patellofemoral pain syndrome ( pfps ) , patellar subluxation , and patellar dislocation are 
 factors that increase knee joint instability and are major causes of anterior knee pain 
 ( akp)1 . the vastus medialis oblique 
 ( vmo ) of the quadriceps is a factor that determines stress on the patellofemoral joint and 
 stabilizes the patellar bone2 . in 
 particular , selective atrophy of the vmo may break the balance of the vastus medialis 
 oblique / vastus lateralis ( vmo / vl ) ratio to cause pfps3 . therefore , selective strengthening of the vmo is necessary to 
 prevent and improve knee joint dysfunction . studies conducted to improve the vmo atrophy that causes pfps have proposed strengthening 
 of the quadriceps using open kinematic chain exercises and closed kinematic chain exercises 
 as a possible solution4 . in particular , 
 squat exercises are known to help the selective strengthening of the vmo , and many studies 
 have investigated the optimum methods for conducting squat exercises4 , 5 . training using 
 unstable surfaces results in greater joint movement and leads to more proprioceptive 
 feedback ; therefore , it can promote muscle activation6 . in the present study , to find the effective parameters of squat exercises , the muscle 
 activities of the vmo , and vl , and the vmo / vl ratio support surfaces were made using hard plate surfaces , foam surfaces , and rubber 
 air discs . the subjects of the present study were 14 healthy adults ( 5 males , 9 females ; average age 
 21.4  years ; average height 167  cm ; average weight 60.3  kg ) without any musculoskeletal or 
 neurologic disorders or any history that might have affected the experimental results . the 
 content of this experiment was sufficiently explained to the subjects , and only those 
 subjects who voluntarily agreed to participate in the experiment were enrolled . the stable surface was the grounds ; the unstable surfaces were 40  47  7  cm foam pads 
 ( balance - pad elite , germany ) , and 13-inch diameter , 3-inch thick rubber air discs ( balance 
 trainer , harbinger , canada ) . the squat exercises to be performed on the different support surfaces were randomly 
 assigned to the subjects . during the squat exercises , subjects ' feet were placed apart at a 
 distance equal to 120% of each subject 's shoulder width . feet positions were determined by 
 foot - shaped marks at the center point of each support surface . the subjects were instructed to keep their arms folded and 
 their trunk upright while performing the exercises . they were also instructed to look at a 
 point in front of them at eye level and to maintain a knee joint flexion angle of 604 . , usa ) was used to measure muscle activity , and a myoresearch 
 xp master 1.07 was used to process the raw data obtained by the measurements . before 
 attaching the electrodes ( t246h , bioprotech , korea ) , the skin at the attachment sites was 
 shaved and cleaned using alcohol swabs . for vl , an electrode was placed at the two thirds of 
 the distance along point the line connecting asis and the lateral side of the patellar bone . 
 for vmo , an electrode was placed at the eight tenths of the distance along point the line 
 connecting asis and the anterior joint space of the anterior border of the medial ligament , 
 which is on the side of the superior medial margin of the patellar bone . the electrodes on 
 the vmo and the vl formed angles of 45 and 15 , respectively , with the femur shaft . electromyogram signals were obtained at a sampling rate of 1,000  hz and processed by a 
 20500  hz band pass filter and a 60  hz notch filter . the raw rmss were divided by the value 
 of maximal voluntary isometric contraction ( mvic ) to convert them to percentage values 
 ( % mvic ) . emg signals were collected for five seconds while the subjects maintained their knee joints 
 at an angle of 60. the first and last seconds of data were discarded , and only the three 
 seconds in the middle were used in the analyses . the 
 ratio of the muscle activity of the vmo to that of the vl was calculated as vmo / vl . spss 
 14.0 for windows was used to compare emg data obtained on the different support surfaces , 
 with a significance level of 0.05 . one - way anova and scheffe 's post hoc test were used to analyze differences in 
 emg data between the different support surfaces . all data are presented as mean  sd the value with different letter suffixes are 
 significantly different ( p<0.05 ) by scheffe 's test unit : % mvic the muscle activity of the vmo was the highest on the rubber air discs , and the differences 
 from the other surfaces were statistically significant ( p<0.05 ) . the muscle activity of 
 the vl was also the highest on the rubber air discs , but the differences from the other 
 surfaces were not statistically significant ( p>0.05 ) . the vmo / vl ratio was the highest on 
 the rubber air discs , and the differences from the other surfaces were statistically 
 significant ( p<0.05 ) ( table 1 ) . the cause of 
 such abnormal motions is the insufficient synergy of the quadriceps1,2,3 . as the number of people playing sports increases , akp has become a 
 pathological condition that frequently occurs in young adults and athletes7 . the subjects of the present study were 
 young adults , who adapted well to the rubber disc training . however , elderly people may be 
 at risk if the rubber air discs , which are highly unstable , are used . kushion et al.8 examined vmo / vl ratios in 
 33 normal adults and reported the vmo / vl ratios ranged from 0.99 to 1.0 . healthy adults ' 
 vmo / vl ratios are close to 1 , and in normal knees , the activities of the vmo and vl are 
 similar . in a study conducted by mariani and caruso9 , the normal group showed the same muscle activity levels for the vmo 
 and the vl at the end range of knee extension , whereas the group with patellar subluxation 
 showed lower vmo / vl ratios , since the muscle activity of the vmo was lower . in the present 
 study , the average vmo / vl ratio on the hard surface was 0.92 , which is close to 1 . kushion et al.8 reported that vmo / vl 
 ratios ranged from 0.99 to 1.0 during leg raise exercises in a supine position . in the 
 present study , a vmo / vl ratio of 0.92 was obtained in the hard plate group . the leg raise 
 exercises in the supine position and squat exercises on hard plates showed similar levels of 
 muscle activation . however , the rubber air disc group showed a vmo / vl ratio of 1.49 . this 
 means that squat exercises on unstable surfaces are more effective at activating the vmo 
 than leg raise exercises . the instability of unstable surface enhances muscle firing and recruitment9 , 10 . 
 in a study of unilateral squat exercises using wobble boards , a vmo / vl ratio of 0.93 was 
 shown , which was higher than the ratio obtained on stable surfaces11 . unstable surfaces are effective at enhancing vmo / vl ratios . in the present study , these results are consistent with 
 arguments indicating that training on unstable surfaces enhances muscle activity more than 
 training on stable surfaces . in the present study , squat exercises were performed on support surfaces with different 
 levels of instability and the vmo / vl ratios were calculated . the vmo / vl ratios were 0.92 on 
 the stable hard surface , 0.88 on the foam surface with low levels of instability , and 1.49 , 
 the highest , on the rubber air discs , which had the highest level of instability . these 
 results mean that squat exercises performed on unstable surfaces with high levels of 
 instability can enhance the activity of the vmo , and that squatting exercises performed on 
 foam surfaces with low levels of instability or hard surfaces are not as effective for the 
 selective activation of the vmo . we consider the reason why the activity of the vmo is 
 particularly highly enhanced on rubber air discs is the fact that unstable surfaces make 
 maintaining ankle joint positions difficult , so that more effort is required to maintain 
 posture6 . these characteristics of 
 exercises on unstable surfaces are considered to better stimulate the receptors in the 
 joints , thus increasing the afferent inputs of the proprioceptive senses and eventually 
 leading to increased reflexive motor responses12 .
[ purpose ] the purpose of the present study was to examine the effects of squat exercises 
 performed on different surfaces on the activity of the quadriceps femoris muscle in order 
 to provide information on support surfaces for effective squat exercises . 
 [ subjects and 
 method ] fourteen healthy subjects performed squat exercises for five seconds each on three 
 different support surfaces : hard plates , foam , and rubber air discs . 
 their performance was 
 measured using electromyography . 
 as the subjects performed the squat exercises on each 
 surface , data on the activity of the vastus medialis oblique and the vastus lateralis , and 
 the vastus medials oblique / vastus lateralis ratio , were collected . 
 [ results ] the activity 
 of the vastus medialis oblique and the vastus medialis oblique / vastus lateralis ratio were 
 found to be statistically significantly higher on rubber air discs than when the squats 
 were performed on hard plates or foam . 
 [ conclusion ] to activate the vastus medialis 
 obilique , and to enhance the vastus medialis oblique / vastus lateralis ratio , unstable 
 surfaces that are highly unstable should be selected .