the knee joint has the capacity to bear weight and a wide range of motion ( rom ) , so it 
 plays a role in providing weight support and activity control1 . however , since the knee joint is constantly influenced by 
 body - weight loading at various angles , it is vulnerable to injury2 . patellofemoral pain syndrome ( pfps ) , an overuse injury 
 characterized by anterior knee pain , is a condition commonly diagnosed in young 
 individuals3 , 4 . although it is a very common condition , its etiology has not been 
 clearly defined yet5 , 6 . it has been reported that the causes for pfps are multifactorial . among the many risk 
 factors including structural abnormalities , kinematic variables , and onset timing of medial 
 and lateral vastii muscles , many studies have reported that 
 lateralization of the patella is observed in patients with pfps , and this abnormal tracking 
 is caused by an imbalance between the vastus medialis oblique ( vmo ) muscle and the vastus 
 lateralis ( vl ) muscle6 , 9 . the ratio of the vmo muscle activity relative to the vl muscle 
 activity in pfps patients is less than that in healthy subjects10 . in the patellofemoral joint , the vmo muscle is in charge 
 of stabilizing the medial side of the patella , so it is thought that knee imbalance may 
 result from weakness of the vmo muscle11 . selective strengthening exercises for the vmo muscle have been investigated to solve 
 problems caused by pfps . among the many exercise methods , closed kinetic exercise such as 
 squat exercise can influence on the reduction of stress in the patellofemoral joint1 , 11 . 
 in order to clarify the type of the exercise method that can selectively strengthen the vmo 
 muscle , the present study investigated the effects of squat exercises with various tools 
 including a gym ball , a wedge , and an elastic band . a total of twenty healthy subjects with no history of neurological , musculoskeletal injury , 
 or pain in the lower limbs were recruited . the mean age , height , and weight of the 
 participants were 21.501.10  years , 166.859.26  cm , and 57.958.44  kg , respectively ( table 1table 1.general characteristics of the subjectsage ( years)height ( cm)weight ( kg)subjects21.51.1166.99.358.08.4values are expressed as the meansd ) . the purpose and procedures of this study were explained to all subjects , and 
 they provided written informed consent prior to their participation . values are expressed as the meansd all subjects performed four types of squat exercise : the conventional squat exercise , the 
 squat exercise with a gym ball , the squat exercise with a wedge , and the squat exercise with 
 an elastic band . the conventional squat exercise ( se1 ) is a static posture held for 6 
 seconds with the legs spread shoulder - length apart and the heels planted firmly on the 
 ground while bending the knees to a 90-degree angle . the squat exercise with a gym ball 
 ( se2 ) is a static posture that is an extension of se1 with a gym ball between the knees 
 while adducting the hips in order to not drop the ball . the squat exercise with a wedge 
 ( se3 ) is an extension of se1 performed on a wedge board . the squat exercise with an elastic 
 band ( se4 ) is also an extension of se1 with an elastic band above the left knee to a 45 
 degree angle . . a sampling rate of 1,000  hz was used for emg signal acquisition , and 
 the signals were full - wave rectified . band - pass filtering at 30500  hz was performed using 
 the myoresearch xp 1.07 ( noraxon u.s.a . , scottsdale , az , usa ) software , and the 
 signals were also notch filtered at 60  hz to remove noise . the maximum voluntary isometric contractions ( mvic ) values of each muscle were used to 
 normalize the values of the muscle activities . statistical analysis was performed using pasw statistics for windows ( version 18.0 ) . in 
 order to assess emg activities according to squat exercises , one - way repeated measures 
 analysis of variance all measurements were expressed as the mean standard 
 deviation , and significance was accepted at values of p<0.05 . the emg activity of the vmo muscle in the squat exercise with a wedge was significantly 
 higher than that in the squat exercise with an elastic band . there were no significant 
 differences although vmo muscle activity in the conventional squat exercise and the squat 
 exercise with a wedge were higher than those in the squat exercise with a gym ball and the 
 conventional squat exercise , respectively . there were no significant differences among the 
 emg activities of the vl muscle ( table 
 2table 2.comparison of activity in the vl and vmo muscle according to the type of the 
 squat exercise ( unit : % mvic)se1se2se3se4vl50.619.351.921.454.124.649.819.7vmo54.731.354.329.956.533.2 * 50.829.1values are expressed as the meansd . * significant difference vs. se4 ( p<0.05 ) ) . pfps is usually described as a common condition related to knee disorder , especially in 
 young active females and athletes8 . the 
 symptoms of pfps mainly appear when ascending stairs or sitting with the knee flexed for a 
 long period of time , and persistence of symptoms can influence the development of 
 osteoarthritis7 , 11 , 12 . according to many studies , it has been considered that weakness of the vmo muscle results 
 in pfps and that this condition is caused by an imbalance in activity between the vmo and 
 the vl muscle13 . in general , 
 strengthening exercise for the quadriceps femoris muscle has been regarded as a common 
 intervention for patients with pfps in the field of physical therapy , and use of open 
 kinetic chain exercise has been preferred6 . however , open chain knee extension produces anterior shear force that 
 induces stress in the patellofemoral joint . it is not appropriate to strengthen the vmo 
 muscle selectively , since the overall strength of the quadriceps femoris muscle increases 
 through open chain knee extension1 , 11 . it is thought that closed kinetic chain exercises are more effective than open kinetic 
 chain exercises due to their stimulating effect on structures surrounding the joint . the 
 squat exercise , a representative lower extremity close kinetic chain exercise , can promote 
 joint stability through co - contraction of agonist and antagonist muscles2 , 6 . tang et al.10 concluded that the vmo / vl 
 ratio in the squat exercise was higher than that in open kinetic chain exercise . in particular , earl et al.11 reported that squat exercise with hip 
 adduction may result in selective strengthening of the vmo muscle . furthermore , it was 
 suggested that squat exercise with a 30 degree wedge may selectively increase the vmo muscle 
 activity14 . the present study applied squat exercises using a gym ball , a wedge , and an elastic band . 
 although the level of emg activity of the vmo muscle in se2 and se4 were less than that in 
 se1 , higher activity of the vmo muscle however , 
 se3 did not induce selective strengthening of the vmo muscle because the activities of the 
 vmo and vl muscle were pretty much the same . judging from the muscle activity of the vmo 
 muscle relative to the vl muscle , se1 was the most effective method of strengthening the vmo 
 muscle selectively . in spite of these results , since the 
 results of the present study did not consider the angle of the wedge , adduction induced by 
 the gym ball , and resistance induced by the elastic band , further study should be performed 
 to make the results more clear .
[ purpose ] the present study investigated the effects of squat exercises with and without 
 a variety of tools including a gym ball , wedge , and elastic band on the vastus lateralis 
 and vastus medialis oblique muscles . [ subjects and methods ] a total of twenty healthy 
 subjects with no history of neurological , musculoskeletal injury , or pain in the lower 
 extremities were recruited . 
 all subjects performed four types of exercise ( conventional 
 squat exercise , squat exercise with a gym ball , squat exercise with a wedge , squat 
 exercise with an elastic band ) . 
 [ results ] there were no significant differences between 
 exercises in comparison of the vastus lateralis muscle activity . in the squat exercise 
 with a wedge , 
 significantly higher activity of the vastus medialis oblique muscle was 
 found compared with in the squat exercise with an elastic band . 
 [ conclusion ] the present 
 study suggests that the conventional squat exercise can be one of the useful interventions 
 for patients with patellofemoral pain syndrome .