one of the most popular athletic games , soccer , requires a basic element of strong physical 
 power almost at the threshold level1 , 
 through which successful athletic performance is marked by technical and tactical 
 activities2 .
physical power is based on 
 muscle strength and durability , quick reaction and agility , and flexibility , and it involves 
 complex components , such as the relationship between the strength of the quadriceps femoris 
 and anaerobic power3 . to maintain balance 
 and allow for quick reactions and agility during soccer
, the iterative interaction among the 
 joints of the lower limb has a significant effect on the players athletic performance4 .
the muscle strength of the knee joint plays a role in supporting the body s weight . during 
 a game
, however , it also supports the center of body mass while performing such dynamic 
 movements as kicking the ball , jumping , and changing direction , which all rely on quick 
 reaction , agility , and coordination5 . 

functional abnormalities of the knee joint can include overuse , repetitive injuries , or 
 unstable proprioception due to repetition of limited movements within the joint s mobility 
 range6 . to maintain the stability of the 
 knee , it is important to evaluate the level of the muscle strength balance , to assess 
 endurance using isokinetic exercises , and then to facilitate muscle development 
 accordingly7 . in soccer ,
the foot and ankle joints have the highest rates of injury , and functional 
 movement impairment can be a cause of instability through insufficient muscle strength and 
 lack of proprioception8 . even when 
 structural instability is excluded , functional instability can cause the weakening of the 
 ankle joint when movement patterns are repetitive , direction is abruptly changed9 , or active movement is carried out 
 intensely10 . in competitive elite 
 sports like soccer
, these problems can manifest themselves as chronic issues such as a 
 higher risk of injury or functional abnormalities11 .
the coordinated activity of the joints and muscles involved in 
 movements of the ankle joint further enhances the functional stability by improving stable 
 weight support as well as motor control function12 . through the measurement of isokinetic variables ,
the aim of this study was to assess the 
 impacts of functional and chronic asymmetry on the muscle strength of the left and right 
 knee and ankle joints in soccer players at a range of athletic performance levels , and to 
 generate useful data for the development of effective functional exercise programs for 
 symmetrical muscle strength .
the 49 study subjects were recruited from 3 different levels of athletic performance : 
 professional soccer players ( k league , n=15 ) , amateur players ( k3 league , n=16 ) , and college 
 soccer players ( u league , n=18 ) .
all of the participants provided written consent to 
 participate in the study , after the study procedure and methods were explained to them . 

table 1table 1.general characteristics of participantsvariablesnpp ( k league)ap ( k3 league)up ( u league)age ( yrs)1526.94.421.11.920.31.5height ( cm)16179.47.3175.54.1180.94.6weight ( kg)1877.58.070.44.676.66.1values are meansd .
pp : professional players , ap : amateur players , up : university players shows the general characteristics of the subjects .
all the subjects understood 
 the purpose of this study and provided their written informed consent prior to 
 participation , in accordance with the ethical standards of the declaration of helsinki .
pp : professional players , ap : amateur players , up : university players the isokinetic muscle functions of the knee and ankle joints were the main experimental 
 variables of the study , and they were measured using the humac norm testing and 
 rehabilitation system ( csmi medical & solution , usa ) .
subjects were seated in the 
 measurement chair , and the torque of the knee joint was aligned with the rotating axis of 
 the dynamometer by adjusting the position with a table - tube cross clamp and a pedestal 
 column clamp . the thigh area and the upper body
were tightly fixed using a strap and a belt 
 so that the quadriceps exercise would not be affected by external force during the flexion 
 and extension exercises of the knee joint . additionally , to isolate the muscle strength of 
 the area of interest
, the ankle was fixed with a strap by adjusting the length of the lower 
 leg and the adjustment axis with an adapter , after which the flexion and extension exercises 
 of the knee were performed .
the range of motion was determined by measuring the maximum 
 flexion from the position in which the joint was extended ( 0 ) in a sitting position .
care 
 was taken so that the joint would not be either hyperextended more than 0 or hyperflexed 
 more than 135. measurements were taken by group . as a warm - up exercise
, the subjects 
 performed flexion and extension exercises of the lower limb 3 times below maximum and 1 time 
 at maximum , at angle speeds of 60/sec and 180/sec .
then , the measurements were collected 5 
 times at the angle speed of 60/sec and 15 times at the angle speed of 180/sec .
after the 
 measurements were obtained at each speed , the subjects were told to repeat the flexion and 
 extension of the knee joint , while resting for approximately 2 minutes in a sitting 
 position .
next , the subjects were seated in the measurement chair in the same manner described above 
 for the knee joint , and the strength of the plantar flexion and dorsiflexion muscles in the 
 ankle were measured at angle speeds of 30/sec and 120/sec .
after completion of a warm - up 
 exercise of plantar flexion and dorsiflexion 3 times below maximum and 1 time at maximum , at 
 the angle speed of 30/sec and 120/sec , measurements were taken 5 times at 30/sec and 15 
 times at 120/sec .
after the performance was measured at each speed , the subjects repeated 
 the flexion and extension exercises , while resting for approximately 2 minutes in a sitting 
 position . for each measurement ,
all of the data analysis was conducted using spss for windows version 20.0 ( spss inc . , 
 usa ) .
means and standard deviations were computed for all the measurement items . to test 
 group differences ,
one - way anova was performed , and when significant differences were found , 
 post - hoc comparisons were performed using the scheffe method .
tables 2table 2.comparison isokinetic peak torque of the knee jointppapupextensor ( nm)60/secright207.823.2205.318.8254.134.0left202.733.1204.920.3246.943.6180/secright123.021.1118.629.3148.121.3left128.027.9120.022.4147.320.8flexor ( nm)60/secright133.628.8129.120.0152.038.1left131.525.2128.117.5145.125.2180/secright90.125.3111.919.295.424.3left91.922.5104.411.997.621.2values are meansd , * p<0.05 , * * p<0.01 , * * * p<0.001 .
pp : professional players , ap : amateur players , up : university players and 3table 3.comparison isokinetic ratio of the knee jointppapup60/sec ( % ) right64.110.964.111.560.013.2left64.97.064.910.559.27.6180/sec ( % ) right72.613.564.111.566.220.3left71.810.764.910.566.713.9values are meansd .
pp : professional players , ap : amateur players , up : university players show the results of the one - way anova on peak isokinetic strength and 
 differences in muscle strength of the knee and ankle joints , and the level of muscle 
 strength on the left and the right , according to level of athletic performance .
peak 
 extensor strength of the knee joint was significantly higher in the university players ( up ) 
 than the professional players ( pp ) or amateur players ( ap ) , both in the right and left knee 
 at the angle speed of 60/sec ( both p<0.001 ) and the angle speed of 180/sec ( both 
 p<0.01 ) .
the peak flexor strength of the knee joint was significantly higher in ap than 
 pp or up , at the angle speed of 180/sec in the right knee ( p<0.05 ) .
values are meansd , * p<0.05 , * * p<0.01 , * * * p<0.001 .
pp : professional players , ap : amateur players , up : university players values are meansd .
pp : professional players , ap : amateur players , up : university players tables 4table 4.comparison isokinetic peak torque of the ankle jointppapupplantar flexion ( nm)30/secright106.828.3125.025.8109.128.1left101.820.9109.221.2105.423.9120/secright51.718.880.823.462.920.8left56.820.076.134.758.717.0dorsi flexion ( nm)30/secright30.38.734.95.822.35.1left29.97.433.65.535.45.1120/secright16.27.019.64.29.92.2left17.25.918.43.717.22.6values are meansd , * p<0.05 , * * p<0.01 , * * * p<0.001 .
pp : professional players , ap : amateur players , up : university playersand 5table 5.comparison isokinetic ratio of the ankle jointppapup30/sec ( % ) right28.54.028.77.021.14.6left30.18.331.77.835.610.3120/sec ( % ) right32.69.726.29.417.76.3left32.311.627.09.530.29.1values are meansd , * * * p<0.001 .
pp : professional players , ap : amateur players , up : university players show the results of the one - way anova on the peak strength of the isokinetic 
 function in the ankle joint , as well as the ratio of muscle strength on the left and right , 
 according to participant s level of athletic performance .
the peak plantar flexor strength 
 of the ankle joint was higher in ap than pp or up at the angle speed of 120/sec in the 
 right ankle ( p<0.01 ) .
the peak dorsiflexor strength was significantly lower in up than pp 
 or ap in the right ankle at the angle speed of 30/sec , and at the angle speed of 120/sec 
 ( p<0.001 ) , and significantly higher in the left ankle at the angle speed of 30/sec 
 ( p<0.05 ) .
up showed a significant asymmetry in muscle strength between the left and right 
 sides compared to pp and ap at the angle speeds of 30/sec and 120/sec ( p<0.001 ) .
the 
 ratio of the muscle strength of the ankle joint in the left and the right was significantly 
 lower for up than pp or ap in the right ankle at the angle speed of 30/sec and at the angle 
 speed of 120/sec ( p<0.001 ) .
values are meansd , * p<0.05 , * * p<0.01 , * * * p<0.001 .
pp : professional players , ap : amateur players , up : university players values are meansd , * * * p<0.001 .
reinforcing muscle strength to maintain the stability of the knee joint is essential for 
 soccer players13 .
deficiencies in 
 quadriceps muscle strength have a direct link to the risk of injury and can cause 
 osteoarthritis in the knee joint14 . it 
 has been reported that injury frequency increases in athletes when there is a difference of 
 10% or more between the strength of the left and the right quadriceps femoris15 , 16 .
one cause of instability of the main joints in the lower limbs is 
 instability of flexion and extension movements , accompanied by loss of muscle strength ; this 
 instability can be manifested in a variety of forms17 . in the present study ,
up had significantly higher isokinetic strength of the quadriceps 
 femoris than pp or ap , and no asymmetry was observed in the left and the right muscle 
 strength .
however , a previous study on the isokinetic strength of elite players and ap 
 demonstrated that the muscle strength rate of the hamstring was higher in elite players18 .
although in that study it was observed , 
 as in this study , that up had significantly higher quadriceps femoris strength , pp showed 
 balance between the quadriceps femoris and the hamstring and a higher muscle strength rate 
 of the hamstring .
it is thought that the muscle strength rate of the hamstring is higher 
 because it enhances the stability of the knee joint to increase speed over short distances 
 and to allow for precise foot techniques .
the muscle strength in the ankles of soccer players also differs depending on players 
 athletic performance level .
a study that compared the isokinetic muscle strength in the 
 ankle joint of up and pp found that it was greater in pp , who were at a higher level of 
 athletic performance than up19 . in this 
 study ,
the plantar flexor strength was higher in ap compared to pp and up , and no asymmetry 
 in muscle strength in the left and the right was observed , whereas the dorsiflexor strength 
 was significantly lower in up compared to pp and ap .
our finding is similar to that of a 
 previous study that reported that muscle strength imbalance in the foot joints , deficiencies 
 in proprioception , mechanical instability , and weakening of the peroneal tendon , which can 
 all be caused by repetitive training , overuse , and damaged soft tissue , are 
 interrelated20 .
thus , it is believed 
 that the tibialis anterior , which is involved in dorsiflexion and inversion , is relatively 
 weak in muscle strength expression , in comparison to the strength of the soleus , 
 gastrocnemius , and peroneal tendon that are involved in plantar flexion and eversion .
muscle balance is an indicator employed to judge the level of athletic performance , and the 
 extent of muscle imbalance due to exercise and incorrect positioning is one factor that 
 influences performance21,22,23 .
soccer is a 
 sport in which a variety of connecting movements , including running , abrupt stopping , 
 changing direction , jumping and landing , kicking and passing , repeatedly occur and these 
 movements require complex functioning of the knee and ankle joints .
the hamstring and the 
 calf muscle work to maintain the stabilized center of body mass or to push the ground 
 surface using reaction force , and the quadriceps femoris and dorsiflexors are used when 
 moving forward , kicking , or adjusting a ball by flexion of the hip joint , extension of the 
 knee , and dorsiflexion of the ankle .
it is not only the balance between the agonist and 
 antagonist muscles , but also the balance between the left and the right , and between the 
 proximal joint and distal joint in a weight - bearing position that are involved during 
 exercise24 , 25 .
as seen in the up in this study , the weakening of dorsiflexors due 
 to muscle strength asymmetry in the right ankle joint is thought to cause the strengthening 
 of the extension muscles of the knee joint in order to maintain the center of body movement 
 and stability . to maintain stability and balance in lower limb movement , it is essential to maintain 
 muscle activation26 .
when a movement is 
 made by a soccer the player , the muscles in the foot are already activated to function 
 before the foot touches the ground27 , and 
 when the knee joint sustains a large shock like landing or when the center of mass moves , 
 the contraction greatly increases in the quadriceps femoris28 .
it is thought that muscle strength asymmetry in the ankle joint 
 may strengthen the muscles of the knee joint as a counterbalance in order to maintain the 
 center of body mass . in the future , to investigate the extent of muscle activation due to 
 asymmetry between left and right , studies must be conducted on muscle contraction as the 
 center moves with the use of kinetic variables .
 [ purpose ] the aim of this study was to collect basic data on the effect of asymmetry on 
 the muscle strength of the left and right knee and ankle joints of soccer players at 
 varying athletic performance levels , to guide the development of improved exercise 
 programs . [ subjects and methods ] forty - nine soccer players at three athletic performance 
 levels participated : 15 professional , 16 amateur , and 18 college . 
 knee extensor and flexor 
 strength were measured at 60/sec and 180/sec , and ankle plantar flexor and dorsiflexor 
 strength were measured at 30/sec and at 120/sec . 
 variables were analyzed by one - way 
 anova . 
 [ results ] college soccer players showed greater muscle strength at 60/sec and 
 180/sec in the knee extension muscles of both the right and the left sides , lower muscle 
 strength at 30/sec and 120/sec in the dorsiflexor of the right ankle , and similar levels 
 of asymmetry between left and right . the maximum muscle strength on the same side 
 significantly differed in the right ankle joint , with asymmetry between left and right at 
 30/sec and 120/sec . [ 
 conclusion ] these findings suggest that muscle strength asymmetry 
 in the ankle joint may lead to counterbalancing muscle strengthening of the knee joint to 
 maintain the center of body mass . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION