patellofemoral pain syndrome ( pfps ) is a common orthopedic disease , and often induces 
 patella pain during climbing and long - distance running1 .
the results of a previous study indicate that athletes have high 
 risks of knee overuse2 .
the quadriceps 
 supplies the power of dynamic patellar movement , and the vastus medialis oblique ( vmo ) and 
 vastus lateralis ( vl ) enable the patella to stabilize during tracking3 .
a lack of muscle strength in the vmo decreases the medial 
 muscular tension the patellar and causes the muscle imbalance to increase patellar 
 maltracking4 .
the ideal ratio of the vmo 
 to vl is 1:1 , but many pfps patients have had a low ratio for both muscles in 
 electromyography studies5 . some studies 
 indicated that increasing the muscle strength of the vmo is a useful method of preventing 
 pfps6 , 7 .
some study results indicated that specific vmo exercise training can 
 effectively prevent and reduce the incidence of pfps7 , 
 8 .
the open kinetic chain is a movement in which the distal end of the extremity is 
 not fixed to a relatively stable surface . in the closed kinetic chain , movement of a joint 
 can not occur without causing predictable movements in the other joints of the extremity9 . a closed kinetic chain exercise for the 
 knee joint can cause quadriceps and hamstring co - contraction to reduce displacement and 
 improve joint stability , but this will increase the intra - articular pressure10 .
this exercise can cause quadriceps 
 contracture , and this is similar to the muscle activity recruitment occurring during 
 weight - bearing in the gait11 .
open 
 kinetic chain exercise uses a lower amount of weight loading for pfps patients than closed 
 kinetic chain exercise .
the results of some studies show that closed kinetic chain exercise has a significant 
 therapeutic effect for pfps patients11,12,13 . 
 however , these studies often focused on the pain decrease and ignored discussing the effect 
 of vmo muscle activity during the exercise training .
sling exercise therapy ( set ) is a new 
 training method for athletes and orthopedic patients utilized in recent years14 . because of its convenience and 
 practicality ,
set has become more and more common in rehabilitation clinics and fitness 
 training centers15 .
however , studies 
 about the effects of using set with open and closed kinetic chains on vmo muscle training in 
 pfps patients are rare .
we followed the theories about open and closed kinetic chain 
 exercises to design two exercises using set and explore the vmo and vl muscle activity .
to 
 our knowledge , this is the first study to investigate this theme , and we think that the 
 results of this study are important for pfps patients as well as physical therapists and 
 athletic trainers providing therapeutic exercise .
this study was a cross - over study and was approved by the institutional review board of 
 china medical university and hospital .
we informed all participants about the experimental 
 process in detail and then obtained their informed consent before the study .
we recruited 
 healthy female college students and randomly assigned them to the two set exercises , 
 comprising sling open chain knee extension ( socke ) exercise and sling closed chain knee 
 extension ( sccke ) exercise ( fig .
a redcord suspension system ( redcord trainer , record , staub , norway ) was used to 
 train the quadriceps muscle by one athletic trainer .
the two exercises were performed for 
 10  min , and 3 repetitions of maximum force were completed in this period .
the socke exercise 
 used set to suspend the knee and maintain knee flexion at 60 degrees .
the sccke exercise 
 used set to suspend the ankle and maintain the hip in full extension .
we asked the 
 individual to extend the knee to strengthen the concentric contraction of the quadriceps . 

these exercises were designed to improve the strength of the vmo for application in the 
 field of sports medicine .
scottsdale , az , usa ) was fixed to the lateral knee joint , which was the axis of the 
 goniometer , and the stationary and movable arms were tied to the lateral midline of the 
 thigh and shank .
the range of motion of the knee was monitored , and the vmo and vl muscle 
 activity was collected by electromyography by one examiner .
socke ( a ) and sccke ( b ) exercises electromyography ( myotrace 400 , noraxon usa inc . ,
scottsdale , az , usa ) was used to 
 calculate the maximal voluntary contraction ( mvc ) .
the vl electrodes were placed on the line 
 from the anterior superior iliac spine to the superolateral boarder of the patella and 10  cm 
 from the patella .
the vmo electrodes were placed at a distance of 4  cm from the superior 
 side of the patella along a line inclined 50 that ran parallel to the line from the 
 anterior superior iliac spine to the superolateral boarder of the patella16 . in the electromyography signal 
 processing , the sampling frequency was set to 1,000  hz .
we used a band - pass filter of 40 to 
 400  hz to reduce external noise and performed full - wave rectification .
then , setting the 
 root mean square at 100  ms smoothed the signal17 . finally , we set the amplitude normalization at 300  ms and 
 normalized the different movement data by transforming mvc into mvc% with the following 
 formula : mvc% = ( mean mvc / maximum force of the resistance test )  100%
, 
 chicago , il , usa ) to compare the activations of the of vmo and vl .
the wilcoxon test was 
 used to compare the differences in the mvo% values of the vmo and vl between the sccke and 
 socke exercises .
the 
 consistencies of activation of the vmo and vl for 3 repetitions of maximum contraction of 
 one exercise within each individual were analyzed by using the intraclass correlation 
 coefficient ( icc ) .
seven healthy college students ( age = 21.3  0.6 ; weight = 51.3  6.3  kg ; height = 161.5  
 4.8  cm ) were recruited and performed 3 repetitions of maximum contraction for each exercise . 

the icc for activation of the vmo and vl for 3 repetitions of maximum contraction was 0.91  
 0.23 , and there were no significant differences in the socke and sccke exercises among the 3 
 repetitions of maximum contraction .
table 
 1table 1.mvc results of the seven participantsnosocke exercisesccke exercisevmo ( % ) vl ( % ) vmo ( % ) vl ( % ) 198.9  2.484.4  2.379.9  3.363.2  1.3289.8  3.189.4  3.193.7  3.298.4  2.8360.6  3.586.3  4.278.6  2.671.2  2.3499.6  3.798.1  3.882.5  3.581.5  1.4573.3  3.289.9  2.782.2  3.386.2  3.3696.5  2.677.1  3.392.1  4.170.1  2.1785.4  1.478.2  2.990.7 
3.274.4  3.2socke , sling open chain knee extension ; sccke , sling close chain knee extension 
 exercise ; vmo , vastus medialis oblique ; vl , vastus lateralis shows that 5 participants ( nos . 1 , 2 , 4 , 6 , and 7 ) had higher mvc% values for 
 the vmo than for the vl during the socke exercise .
five participants ( nos . 1 , 3 , 4 , 6 , and 
 7 ) had higher mvc values for the vmo than for the vl during the sccke exercise .
the mean 
 mvc% values for the vmo ( 86.30  14.61 ) and vl ( 86.20  7.25 ) during the socke exercise were 
 higher than those for the vmo ( 85.67  6.27 ) and vl ( 77.85  11.81 ) during the sccke 
 exercise .
no significant differences in mvc% values for the vl and vmo were found between 
 the exercises ( p > 0.05 ) . however , the mvc% values for the vmo and vl during the socke 
 exercise were higher than during the sccke exercise .
the mvc% of the vl was higher than that 
 of the vmo during both exercises .
we also found that the ratio of the vmo to vl was 1.0  
 0.19 during the socke exercise and 1.11  0.15 during the sccke exercise .
socke , sling open chain knee extension ; sccke , sling close chain knee extension 
 exercise ; vmo , vastus medialis oblique ; vl , vastus lateralis
this study compared the vmo and vl muscle activities during the sccke and socke exercises . 

our results showed that both exercises had a recruitment effect on the vmo muscle . in 
 addition , the socke exercise had higher mvc% values for the vmo and vl than the sccke 
 exercise .
many studies have used closed kinetic chain and weight - bearing exercises , e.g. , 
 double - leg squat and lunge exercise , to improve the vmo strength5 , 18 , 19 .
their results showed that these exercises can 
 significantly increase the activation of the vmo and vl .
some studies showed that closed 
 kinetic chain exercises can cause greater activation in the vmo than in the vl5 , 6 , 19 .
however , combining closed kinetic chain 
 exercise with weight - bearing exercise often provides a large increase in pulling strength , 
 and increases the intra - articular pressure .
we found that the using set in the sccke exercise can also cause 
 greater activation in the vmo than in the vl .
set can reduce the effect of body weight on 
 the knee intra - articular pressure and can be used to train knee extension with non - weight 
 bearing . the sccke exercise may be useful and helpful in training vmo activity for pfps 
 patients
. we also found the socke exercise to be more effective in activating the vmo than 
 the sccke exercise .
some studies reported that open kinetic chain exercise has little effect 
 on ptps rehabilitation , because the activation of the vmo was not enough to improve patellar 
 tracking20 , 21 .
however , our study results showed that use of set in the socke 
 exercise can increase the muscle activity of the vmo by more than that of the vl .
we thought suspension of the 
 knee by set would decrease the postural stability during the sccke exercise .
this can 
 challenge the stability of the trunk core muscles to facilitate knee extensor contracture . 

so , our study results showed that the socke exercise , which easily causes quadriceps muscle 
 contraction , can be more effective in training the vmo than the sccke exercise .
a previous electromyography study showed that the ideal ratio of the vmo to vl was 1:1 in 
 knee extension exercise for the asymptomatic knee joint5 . in our study , the ratios of the vmo to vl in the socke and sccke 
 exercises were over the ideal ratio of 1:1 .
the socke exercise is more suitable training the 
 vmo and vl of pfps patients than the sccke exercise because it has an approaching ideal vmo 
 to vl ratio .
this means that the signals 
 detected for muscle fibers were the same for the socke and sccke exercises .
we found that 
 large muscle fibers were recruited for vmo activity during the socke exercise , and the ratio 
 of the vmo to vl was 1.11:1 .
previous studies showed high ratios of the vmo to vl at knee 
 extension from 60 to 180 degrees during isokinetic open chain exercise21 , 22 .
we used set to 
 suspend the lower extremity and maintain extension of the knee from 60 to 180 degrees during 
 the socke and sccke exercises .
furthermore , our results showed that both exercises can 
 recruit vmo activity , presenting ratios of the vmo to vl higher than 1:1 .
so , both exercises 
 can improve the vmo muscle contraction of pfsp patients , and the beneficial effect on vmo to 
 vl ratio of the socke exercise is better than that of the sccke exercise .
first , the healthy individuals in this 
 study did not have pain and symptoms related to the patella and did not conform to the 
 muscle activity of a ptps patient .
second , the small sample size , 7 participants only , means 
 that the findings can not be generalized to all ptps patients .
so , we suggest that a larger 
 sample of symptomatic ptps patients is required in a future study .
 [ purpose ] the muscle strength of the quadriceps muscle is critical in patellofemoral pain 
 syndrome . 
 the quadriceps muscle supplies the power for dynamic patellar movement , and the 
 vastus medialis oblique ( vmo ) and vastus lateralis ( vl ) enable the patella to stabilize 
 during tracking . 
 we followed the theories about open and closed kinetic chain exercises to 
 design two exercises , sling open chain knee extension ( socke ) exercise and sling closed 
 chain knee extension ( sccke ) exercise . 
 the purpose of our study was to research the 
 changes in quadriceps muscle activity during both exercises . 
 [ methods ] electromyographic 
 analysis was used to explore the different effects of the two exercises . 
 the mvc% was 
 calculated for the vmo and vl during exercise for analysis . 
 [ results ] we found that the 
 mean mvc% values of the vmo and vl during the socke exercise were higher than those during 
 the sccke exercise . 
 the ratio of the vmo to vl was 1.0  0.19 during the socke exercise 
 and 1.11  0.15 during the sccke exercise . 
 [ conclusions ] the socke exercise is targeted at 
 quadriceps muscle training and has a recruitment effect on the vmo . 
 the beneficial effect 
 of the socke exercise is better than that of the sccke exercise . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION