during concentric contraction , the quadriceps extends the knee which is commonly used to 
 raise the center of mass , such as when running uphill , jumping , or standing from sitting . in 
 particular , the quadriceps pulls the patella upward and laterally , and generates high power 
 through a mechanical advantage from the patella1 .
knee pain occurs as the patella slides abnormally when there is 
 imbalance between the quadriceps vastus medialis and lateralis during concentric 
 contractions to extend the knee2 .
therefore , both balance and 
 stability between the vastus medialis and lateralis must be acquired to correct alignment of 
 the patella6 , and to prevent and treat 
 patellofemoral pain syndrome .
some studies have analyzed differences in muscle activities or 
 onset timing between the vastus medialis and lateralis7,8,9,10 .
in particular , weakness 
 is often observed in the vastus medialis , which is the first muscle to show weakness among 
 the quadriceps muscles11 .
hence , many 
 studies have attempted to suggest the best angular velocity during isokinetic exercise to 
 selectively strengthen the vastus medialis .
a few studies based on knee alignment have been conducted on the mechanical characteristics 
 of the quadriceps . moreover , some studies have measured the cross - sectional areas of the 
 vastus medialis and lateralis according to knee alignment and q - angle12 , 13 , and the 
 difference in onset timing between the vastus medialis and lateralis according to knee 
 alignment during isometric contraction has also been investigated14 . a significant difference was observed between 
 cross - sectional areas of the vastus medialis and the lateralis according to knee alignment 
 measured by the q - angle .
more specifically , sogabe12 reported that subjects with genu varum have a larger vastus 
 medialis and a smaller vastus lateralis , tsakoniti13 reported that subjects with a smaller q - angle have a larger vastus 
 lateralis and a smaller vastus medialis .
thus , knee alignment affects the imbalance between 
 the vastus medialis and lateralis , and genu varum is related to a smaller q - angle and genu 
 valgum is related to a larger q - angle .
therefore , the relationship between knee alignment or 
 q - angle and the quadriceps needs to be investigated . supporting this notion
, park14 demonstrated that knee alignment affects 
 onset timing between the vastus medialis and lateralis during isometric contraction . in 
 addition
, balance training considering the knee alignment in term of the vastus medialis and 
 lateralis can be used to relieve knee pain and strengthen the quadriceps .
however , no 
 studies have yet determined the onset timing during dynamic isometric contraction . in this study , we investigated the difference in onset timing between the vastus medialis 
 and lateralis , according to knee alignment , as well as the best isokinetic angular velocity 
 for performing an isokinetic concentric contraction .
the onset timing of the vastus medialis 
 and lateralis in subjects with genu valgum or genu varum was measured during concentric 
 contractions performed at three angular velocities using equipment for measuring isokinetic 
 muscle strength .
fifty - two healthy young adults with no orthopedic or neurologic diseases and no knee pain 
 were enrolled , and they were classified into three groups according to their knee 
 alignments .
if a subject had > 4  cm intermalleolar distance , they were placed in 
 the genu valgum group .
if a subject had > 4  cm intercondylar distance , they were placed 
 in the genu varum group .
we also determined age , height , weight , 
 and q - angle as general characteristics ( table 
 1table 1.anthropometric data ( m  sd)controlgenu varumgenu valgumn ( male : female)20 ( 7:13)20 ( 5:15)12 ( 8:4)age ( years)21.4  1.7321.3  1.4523.7  3.22height ( cm)165.4  8.95165.0  7.45171.7  8.51weight ( kg)58.0  13.1055.4  8.0567.3  14.05q - angle ( )**13.15  1.6618.60  1.277.33  1.53distanceic ( cm)0.42  1.545.06  0.75-im ( cm)--6.13  1.03 m  sd : mean  standard deviation . * p < 0.05 , * * p < 0.01 .
distance : 
 intercondylar distance of the knee in the genu varum group , intermalleolar distance of 
 the ankle in the genu valgum group ) .
the subjects understood the experimental purpose and methods and agreed 
 voluntarily to participate in the study .
all the participants read and signed an informed 
 consent form approved by the local ethics committee of the catholic university of daegu .
m  sd : mean  standard deviation . * p < 0.05 , * * p < 0.01 .
distance : 
 intercondylar distance of the knee in the genu varum group , intermalleolar distance of 
 the ankle in the genu valgum group an eight - channel wireless surface electromyograph ( wemg-8 , laxtha , daejeon , korea ) was used 
 to measure the onset timing of the vastus medialis and lateralis during isokinetic 
 concentric contraction .
the surface electrodes ( ag / agcl 2223 , 3 m , seoul , korea ) were 
 attached to point at 50 relative to the femoral longitudinal axis and over 5  cm from the 
 superiomedial patella for the vastus medialis , which was the diagonal direction of the 
 lateral 15 of the midline at 3 to 5  cm above the patella for the vastus lateralis .
the 
 ground electrode was attached to a point on the skin above the tibial tuberosity at 6 to 
 12  cm from the inferior patella15 .
the 
 emg signal sampling rate was 1,024  hz , and it was filtered using a 13430  hz bandpass filter 
 and a 60  hz notch filter .
muscle 
 onset timing was the time in the first 25  ms of contraction when the signal exceeded a value 
 three standard deviations above the mean emg baseline signal measured in the 200  ms before 
 the isometric contraction .
the value obtained by subtracting the vastus lateralis onset time 
 from the vastus medialis onset time was the difference in the onset timing between the 
 vastus medialis and lateralis16 . a biodex 
 system iv ( biodex medical systems , inc .
, shirley , ny , usa ) was used to provide isokinetic 
 resistance for the quadriceps at angular velocities of 30 , 60 and 90/sec .
the subjects sat 
 on the biodex , with their thighs , chest , and ankles fixed with straps .
subsequently , they 
 performed knee extensions by pushing a lever arm placed over the ankle .
onset timing was 
 measured three times at each angular velocity during concentric contraction , and the mean 
 values were used in the analysis .
the kruskal - wallis test was used to determine the significance of differences in onset 
 timing between the vastus medialis and lateralis of the various knee alignments .
tukey s and 
 duncan s post - hoc tests were converted as rank cases ; hence , one - way analysis of variance 
 was used .
significant differences in onset timing were observed between the vastus medialis and 
 lateralis at 30 , 60 , and 90/sec of angular velocity during concentric contractions among 
 the knee alignments ( p < 0.05 ) .
negative onset timing values indicate that the onset time 
 for the vastus lateralis was delayed compared to that of the vastus medialis , whereas 
 positive values indicate a delayed vastus medialis onset compared to that of the vastus 
 lateralis . for the genu varum group ,
our results show the onset timing was negative ; thus , 
 firing of the vastus lateralis was delayed compared to that of the vastus medialis .
onset 
 timing was positive in the genu valgum group ; thus , firing of the vastus medialis was 
 delayed compared to that of the vastus lateralis .
no differences in onset timing were observed between the vastus medialis and lateralis 
 among the angular velocities used for during concentric contractions in any of the three 
 groups ( table 2table 2.onset timing ( ms ) between the vastus medialis and lateralis at different angular 
 velocities of concentric contraction ( m  sd)controlgenu varumgenu valgum30/sec**4.02  3.044.15  5.2334.27  48.0160/sec**2.73  2.912.62  3.3010.74  7.4990/sec**3.36  3.002.61  3.5233.40  28.82 m  sd : mean  standard deviation . * p < 0.05 , * * p < 0.01 ) .
m  sd : mean  standard deviation . * p < 0.05 , * * p < 0.01
differences in onset timing between the vastus medialis and lateralis during isokinetic 
 concentric contractions were measured .
significant differences in onset timing were observed 
 among the three groups at 30 , 60 and 90/sec of angular velocity among the three groups . 

onset timing between the vastus medialis and lateralis was calculated by subtracting the 
 onset time of the vastus lateralis from that of the vastus medialis . when the value was 
 negative , firing of the vastus lateralis was delayed compared to that of vastus medialis ; 
 when the value was positive , firing of the vastus medialis was delayed ; and when the value 
 was 0 , the two muscles contracted simultaneously .
the value was negative in the genu varum 
 group in the present study ; thus , the vastus lateralis firing was delayed compared to that 
 of the vastus medialis . in contrast , the value was positive in the genu valgum group , so the 
 vastus medialis firing was delayed compared to that of the vastus lateralis .
these results 
 were observed at all angular velocities ; thus , angular velocity did not affect quadriceps 
 activity .
in particular , differences in onset timing in the genu valgum group were five to 
 ten times that of the control group .
thus , delayed firing of the vastus medialis in the genu 
 valgum group was longer than that in the vastus lateralis of the genu varum group . more 
 specifically , the firing delay in the vastus medialis of the genu valgum group was more 
 serious than the delay in the vastus lateralis on the genu varum group .
this result is in 
 agreement with park14 , who analyzed 
 differences in onset timing between the vastus medialis and lateralis during isometric 
 contractions .
however , our result was different from that of karst and willett17 , who compared onset timing between the 
 vastus medialis obliquus and vastus lateralis in subjects with patellofemoral pain syndrome 
 and those without pain during active nonweight - bearing knee extension ; they found no 
 significant differences between the groups .
the quadriceps pulls the patella both upward and laterally1 , and the pulling direction is referred to as the q - angle or 
 quadriceps angle18 .
the balance of 
 physiological characteristics between the vastus medialis and lateralis induces the patella 
 to slide normally ; however , delayed firing or a weak vastus medialis causes excess lateral 
 sliding of the patella , thereby inducing genu valgum and lateral patellar subluxation . 

therefore , the quadriceps can not stabilize and protect the patella sufficiently , ultimately 
 causing patellofemoral pain syndrome2,3,4,5 .
there is much controversy over whether or not patellofemoral pain syndrome is caused by 
 delayed firing of the vastus medialis .
many studies have claimed that delayed firing of the 
 vastus medialis is a cause of patellofemoral pain syndrome15 , 19 , whereas others claim 
 that patellofemoral pain syndrome does not delay firing of the vastus medialis17 , 20 .
our subjects were not patients with patellofemoral pain syndrome ; 
 however , knee alignment or q - angle affects the quadriceps as follows .
first , the subjects in 
 the genu valgum group had an increased q - angle in this study , and an increased q - angle is a 
 cause of patellofemoral pain syndrome4 ; 
 therefore , patellofemoral pain syndrome affects genu valgum3 .
the vastus medialis is the quadriceps muscle most often seen to weaken , and various 
 interventions to selectively strengthen the vastus medialis have been studied to prevent and 
 treat pain or injury induced by weakness of the vastus medialis21,22,23 .
thus , a selective training program for the vastus 
 medialis is needed for people with genu valgum ; however , such a program may cause more 
 severe problems . accordingly
, it is necessary to consider knee alignment and quadriceps 
 firing prior to applying a rehabilitation program to strengthen the vastus medialis . in 
 other words , a rehabilitation program to balance the vastus medialis and lateralis
no significant differences in onset timing were observed between the vastus medialis and 
 lateralis among different angular velocities .
it was difficult to compare our results with 
 those of prior studies because only a few studies have reported differences in onset timing 
 between the vastus medialis and lateralis during concentric contraction .
na et al.24 reported that vastus lateralis activity 
 tended to be higher at an angular velocity of 60/sec during isokinetic concentric 
 contractions , and that vastus medialis activity tended to be higher at 120/sec ; however , 
 they did not find significant differences .
han and lee25 found that the thicknesses of the vastus medialis and rectus 
 femoris increased regardless of angular velocity .
muscle thickness is related to muscle 
 strength26 ; thus , it may be related to 
 muscle emg characteristics , such as muscle activity or onset time .
one of the limitations of this study was that the subjects were healthy young adults with 
 no knee pain ; thus , future studies should enroll patients with knee pain or knee joint 
 instability . moreover , some general characteristics , such as mean height and weight in the 
 genu valgum group , were different from those in the other group due to a higher proportion 
 of males .
however , the onset times of the vastus medialis and lateralis were compared 
 individually , and not between subjects ; which may have eliminated potential gender 
 differences .
future studies should control subjects well . in addition , experiments using 
 various angular velocities are necessary . moreover , if changes in muscle activities are 
 measured during activities of daily living , such as gait , the results would help the 
 prevention or treatment of injuries during daily activities . in conclusion ,
genu varum and valgum , which are caused by lower limb malalignment of the 
 musculoskeletal system , affect quadriceps firing .
no differences were detected between the 
 genu valgum and varum groups at different angular velocities .
selective rehabilitation 
 training of the quadriceps femoris should be considered to prevent pain or knee malalignment 
 deformities .
 [ purpose ] the purpose of this study was to investigate the difference in onset timing 
 between the vastus medialis and lateralis among the different knee alignments , as well as 
 the best isokinetic angular velocity for an isokinetic concentric contraction . 
 [ subjects ] 
 fifty - two adults ( 20 with genu varum , 12 genu valgum , and 20 controls ) were enrolled in 
 this study . 
 subjects with > 4  cm between the medial epicondyles of the knee were placed 
 in the genu varum group , whereas subjects with > 4  cm between the medial malleolus of 
 the ankle were placed in the genu valgum group . 
 [ methods ] surface electromyography was 
 used to measure the onset times of the vastus medialis and vastus lateralis during 
 concentric contractions at 30 , 60 , and 90/sec . [ results ] the vastus lateralis showed more 
 delayed firing than the vastus medialis in the genu varum group , whereas vastus medialis 
 firing was delayed more than that of the vastus lateralis in the genu valgum group . 
 no 
 differences in onset timing were observed between the vastus medialis and lateralis 
 according the different angular velocities during concentric contractions in all three 
 groups . 
 [ conclusion ] genu varum and valgum affect quadriceps firing . 
 therefore , selective 
 rehabilitation training of the quadriceps femoris should be considered to prevent pain or 
 knee malalignment deformities . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION