of the 
 flexion of the knee joint . therefore , squat exercise with fortified bending of the trunk 
 will result in a reduction in the muscle activation of the knee joint extensor along with 
 greater action of the hip joint extensor .
when the upper body is bent forward , the load on the trunk increases significantly . bending 
 of the upper body while in a standing posture will increase the compression on the 
 intervertebral disc by almost 216% in comparison to an erect posture20 .
a bent posture induces greater action of the muscles used 
 to maintain the balance of the upper body21 .
this can be one of the main causes of problems in the lower 
 back . according to albayrak et al.22 ,
the 
 activation of the erector muscles of the spine and hamstring increased significantly when 
 the upper body was bent forward without the support of the trunk . in this study ,
the extent 
 of the activation of the erector muscles of the spine and hamstring increased as the lower 
 back was tilted forward during the squat exercise .
the erector muscles of the spine 
 underwent eccentric contraction in order to adjust the tilting of the trunk forward during 
 squat exercise .
accordingly , although the actions of the quadriceps muscle of the thigh 
 increase , the activities of the erector muscles of the spine and hamstring are relatively 
 reduced when a squat exercise is executed while the trunk is kept erect without bending 
 forward . on the other hand , the actions of the erector muscles of the spine and hip joint 
 extensor increase relatively while the actions of the quadriceps muscle of the thigh 
 decrease when the trunk is bent by 30. therefore , a repetitive squat exercise while the 
 trunk is bent forward can increase the load on the lower back and induce excessive fatigue 
 of the erector muscles of the spine .
moreover , if additional load is exerted , this load and 
 fatigue will increase even further . in conclusion
, squat exercise should be executed while maintaining an erect trunk posture 
 in order to strengthen the quadriceps muscle while reducing the load on the lower back . 
 moreover , although the execution of squat exercise while the trunk is in a bent posture can 
 actually fortify the strength of the biceps femoris and gluteus maximus , the load on the 
 lower back will be increased substantially with this posture .
the limitations of this study include the failure to uniformly adjust the distribution of 
 the weight of the left and right lower limbs during squat exercise and the failure to 
 observe the changes in the extent of the activation of the abdominal muscles in relation to 
 the extent of the bending of the trunk . in order to supplement these limitations , examining 
 the extent of activation of the abdominal muscles together while controlling to ensure the 
 weight on the left and right are the same would be necessary in future research .
[ purpose ] the purpose of this study was to investigate the activation of back and lower 
 limb muscles in subjects who were performing a squat exercise at different angles of trunk 
 flexion . 
 [ subjects and methods ] twenty healthy subjects ( age 21.1 1.8  years , height 168.7 
  8.2  cm , weight 66.1  12.3  kg ) volunteered . 
 the activation of the erector spinae muscle , 
 rectus femoris muscle , gluteus maximus muscle and biceps femoris muscle was observed while 
 the subjects performed squat exercises with a trunk flexion of 0 , 15 , and 30. [ results ] 
 the erector spinae muscle , gluteus maximus muscle , and biceps femoris muscle were 
 activated more during the squat exercise with the trunk flexion at 30 than the exercise 
 with the trunk flexion at 0. the rectus femoris muscle showed a tendency to decrease as 
 the truck flexion increased . 
 [ conclusion ] squat exercise be executed while maintaining an 
 erect trunk posture if one wishes to strengthen the quadriceps muscle while reducing the 
 load on the lower back .