approximately 91% of anterior cruciate ligament ( acl ) injuries occur during sports 
 activities1,2,3,4 ; most are caused by noncontact injury mechanisms , e.g. , landing from 
 a jump and rapid deceleration5,6,7,8 .
the position of the knee in noncontact acl injury is characterized 
 by slight flexion ( < 30 ) , valgus of the knee , and internal rotation of the tibia1 , 9,10,11 . 

thus , training programs to prevent noncontact acl injuries should consider the anatomical 
 and neurophysiological factors for preventing slight flexion and valgus of the knee joint . 

recent research has reported an association between the position of acl injuries and hip 
 joint kinematics12 .
hip and knee flexion 
 angles upon landing are important factors determining the load on the knee , with slight 
 flexion angles resulting in a greater load13 . moreover ,
the decrease in hip flexion angle during rapid 
 deceleration results in a leg position that increases the quadriceps force , leading to 
 anterior tibial displacement and increased risk of acl injury14 .
these findings indicate that the shape of the femur may affect 
 both kinematics of the hip and knee joints upon landing as well as muscle activity for 
 protecting the joints .
anteversion and the neck  shaft angle are morphological factors of the femur that affect the 
 kinematics of the hip and knee joints upon landing .
femoral anteversion is the angle formed 
 by the axis of the femoral neck and the horizontal axis of the femoral condyles .
the angle 
 of torsion decreases up to approximately 6  years of age , and the femoral head changes so 
 that it faces more medially in the acetabulum15 .
thus , when femoral anteversion is larger , the femoral head faces 
 anteriorly in the acetabulum , resulting in reduced congruity of the hip joint .
improved 
 congruity of the hip joint may occur with excessive internal rotation of the hip16 and valgus of the knee17 .
however , the association between neuromuscular 
 control and hip and knee joint kinematics upon single - leg landing due to differences in 
 femoral anteversion remains unclear .
this study aimed to clarify the relationship between femoral anteversion and hip and knee 
 joint kinematics and muscle activity of the lower extremity upon single - leg landing in terms 
 of risk factors for acl injury .
we hypothesized that increased femoral anteversion leads to 
 an acl injury - risk position and , therefore , must be considered a risk factor for acl 
 injury .
sixteen healthy female college students ( age , 20.8  1.0  years ; height , 160.9  3.8  cm ; 
 weight , 54.1  5.8  kg ) participated in this study .
the purpose of the study and the 
 measurements involved were explained to the subjects beforehand , both verbally and in 
 writing , and their consent was obtained .
this study was approved by the ethics committee of 
 juntendo university graduate school of health and sports science .
subjects performed a single - leg landing task by standing on both legs on a 30-cm high 
 platform .
they then jumped 30  cm forward , landing on the left leg . to minimize the effects 
 of efforts for maintaining balance , subjects were instructed to cross their arms on their 
 chest .
inability to maintain this posture upon landing and allowing the opposite leg to 
 contact the ground were considered failures .
femoral anteversion was measured using craig 's test . for measurement , the knee joint of the 
 tested leg
the examiner palpated the 
 greater trochanter while passively rotating the hip until the most prominent part of the 
 greater trochanter reached its most lateral position . the angle between the shaft of the 
 tibia and a line perpendicular to the floor
the angle of the hip and knee joints upon single - leg landing was measured using the vicon 
 mx three - dimensional ( 3d ) motion analysis system ( vicon motion systems , oxford , uk ) , eight 
 infrared cameras , and a frequency of 100  hz .
measurements were performed after the 
 measurement error was confirmed to be 0.7  mm or less .
ground reaction force was calculated synchronously at 1500  hz using an amti or6 - 7 force 
 platform ( amti , watertown , ma , usa ) .
infrared reflective markers were affixed to the body 
 according to the marker positions in the plug - in gait lower body model ( vicon motion 
 systems ) .
markers were placed at 16 sites : both the anterior superior and posterior superior 
 iliac spines , the center of both thighs externally , the lateral joint line of both knees , 
 the center of both shanks externally , the lateral malleolus of both ankles , the center of 
 both heels , and the head of the second metatarsal of both feet .
marker trajectories were filtered with a woltring low - pass filter and a 20-hz cut - off 
 frequency .
ground reaction force data were filtered with a fourth - order butterworth low - pass 
 filter with zero lag and a 6-hz cut - off frequency .
the hip and knee joint angles in the 
 sagittal and frontal planes were calculated using a plug - in biomechanical modeler ( vicon 
 motion systems ) .
the angle of hip flexion and extension were defined as the angles formed by 
 the pelvic axis and femoral axis in the sagittal plane , with flexion denoted as + and 
 extension denoted as . the angle of hip adduction and abduction were defined as the angles 
 formed by the pelvic axis and femoral axis in the frontal plane , with adduction denoted as + 
 and abduction denoted as . the angle of knee flexion and extension were defined as the 
 angles formed by the femoral axis and shank axis in the sagittal plane , with flexion denoted 
 as + and extension denoted as . the varus and valgus angles of the knee were defined as the 
 angles formed by the femoral axis and shank axis in the frontal plane , with varus denoted as 
 + and valgus denoted as . the measurement interval was calculated from the initial ground 
 contact upon landing to 100  ms immediately after ground contact .
initial ground contact was 
 defined as the point at which the vertical ground reaction force exceeded 8 n. muscle activity of the lower extremity was measured using a telemyo 2400 surface 
 electromyography system ( noraxon , scottsdale , az , usa ) and a 1500-hz sampling frequency .
the 
 electromyography ( emg ) data were collected from four muscles : the rectus femoris ( rf ) , 
 gluteus maximus ( gm ) , semitendinosus ( st ) , and biceps femoris ( bf ) . the skin over the belly 
 of each muscle was prepared for electrode placement by dry shaving and cleaning the area 
 with alcohol to reduce surface impedance .
blue sensor m-00-s surface electrodes ( ambu , 
 ballerup , denmark ) affixed in the direction of the muscle fibers with a 2-cm interelectrode 
 distance .
the position of the electrode for rf was the midpoint of a line connecting the 
 anterior superior iliac spine and the top of the patella , that for the gm was the midpoint 
 of a line connecting the sacrum and the greater trochanter of the femur , that for the st was 
 the midpoint of a line connecting the ischial tuberosity and the medial epicondyle of the 
 tibia , and that for the bf was the midpoint of a line connecting the ischial tuberosity and 
 the lateral epicondyle of the tibia .
the emg data for each muscle were obtained from 100  ms before ground 
 contact to 100  ms immediately after ground contact .
preactivity plays a key role in maintaining knee joint stability after the impact of landing 
 because the feedback mechanism alone is not enough to provide knee joint stability21 , 22 .
thus , the activity approximately 100  ms before ground contact is 
 crucial23 , 24 .
acl strain upon landing reportedly peaks at approximately 40  ms 
 after landing , and the ground reaction force after landing reportedly peaks at approximately 
 100  ms after landing4 , 25 , 26 .
thus , muscle activity 
 100  ms after landing was measured because acl injury most likely occurs within 100  ms of 
 landing .
measurements of the maximum voluntary contraction ( mvc ) utilized the test positions 
 for manual muscle testing reported by daniels and worthingham .
normalization ( % mvc ) of the 
 emg data for each muscle was accomplished using the emg data for 1  s during the waveform of 
 mvc for 5 s. all measurements were performed for the left leg , and the mean of three 
 attempts was used in the analysis .
the mean for the 16 subjects was calculated on the basis of measurements of femoral 
 anteversion ; the subjects were divided into two groups : the high group with six subjects at 
 the upper end of the mean and the low group with six subjects at the lower end of the mean . 

although the femoral anteversions of the two groups were significantly different , there were 
 no significant differences between the groups for age , height , and weight ( table 1 ) .
all statistical tests were performed using the spss 11.0  j for windows statistical software 
 ( spss inc . ,
the low and high groups were compared using unpaired t - tests 
 for the kinematics and emg data . the alpha level for determining statistical significance 

gm , gluteus maximus ; rf , rectus femoris ; st , semitendinosus ; bf , 
 biceps femoris mean  sd . * p<0.05 .
gm , gluteus maximus ; rf , rectus femoris ; st , semitendinosus ; bf , 
 biceps femoris table 2 shows the mean hip and knee joint 
 angles in the sagittal and frontal planes upon single - leg landing .
the hip joint flexion 
 from initial ground contact upon landing to 100  ms immediately after ground contact was 
 significantly lower in the high group than in the low group ( p < 0.05 ; table 2 ) .
significant differences between the two 
 groups for hip joint adduction and abduction were not observed ( p < 0.05 ; table 2 ) .
the knee joint flexion from 80 to 100  ms 
 after initial ground contact was significantly higher in the high group than in the low 
 group ( p < 0.05 ; table 2 ) .
the knee valgus 
 from 70 to 100  ms after the initial ground contact was significantly higher in the high 
 group than in the low group ( p < 0.05 ; table 
 2 ) .
muscle activity 100  ms before ground contact indicated that the % mvc of the gm was 
 significantly less in the high group ( 0.5%  0.5% ) than in the low group ( 1.5%  0.6% , p 
 < 0.05 ; table 3 ) .
the % mvc of the rf was 
 significantly greater in the high group ( 1.7%  0.5% ) than in the low group ( 0.9%  0.4% , p 
 < 0.05 ; table 3 ) .
significant differences 
 between the groups in % mvc of the st and bf were not observed ( table 3 )
. muscle activity 100  ms after ground contact indicated 
 that the % mvc of the rf was significantly greater in the high group ( 5.8%  0.7% ) than in 
 the low group ( 4.0%  1.4% , p < 0.05 ; table 
 4 ) .
significant differences between the two groups in % mvc of gm , st , and bf were not 
 observed ( table 4 ) .
abnormal alignment of the lower extremity has been proposed as a risk factor for acute and 
 chronic lower extremity injuries such as acl injuries , patellofemoral syndrome , and plantar 
 fasciitis .
in addition , it has been suggested that biomedical changes caused by abnormal 
 alignment of the lower extremity may influence joint load and muscles as well as 
 neuromuscular control and function27 .
increased femoral anteversion causes anterior displacement of the femoral head in the 
 acetabulum and a decrease in congruity of the hip joint .
thus , improvement in the congruity 
 of the hip joint may occur with excessive internal rotation of the hip16 .
excessive internal rotation of the hip leads to knee 
 valgus alignment as a result of a kinematic chain17 ; thus , an increase in femoral anteversion may cause acl injury .
our 
 study results showed that the valgus of the knee in the high group increased after 
 single - leg landing .
this was because the congruity of the acetabulum and the head of the 
 femur may have improved to protect the hip joint from the impact of landing ; excessive 
 internal rotation of the hip may result in a kinematic chain .
this study focused on joint 
 kinematics in the frontal plane as well as the kinematics of the hip and knee joints in the 
 sagittal plane .
the hip and knee joint kinematics in the high group were characterized by 
 lower hip joint flexion and higher knee joint flexion after single - leg landing . on the basis of video image analysis of acl injuries ,
a recent study reported that acl 
 injury occurs when the trunk is positioned behind the leg28 .
in other words , body position in the sagittal plane affects 
 kinetic and kinematic components of the leg , presenting a risk of acl injury .
blackburn and 
 padua demonstrated that increased trunk extension upon landing increased the ground reaction 
 force and the quadriceps forces29 . 

similarly , kulas et al . reported that increased trunk extension upon landing caused an 
 increase in quadriceps forces and a decrease in hamstring forces30 .
increased quadriceps forces lead to anterior tibial 
 displacement , increasing the load on the acl28 , 31 .
increased trunk extension is associated 
 with a decrease in the angle of hip joint flexion32 .
thus , increase of the femoral anteversion suggests a physical 
 characteristic that probably leads to acl injury .
the leg position upon landing is 
 maintained by soft tissues such as muscles , the joint capsule , and ligaments .
however , the 
 feedback mechanism alone is not enough to provide knee joint stability and prevent 
 injury21 , 22 .
once the foot contacts the ground , the ground reaction force and acl strain peak .
cerulli 
 et al . reported that acl strain peaks approximately 40  ms after ground contact25 , whereas schmitz et al . reported that it 
 increased approximately 100  ms after ground contact4
thus , acl injury occurs 40100  ms after ground contact , the time 
 during which acl strain peaks .
however , the feedback mechanism can not provide joint 
 stability at this point ; therefore , the feedforward mechanism is important for joint 
 stability23 , 24 .
the results concerning emg activity in this study indicated lower 
 muscle activity of the gluteus maximus and greater muscle activity of the rectus femoris 
 before ground contact in the high group than in the low group .
thus , the decrease in gluteus maximus activity before ground 
 contact indicates that soft tissues such as ligaments are involved in limiting excessive hip 
 internal rotation and that the ability to resist valgus of the knee diminished .
the rectus 
 femoris resists knee flexion moment32 and 
 produces anterior tibial displacement32 , 35 , 36 .
moreover , this muscle leads to valgus of the knee and internal 
 rotation of the tibia37 ; thus , the rectus 
 femoris increases the load on the acl .
therefore , increased rectus femoris activity before 
 and after ground contact may cause knee extensor moment and anterior tibial displacement . 

therefore , activity in these muscles may promote valgus of the knee and anterior tibial 
 displacement , both of which predispose to acl injury . based on hip and knee joint kinematics upon single - leg landing and muscle activities 
 immediately before and after ground contact , the above findings suggest that increased 
 femoral anteversion may be a physical characteristic that probably leads to acl injury . 

furthermore , femoral anteversion may help to identify individuals who are likely to develop 
 acl injury and may be considered a risk factor for acl injury .
the kinematics of hip and knee joints upon single - leg landing were 
 analyzed using the plug - in gait model .
measurement of femoral anteversion is reportedly more 
 reliable than techniques using x - rays38 . 

however , studies using more accurate measurement techniques involving sectional computed 
 tomography images are required .
in addition , kadaba et al . indicated that disparities in the 
 placement of infrared reflective markers in the plug - in gait model can lead to errors ; i.e. , 
 errors in measurement of joint motion other than knee joint flexion / extension39 .
future studies should validate the knee 
 joint motion in detail using the point cluster technique40 , which allows more accurate measurement of the 3d motion of the 
 knee .
   [ purpose ] increased femoral anteversion may occur with hip internal rotation and valgus 
 knee alignment upon landing and is considered a risk factor for anterior cruciate ligament 
 injury . 
 we examined the relationship between femoral anteversion and joint motion and 
 muscle activity of the lower extremity in terms of the risk factors for anterior cruciate 
 ligament injury . 
 [ subjects ] sixteen healthy females were divided on the basis of femoral 
 anteversion into low and high groups . [ methods ] femoral anteversion was assessed using 
 craig 's test . 
 we performed kinematic analysis and measured the electromyography activity 
 of the lower extremity upon left single - leg landing . 
 [ results ] the high group had a 
 significantly lower hip flexion angle and higher knee flexion and valgus angles than the 
 low group . 
 the rectus femoris showed significantly greater electromyography activities in 
 the high group than in the low group . 
 [ conclusion ] these results suggest that increased 
 femoral anteversion results in lower hip flexion angle , higher knee valgus alignment , and 
 greater rectus femoris muscle activity , leading to anterior tibial displacement upon 
 single - leg landing . 
 increased femoral anteversion may be a potential risk factor for 
 anterior cruciate ligament injury . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION