dynamic knee stability is defined as the state responding to the proprioceptive feedback 
 alerted by recurring postural instability caused by limited range of motion ( rom ) and 
 restricted movement1 .
pain and functional 
 instability are the most common pathological conditions of the knee joint . mechanical knee 

joint injuries are primarily associated with meniscal lesions , and knee pain and mechanical 
 disorders are easily overlooked2 . 

recurring disorders arising from diminished rom and muscle strength are reportedly caused by 
 altered muscular reflexes of the tissues surrounding the knee joint due to early trauma or 
 chronic damage and insufficient proprioception3 .
muscular stability , especially that of the quadriceps , is essential 
 for functional recovery of the knee joint4 , 5 , and knee bone malalignment , overstress , and 
 muscular weakness around the hip joints are the most common causes of functional 
 abnormalities6,7,8
. knee instability can lead 
 to lower - limb collapse due to the pain control mechanism of the quadriceps femoris under 
 patellofemoral joint loading in an upright standing position .
pain in the knee joint can be 
 caused by accumulated injuries , traumatic patellar dislocation , knee bone malalignment , 
 increased compressive pressure due to obesity or load - lifting , and primary knee 
 osteoarthritis , which can lead to secondary knee osteoarthritis and is likely to be 
 accelerated by obesity9 .
the ankle joint has various functions , such as weight - bearing , static and dynamic motion 
 and control , walking , running , jumping , landing from a jump , and shock absorption .
it also 
 plays an important role in ensuring a stable base of support and flexibility through 
 biomechanical compensation in motions coordinated by the knee and hip joints10 .
functional ankle instability is a 
 condition of ankle joint weakness (  give way  feeling ) without a structural problem due to 
 recurrent ankle sprains or high - intensity track and field exercises .
not only does it cause 
 recalcitrant chronic problems for athletes , it can also manifest as lesions with many 
 different forms11 , 12 .
hertel noted that chronic ankle instability is associated with 
 several functional impairments , such as decreased muscle strength , affected proprioception , 
 inadequate neuromuscular control , and postural control loss13 .
the results of recurrent and chronic ankle sprains can be 
 classified into diminished rom , anthropometric changes , ligament laxity , and regenerative 
 changes .
functional ankle instability can be classified as changes that affect stability due 
 to the lack of dynamic postural control during functional motions including walking14 , 15 .
appropriate muscle contractility is an efficient indicator of 
 muscular function for athletes and non - athletes alike16 , 17 and can be safely and 
 simply applied to muscle imbalance correction and muscle damage recovery interventions .
based on these theoretical factors , this study aimed to establish basic data that may be 
 efficiently applied to the development of exercise programs designed to improve the 
 functional stability of the knee and ankle joints . to this end , we performed isokinetic 
 measurements and evaluated knee and ankle muscle strength in functionally intact and 
 affected states to identify problems during functional activities in the lower limb joints 
 and muscles .
for the experiments in this study , we enrolled 12 male high school athletes ( five 
 basketball and seven volleyball players ) who had been treated in the hospital for functional 
 instability of the knee and ankle joints in the preceding 12 months as well as 12 age- and 
 gender - matched healthy controls .
all 24 subjects were right - handed . the subjects provided 
 written informed consent prior to participating .
the study was approved by the hufs 
 university and the institution in which it was performed and complied with the ethical 
 standards of the declaration of helsinki .
the subjects general characteristics are shown in 
 table 1table 1.subjects general characteristicsgroupnage ( yrs)height ( cm)weight ( kg)athlete1216.81.9181.15.571.18.6control1220.31.5177.36.872.27.5total2418.62.4179.26.371.67.9values are mean  sd . the isokinetic strength variables of the knee and ankle joints were measured using the 
 humac norm testing & rehabilitation system ( csmi , usa ) . the experimental procedure and 
 measurement setup for the knee joint
the subject was seated on the 
 measurement chair , and the knee positions were adjusted using a table - tube cross - clamp and a 
 pedestal column clamp to align the axis of rotation of the knee joints with that of the 
 isokinetic dynamometer .
the thighs and trunk were firmly fastened using straps and belts to 
 prevent the parts other than the target joints from exerting external force on the muscular 
 motions of the thighs during the knee flexion and extension exercises . during the knee 
 flexion and extension exercises ,
muscle activation of the target region was facilitated by 
 adjusting the lengths for the lower legs and the adjustment axis using a long input adapter 
 and an adjusting arm followed by fixing of the ankle region with velcro straps .
the subject 
 was requested to execute knee joint flexion and extension from 135 to 0 and from 0 to 
 135 , respectively , whereby the rom was controlled to prevent hyperextension or 
 hyperflexion .
prior to the measurement sessions , the subjects performed pre - speed warm - ups 
 in which they performed three submaximal and one maximal flexion and extension exercise at 
 loading speeds of 60/sec ( five repetitions ) , 180/sec ( 10 repetitions ) , and 240/sec ( 20 
 repetitions ) with a 2-min rest after each measurement .
ankle joint plantar- and dorsiflexion strengths were measured at 30/sec and 120/sec as 
 described above .
measurements were performed in each group after pre - speed warm - ups in which 
 the subjects underwent three submaximal and one maximal ankle plantar- and dorsiflexion 
 exercises at speeds of 30/sec ( five repetitions ) and 120/sec ( 15 repetitions ) with a 2-min 
 rest after each measurement .
the researcher verbally encouraged the subjects to perform the 
 exercises to their maximum capacity for all measurements .
the measurement results were processed by spss for windows version 20.0 software ( spss 
 inc . , usa ) .
mean and standard deviation were calculated for each measurement variable , and 
 intergroup differences were analyzed using independent t - tests with the statistical 
 significance level set to p < 0.05 .
the peak torque measurements for the knee joint revealed statistically significant 
 intergroup differences between the functionally affected athlete group and the healthy 
 control group for right extensor strength at 60/sec , 180/sec , and 240/sec as well as 
 right flexor strength at 240/sec , while the remaining variables showed no statistically 
 significant intergroup differences ( table 
 2table 2.comparison of peak torque of the knee jointvariableathlete group control group extensors ( nm ) at 60right232.172.1179.828.6left156.369.1178.134.2flexors ( nm ) at 60right135.532.2119.726.3left96.438.2108.423.9extensors ( nm ) at 180right134.642.8100.519.1left90.041.496.318.7flexors ( nm ) at 180right89.624.873.015.8left72.632.467.614.5extensors ( nm ) at 240right112.027.879.414.6left80.227.080.613.5flexors ( nm ) at 240right85.225.067.19.3left67.530.159.812.9values are mean  sd ; * p < 0.05 , * * p < 0.01 ) . for peak torque / body weight ( ptbw ) ,
intergroup differences were observed for 
 right extensor strength at 60/sec , 180/sec , and 240/sec and for right flexor strength at 
 180/sec and 240/sec ( table 3table 3.comparison of peak torque / body weight of the knee jointvariableathlete group control group extensors ( % ) at 60right325.781.0251.541.3left217.874.0222.870.4flexors ( % ) at 60right193.332.6166.435.6left133.841.9137.647.6extensors ( % ) at 180right188.850.3140.528.7left124.742.8123.246.7flexors ( % ) at 180right125.326.498.525.0left99.935.986.332.0extensors ( % ) at 240right157.534.3111.924.5left112.328.7103.237.1flexors ( % ) at 240right118.827.993.613.8left92.833.176.927.8values are mean  sd ; * p < 0.05 , * * p < 0.01 )
. comparison of left / right peak torque ratios did not yield statistically 
 significant differences for any of the variables at exercise speeds of 60/sec , 180/sec , 
 and 240/sec ( table 4table 4.comparison of bilateral balance ratio of the knee jointvariableathlete group control group ratio ( % ) at 60right61.614.467.016.0left63.516.161.28.8ratio ( % ) at 180right68.813.672.314.8left81.622.571.012.8ratio ( % ) at 240right77.317.986.417.4left84.526.775.117.0values are mean  sd ) .
values are mean  sd ; * p < 0.05 , * * p < 0.01 values are mean  sd ; * p < 0.05 , * * p < 0.01 the peak torque measurements for the ankle joint revealed statistically significant 
 intergroup differences between the affected and healthy groups for left dorsiflexor strength 
 at 30/sec and 120/sec and for right plantar flexor strength at 120/sec ( table 5table 5.comparison of peak torque of the ankle jointvariableathlete groupcontrol groupplantarflexors ( nm ) at 30right73.526.466.921.3left51.624.268.021.1dorsiflexors ( nm ) at 30right30.35.029.63.6left19.44.429.75.9plantarflexors ( nm ) at 120right39.914.738.818.5left29.715.435.212.2dorsiflexors ( nm ) at 120right16.03.013.32.2left9.82.313.84.7values are mean  sd ; * p < 0.05 , * * * p < 0.00 ) . for ptbw , intergroup differences were observed for left plantar flexor 
 strength at 30/sec , left dorsiflexor strength at 30/sec and 120/sec , and right 
 dorsiflexor strength at 120/sec ( table 
 6table 6.comparison of peak torque / body weight of the ankle jointvariableathlete groupcontrol group30plantarflexors ( % ) right103.432.592.325.0left71.126.194.727.4dorsiflexors ( % ) right42.86.241.54.9left27.55.842.511.2120plantarflexors ( % ) right56.819.453.724.5left41.518.550.420.5dorsiflexors ( % )
right23.05.018.53.6left14.32.919.88.0values are mean  sd ; * p < 0.05 , * * * p < 0.00 ) .
comparison of left / right peak torque ratios did not yield any statistically 
 significant differences at 30/sec and 120/sec ( table 
 7table 7.comparison of bilateral balance ratio of the ankle jointvariableathlete group control group ratio ( % ) at 30right46.017.448.015.3left43.214.251.730.8ratio ( % ) at 120right43.314.544.526.8left42.924.446.426.2values
p < 0.05 , * * * p < 0.00 values are mean  sd ; * p < 0.05 , * * * p < 0.00
peak torque of the lower limbs and bilateral balance ratios are most commonly used to 
 assess the isokinetic strength of the lower limb muscles , detect injuries , and predict 
 risk18 .
jumping is one of the most 
 common motions , and the landing or directional change after jumping motion should be 
 executed fluidly and stably under conditions of adequate muscle activation with components 
 of complex muscle extension and flexion movements19 . jumping height depends on flexion and extension of the knee and 
 ankle joints , and repeated excessive use of the lower limb joints is considered responsible 
 for 5080% of injuries20 .
extension is primarily 
 performed by the quadriceps femoris muscle , which is also responsible for the weight - bearing 
 and postural balance used for body axis alignment , stability , and walking21,22,23 .
damage to the knee joint in particular is 
 likely to increase postural imbalance and falling risk by inducing abnormal responses in the 
 muscles surrounding the knee including loss of muscle strength and subsequent fatigue .
kim 
 performed a large - scale 2  year prospective cohort study of 1,060 korean male professional 
 volleyball players that focused on injury sites and types and found that the lower 
 extremities sustained 63.4% of all injuries , with the knee showing the highest risk of 
 injury , followed by the ankle , lower leg , thigh , and foot . in a 4  year study on injuries 
 among female college volleyball players24 , agel et al . also reported lower extremity injury rates of 58.7% 
 during the season and 55.9% during training25 . given that muscle performance in jumping significantly relies upon 
 lower - extremity extension strength in the knee and then the ankle joint , identifying muscle 
 performance abnormalities may be useful for determining the causes of imbalance - induced 
 functional instability and creating corrective strategies26 . in a study in soccer players , choi et al .
reported significant 
 differences between knee extensor and flexor isokinetic strength ( 247.9  34.4 nm vs. 165.1 
  29.4 nm ) and ascribed higher flexor strength in high - performing players to the fact that 
 knee flexors play an important role in enhancing sprinting speed and stability in the 
 execution of various motions involving the knee joint27 .
chun et al . compared the isokinetic muscle strength 
 characteristics of elite soccer players and non - athletes and reported that , the control 
 group ( 220.7  16.2 nm ) showed lower right extensor strength at 60/sec compared to college 
 ( 260.4  16.1 nm ) and professional soccer players ( 256.9  15.4 nm ) , respectively28 .
in addition , similar results were 
 observed at 180/sec suggesting that college and professional soccer players showed higher 
 right extensor strength compared to the control group .
these results were similar to the 
 results of the present study that yielded statistically significant differences between the 
 athlete and control groups .
individuals with chronic ankle instability have diminished rom due to lower - extremity 
 muscle atrophy and postural imbalance ; thus , they encounter problems with balance and 
 walking , motions that comprise the basis for all physical activities .
furthermore , they are 
 at high risk of recurrent injuries and functional movement limitations such as postural 
 sway . a recent meta - analysis of ankle sprains and chronic ankle instability highlighted the 
 need for an in - depth investigation of functional impairment causes and noted that diminished 
 dynamic and static postural control ability can trigger earlier recurrence of ankle 
 sprains11 , 29,30,31 .
reported that the isokinetic peak torque of the ankle 
 joint in concentric contraction showed significant differences between ankle instability and 
 stability groups in a study of the impact of functional ankle instability on the strength of 
 the muscle surrounding the ankle , balance ability , and functional performance ; specifically , 
 differences were observed in plantar flexor strength at 30/sec , 60/sec , and 120/sec and 
 in plantar flexor and dorsiflexor strength at 30/sec with respect to ptbw32 .
these results verified the high risk of 
 recurrent functional instability due to ankle joint weakening .
moreover , the results are in 
 line with those of the present study in which significant intergroup differences were 
 demonstrated in the peak torque of left ankle dorsiflexor strength at 30/sec and in peak 
 torque of bilateral dorsiflexor strength and ptbw at a loading speed of 120/sec .
gribble and robinson compared the muscle strength of the ankle , knee , and hip joints with 
 regard to the biomechanical and dynamic chain relationships to chronic ankle 
 instability33 and reported that the 
 affected side showed significantly lower values than the healthy side in the isokinetic 
 evaluation of plantar flexor ( ankle ) and flexor and extensor ( knee ) strength . in the present 
 study ,
significant differences were demonstrated in peak torque , ptbw , and peak torque 
 ratios in the plantar- and dorsiflexors and in the peak torque , ptbw , and peak torque ratios 
 comparing plantar and dorsiflexors ; significant differences between the healthy and affected 
 sides were observed ( p < 0.001 ) .
these results are consistent with the values for 
 isokinetic variables in a study by kim and jeon of 175 subjects with symptoms of chronic and 
 functional ankle instability34 .
these 
 results also allow the conclusion that enhancing plantar and dorsiflexor strength will 
 stimulate postural balance recovery35 and 
 be conducive to securing ankle joint stability36 , 
 37 . in conclusion , isokinetic
evaluation stimulates muscle contraction at loading speeds based 
 on rom and carries a very low risk of exercise - induced injury .
these valuable basic data 
 will assist in the development of efficient and complex intervention programs to improve 
 knee and ankle joint function according to injury degree and will ensure safety by 
 correcting lower - extremity instability and muscle strength losses due to various causes .
 [ purpose ] to collect basic data for exercise programs designed to enhance functional knee 
 and ankle joint stability based on isokinetic measurement and muscle strength evaluations 
 in normal and impaired functional states . 
 [ subjects and methods ] twenty - four subjects were 
 randomly assigned to the athlete group and the control group ( n = 12 each ) . 
 data were 
 collected of isokinetic knee extensor and flexor strength at 60/sec , 180/sec , and 
 240/sec and ankle plantar and dorsiflexor strength at 30/sec and 120/sec . [ results ] 
 significant intergroup differences 
 were observed in peak torque of the right extensors at 
 60/sec , 180/sec , and 240/sec and the right flexors at 240/sec . 
 significant differences 
 were observed in peak torque / body weight in the right extensors at 60/sec , 180/sec , and 
 240/sec and in the right flexors at 180/sec and 240/sec . 
 significant peak torque 
 differences were noted in the left ankle joint dorsiflexor at 30/sec and 120/sec , right 
 plantar flexor at 120/sec , left plantar flexor at 30/sec , left dorsiflexor at 30/sec 
 and 120/sec , and right dorsiflexor at 120/sec . 
 [ conclusion ] isokinetic evaluation 
 stimulates muscle contraction at motion - dependent speeds and may contribute to the 
 development of intervention programs to improve knee and ankle joint function and correct 
 lower - extremity instability . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION