this study was performed according to the protocol approved by the institutional review board at eulji university hospital . all subjects provided informed consent to participate in this study .
this study was conducted in volunteers with normal gait who had no history of musculoskeletal disease and injury or pain in the lower limbs , and no abnormalities on the anteroposterior hip - to - ankle radiographs with the patient standing and physical examinations .
the study group was selected from among medical students who had volunteered to participate in this study .
a total of 30 students , 15 males and 15 females , participated in this study .
subjects ' height , weight , foot length , foot width , ankle width , knee width , and lower limb length were measured for the preliminary data of gait analysis .
deformities of the lower limbs were checked on the scanogram of lower extremities . to check for the torsional abnormalities ,
degrees of rotational profiles and anteversion by palpation of the greater trochanter in the prone position were measured .
the 3d motion capture of dynamic knee kinematics was estimated using an optical tracking system ( motion analysis co. , santa rosa , ca , usa ) , which required at least three non - collinear markers on each bone .
this system was composed of six synchronized infrared cameras placed circumferentially around the subject being examined .
the frequency of motion capture was set at 120 hz and the walking length for motion capture was set at 7 m. we used three software programs for data processing ; eva real time ( evart ver .
4.2 , motion analysis co. ) , orthotrak ( motion analysis co. ) , and skeletal builder ( skb , motion analysis co. ) .
two virtual planes were formed using the skb program in this study , and the relative movement between these two virtual planes was calculated . then , 9-mm passive reflective markers ( motion analysis co. ) were attached by using the helen - hayes method,15 ) and additional markers were attached to the medial and lateral condyles of the proximal tibia , on which the medial and lateral collateral ligaments are attached ( fig .
while positioning the four markers on the medial and lateral condyles of the femur and tibia , the marker position was checked using c - arm fluoroscopy during the early stage of the study in all cases by an orthopaedic resident .
the marker on the lateral tibial condyle was positioned just over the top of the fibular head , while the marker on the medial tibial condyle was positioned over the bony surface on which the medial collateral ligament is inserted .
it was mostly at the same level as that of the lateral tibial condyle marker ( fig .
2 ) . to capture the data , the subjects were asked to stand in the center of the capture volume , and the markers were attached .
the static data was captured while the subjects stood at the center of walking volume . to capture the dynamic data ,
the subjects were directed to walk along the walkway ( 7 m ) for more than five times .
the mean value was obtained by gathering the data three times . for assessing gait velocity , the subjects were instructed to walk at a constant pace of 90 steps per minute .
before dynamic data were obtained , the subjects were given several practice trials to ensure that they understood the proper walking technique and timing .
the temporal parameters of gait were measured using the orthotrak program , and kinematic data was obtained in 3ds of the sagittal plane , coronal plane , and transverse plane . the data of each subject was displayed on an excel spreadsheet by dividing the gait cycle into 100-time periods ( % ) .
the data was also displayed in several graphs using the orthotrak program to confirm whether the kinematics and kinetics of the subjects were similar to the gait of normal adults . for this study , two virtual planes ( the femoral coronal plane [ pf ] and tibial coronal plane [ pt ] )
the lower leg marker was attached on the front side of proximal one - third of the lower leg .
3 ) almost matched the flexion - extension axis of the proximal tibial articular surface .
the relative movement of the virtual pt , as to the virtual pf , was measured by the degree of freedom in 3ds .
the coordinate system was defined as follows : the valgus / varus angle ( + /-
) was measured on the coronal plane ( x - axis ) , while the internal rotation / external rotation angle ( + /-
) was measured on the transverse plane ( y - axis ) . the flexion / extension angle ( + /-
) was measured on the sagittal plane ( z - axis ) . the wilcoxon paired t - test
was used to analyze demographic data , gender - based difference in healthy knee kinematics , and dynamic knee joint kinematics between the dominant and non - dominant lower limbs during gait .
the wilcoxon paired t - test was used to analyze demographic data , gender - based difference in healthy knee kinematics , and dynamic knee joint kinematics between the dominant and non - dominant lower limbs during gait .
the mean heights were 174 cm ( range , 165 to 185 cm ) in males and 161 cm ( range , 156 to 168 cm ) in females , respectively .
the mean weights were 71 kg ( range , 59 to 80 kg ) and 52 kg ( range , 42 to 61 kg ) in males and females , respectively .
the mean age of males and females was 20 years ( range , 18 to 25 years ) and 20 years ( range , 18 to 24 years ) , respectively .
with respect to the linear parameters of gait and kinematic data , there was no statistically significant difference between the male and female groups ( table 1 ) .
the mean changes in the rotational movement in three planes ( sagittal , coronal , and transverse planes ) between both knees and the mean changes in the knee angle in three planes between genders are summarized in table 2 .
there was no statistically significant difference between the two groups in all three planes ( p > 0.05 ) .
the knee joint was fully extended at initial contact in the sagittal plane , and flexed approximately 13 during the loading response .
then , the knee joint showed rapid flexion in the pre - swing phase , and it reached peak flexion of approximately 60 during the initial swing . on entering into the mid - swing phase , the knee joint started to extend , showing full extension at the end of the swing phase .
it was almost maintained in the neutral position without any significant change in the course of the stance phase in the coronal plane .
the tibiofemoral joint in the initial swing phase showed valgus of approximately 8. the knee joint returned to the neutral position in the late - swing phase .
while the knee joint was being flexed approximately 15 during the loading response , the tibia was externally rotated around the femur .
this amount of tibial rotation was maintained during the mid - stance phase ( fig .
the knee joint occurred in the pre - swing phase , the tibia started to rotate internally around the femur ( a reversal of the screw - home movement ) .
internal rotation was maintained until it reached peak knee flexion in the initial swing phase .
as the knee reached full extension in the lateswing phase , the tibia was rotated externally by 17 ( fig .
4c ) ; thus , it almost reached the neutral position ( screw - home movement ) .
the mean heights were 174 cm ( range , 165 to 185 cm ) in males and 161 cm ( range , 156 to 168 cm ) in females , respectively .
the mean weights were 71 kg ( range , 59 to 80 kg ) and 52 kg ( range , 42 to 61 kg ) in males and females , respectively .
the mean age of males and females was 20 years ( range , 18 to 25 years ) and 20 years ( range , 18 to 24 years ) , respectively .
with respect to the linear parameters of gait and kinematic data , there was no statistically significant difference between the male and female groups ( table 1 ) .
the mean changes in the rotational movement in three planes ( sagittal , coronal , and transverse planes ) between both knees and the mean changes in the knee angle in three planes between genders are summarized in table 2 .
there was no statistically significant difference between the two groups in all three planes ( p > 0.05 ) .
the knee joint was fully extended at initial contact in the sagittal plane , and flexed approximately 13 during the loading response .
then , the knee joint showed rapid flexion in the pre - swing phase , and it reached peak flexion of approximately 60 during the initial swing .
on entering into the mid - swing phase , the knee joint started to extend , showing full extension at the end of the swing phase .
it was almost maintained in the neutral position without any significant change in the course of the stance phase in the coronal plane .
the tibiofemoral joint in the initial swing phase showed valgus of approximately 8. the knee joint returned to the neutral position in the late - swing phase .
while the knee joint was being flexed approximately 15 during the loading response , the tibia was externally rotated around the femur .
this amount of tibial rotation was maintained during the mid - stance phase ( fig .
the knee joint occurred in the pre - swing phase , the tibia started to rotate internally around the femur ( a reversal of the screw - home movement ) .
internal rotation was maintained until it reached peak knee flexion in the initial swing phase .
as the knee reached full extension in the lateswing phase , the tibia was rotated externally by 17 ( fig .
4c ) ; thus , it almost reached the neutral position ( screw - home movement ) .
the principal findings of this study were that while the knee joint was flexed approximately 15 during the loading response , the tibia was externally rotated around the femur about 6. the screw - home movement in the tibiofemoral joint occurred due to the unique anatomical shape.16 ) during the last 20 of knee extension , anterior tibial glide persists on the tibial medial condyle because its articular surface is longer than that on the lateral side .
prolonged anterior glide on the medial side produces external tibial rotation , the screw - home mechanism . when the knee begins to flex ( 0 of extension to 20 of flexion )
, posterior tibial glide begins first on the longer medial condyle , and produces relative tibial internal rotation , a reversal of the screw - home mechanism .
when the knee joint is extended , the anterior and posterior cruciate ligaments get tangled and tightened if the tibia is rotated externally with respect to the femur ( screw - home movement ) .
consequently , the knee joint becomes locked in a position ; the tibia becoming more stable as compared to the femur .
therefore , the screw - home movement stabilizes the knee joint in the extended position .
many authors have primarily assessed the screw - home movement statically during passive flexion - extension of the knee joint , and little has been studied dynamically during normal gait . in this study
, we attempted to measure the amount and direction of tibial rotation with respect to the femur during gait using optical tracking system and software .
tibial rotation in the transverse plane during gait does occur to a lesser degree than the amount of flexion - extension in the sagittal plane .
therefore , measurement of tibial rotation using conventional gait lab tools has been considered to be not very accurate compared to measurement of flexion - extension . in this study ,
tibial rotation during gait was measured using additional markers and skb software to directly compare the rotation angles between the femur and the tibia . during normal gait
, the tibia was expected to rotate internally ( a reversal of the screw - home movement ) while the knee started to flex during the loading response and in the pre - swing phase .
conversely , the tibia was expected to rotate externally ( screw - home movement ) as the knee started to extend in the late - swing phase . in this study , as measured using the skb program , the screw - home movement occurred as expected during the pre - swing phase and the late - swing phase ( the open chain movement ) of the gait cycle . however , in this study , the tibia rotated externally with respect to the femur , rather than internally , while the knee joint started to flex during the loading response ( closed chain movement ) .
this was the paradoxical screw - home movement that reversed the geometry of contacting joint surfaces , and it may use the energy to internally rotate the femur on the tibia fixed on the ground to provide stability to the knee joint during the remaining stance phase .
this is believed to be one of the energy - saving mechanisms of gait , as less energy is used for knee joint locking during the double support period than during the single support period.1617 ) kozanek et al.17 ) studied the 3d gait analysis and reported that the tibia showed external rotation during the early stages of stance phase as the knee joint started to flex during walking , which was similar to the finding of this study .
koo and andriacchi19 ) implied that the knee joint mechanics might differ depending on the movement condition by suggesting that the axis of knee joint rotation shifted laterally during gait unlike in the non - weight - loading state . in this study , the amount of tibial rotation was measured using the skb program to be about 6 during the loading response and 17 in the pre - swing and late - swing phases .
karrholm et al.20 ) reported that the rotational movement of the tibia ranged from 9.9 ( internal rotation ) to 1.6 ( external rotation ) during the active extension movement of the knee joint among healthy people using the roentgen stereophotogrammetric analysis method .
in addition , lafortune et al.21 ) conducted a study by inserting the intra - cortical traction pins into the tibia and femur , and they reported that during the stance phase , the tibiofemoral joint exhibited no external rotation , and during the swing phase , external rotation was about 4. this difference in the range of the rotational movement might be due to the difference in the loads applied to the knee joint , the difference in the measuring tools , and the accuracy of the measurement method . therefore , a careful analysis of such study results is required .
the 3d gait analysis using optical tracking system allows quantitative and non - invasive measurement of the joint movement during the actual gait cycle .
the kinematic analysis of the tibiofemoral joint can be conducted for three rotational movements ( internal / external rotation , varus / valgus , and flexion / extension ) and three translational movements ( anterior / posterior translation , medial / lateral translation , and vertical compression / distraction).20 ) measuring these knee movements during gait might provide useful information to assess different morbid gait patterns as well as to determine the efficacy of treatments including surgical reconstruction and rehabilitation .
more cases are needed to enhance the statistical power . for measuring the rotational movement between the tibia and the femur in this study ,
additional markers were attached , in addition to the helen - hayes marker set , to the medial and lateral condyles of the femur and tibia . however , the markers attached to the inner side of the thigh and leg caused the subjects to walk with the hip in 3 of abduction so that the internal markers would not come in contact with each other .
however , these markers enabled direct measurement of the rotation angle between the femur and the tibia .
we think that this difference was due to a compensatory mechanism during gait in order to provide a clear space for the markers attached to the inner side of the lower limbs .
this is the first study to measure the direction and degree of the screw - home movement during gait of a normal person by utilizing the optical tracking device . as a result , the screw - home movement occurred in the normal direction in the pre - swing and late - swing phases ; however , the paradoxical screw - home movement occurred during the loading response phase
. this may be an important mechanism that provides stability during the remaining stance phase .
obtaining standardized values of the knee joint during gait using 3d motion capture analysis can be useful in diagnosing and treating the pathological knee joints .
 backgroundthe purpose of this study was to evaluate the screw - home movement at the tibiofemoral joint during normal gait by utilizing the 3-dimensional motion capture technique.methodsfifteen young males and fifteen young females ( total 60 knee joints ) who had no history of musculoskeletal disease or a particular gait problem were included in this study . 
 two more markers were attached to the subject in addition to the helen - hayes marker set . 
 thus , two virtual planes , femoral coronal plane ( pf ) and tibial coronal plane ( pt ) , were created by skeletal builder software . 
 this study measured the 3-dimensional knee joint movement in the sagittal , coronal , and transverse planes of these two virtual planes ( pf and pt ) during normal gait.resultswith respect to kinematics and kinetics , both males and females showed normal adult gait patterns , and the mean difference in the temporal gait parameters was not statistically significant ( p > 0.05 ) . in the transverse plane , 
 the screw - home movement occurred as expected during the pre - swing phase and the late - swing phase at an angle of about 17. however , the tibia rotated externally with respect to the femur , rather than internally , while the knee joint started to flex during the loading response ( paradoxical screw - home movement ) , and the angle was 6.conclusionsparadoxical screw - home movement may be an important mechanism that provides stability to the knee joint during the remaining stance phase . obtaining the kinematic values of the knee joint during gait can be useful in diagnosing and treating the pathological knee joints . 
METHODS
Statistical Analysis
RESULTS
Physical Examination, Gait Linear Index, and Kinematics
Relative Movement of Virtual Tibial Coronal Plane for the Virtual Femoral Coronal Plane
DISCUSSION