anterior cruciate ligament ( acl ) injury is one of the most common sports injuries , which 
 frequently occurs during jumping , landing , and cutting motions1 , 2 .
acl reconstruction has 
 become a standard treatment option for athletes who have functional limitations of the lower 
 extremities for cutting and pivoting motions and wish to return to athletic activities3 . in order to assess functional recovery and 
 risk for re - injury after acl reconstruction
, the functional performance test ( fpt ) has been 
 used as a performance - based measurement tool to determine the time to return to play . in 
 addition
there are several types of fpt batteries including hop tests such 
 as the single / triple hop test for distance , side hop test , and figure - of-8 hop test .
motion 
 characteristics during these hop tests consist of jumping , landing , and cutting activities 
 that exert impact loads on the joints of the lower limbs4,5,6,7,8 .
previous studies on hop test results have shown that several 
 kinematic features such as smaller knee flexion angle , larger knee valgus , and internal 
 rotation angles are related to the risk for acl injuries9,10,11,12 . however , 
 three - dimensional kinematic and kinetic characteristics of the limb during side - hopping has 
 not been clarified .
consequently , there is a lack of biomechanical evidence to support the 
 significance of a side hop test as an fpt . in consideration of factors affecting the motion pattern during various hopping tasks ,
the 
 significance of hip function and its effect on knee kinematics and kinetics has been 
 addressed in recent literature . in these studies , weaknesses of the hip abductor , external 
 rotator , and extensor were depicted as risk factors inducing dynamic frontal knee 
 motion13,14,15
. however , no analytical 
 data are available regarding the hip / knee interaction during side - hopping16,17,18,19 .
this study was conducted to investigate kinematic and kinetic sequences of the knee during 
 side hop motion using a three - dimensional motion analysis system and to examine the 
 correlation between clinical parameters of hip function , such as range of motion ( rom ) and 
 muscle strength , on knee motion .
it has been hypothesized that knee kinematic and kinetic 
 patterns characteristic to side - hopping motion are related to acl injuries and that hip 
 function affects the motion pattern of the knee during side hop .
twenty - one healthy , young ( 2026 years ) male college rugby players were initially enrolled 
 in this study .
based on the medical history and results of physical examinations , the limbs 
 that met the following criteria were excluded from the study : history of major 
 musculoskeletal injuries or surgeries in the corresponding lower limb , history of acute 
 lower limb injuries within 3 months , pain or swelling at the ankle or knee affecting the 
 hopping motion . consequently , 16 lower extremities ( 8 right and 8 left limbs ) in 11 athletes 
 were included in the study .
the height of the participants ranged from 1.65 to 1.81  m and 
 weight ranged from 69.1 to 89.5  kg .
the subjects were informed of the experimental 
 protocols , and signed informed consent forms were obtained from the study participants .
the 
 study design was approved by the konan women s university research ethics committee before 
 the study .
three - dimensional analysis of knee motion during side hop and measurements of hip rom and 
 muscle strength were conducted for each subject .
a cross - sectional study design was used to 
 analyze the relationship between the values obtained from the tests . in the first session
, a three - dimensional motion analysis of the knee during side hop 
 movement was performed using a mac3d automatic digitizing motion analysis system equipped 
 with eight cameras ( motion analysis corporation , santa rosa , ca , usa ) and two force plates 
 ( amti japan ltd . ) . for assessment of the side hopping motion ,
the ior ( istituto orthopedico rizzoli ) full - body 
 model was applied for definition of bilateral upper- and lower - body segments including the 
 coda model for the pelvis20,21,22,23 .
eighty - four spherical reflective markers ( diameter : 
 12  mm ) were attached to the subjects skin directly on the anatomical bony landmarks of the 
 upper and lower body using double - sided adhesive tape .
the locations of the markers were as 
 follows : 1 ) head segment ( 4 markers ) .
2 ) spinous process of the 7th cervical vertebrae , second thoracic vertebrae and 7th 
 thoracic vertebrae level ( midpoint between the inferior angles of the most caudal points of 
 the two scapulae ) , incisura jugularis , xiphoid process , and the 1st , 3rd , and 5th lumbar 
 vertebrae for the neck and thoracic segment .
3 ) acromion , deltoid tuberosity , lateral epicondyle of the humerus , dorsal aspect of the 
 forearm , styloid processes of the ulna and radius and head of the third metacarpus for the 
 bilateral upper - extremity segments .
5 ) the greater trochanter , the lateral and medial femoral epicondyles with 4 cluster 
 markers attached to the lateral aspect of the thigh for the bilateral thigh segments .
6 ) the head of the fibula , the tibial tuberosity , the lateral and medial malleoli with 4 
 cluster markers attached to the lateral aspect of the leg for the bilateral shank 
 segments .
7 ) proximal posterior surface of the calcaneus and distal attachment of the achilles tendon 
 on the calcaneus , the sustentaculum tali , the peroneal tubercle , the tuberosity navicular , 
 the 1st , 2nd and 5th metatarsal heads , proximal distal phalanx , bases of the 1st and 2nd 
 metatarsals , styloid process of the 5th metatarsal for the bilateral foot segments
. a series of subject calibration trials in anatomical standing posture was performed to 
 determine anatomical landmarks and lower limb joint coordinate systems24 .
for the side hop movement , the subjects were instructed to first stand on two force plates 
 with both feet parallel and 30  cm apart and then asked to stand on one leg with the other 
 leg maintained in a flexed knee position at approximately 60 , with the arms abducted widely 
 for body balance control .
side - hopping started with this posture and was performed as 
 accurately and quickly as possible on the two force plates placed between interval line 
 tape - markers with 30  cm apart in accordance with previous investigations8 .
the subjects performed several trials of 
 the side hop for both legs until they could successfully perform 10 repetitions for each 
 leg .
trials were excluded if the entire foot did not contact the force plates or if any 
 markers were lost during side - hopping .
the motions of the reflective markers were tracked with 8 synchronized cameras at 200  hz , 
 and the force plates were time synchronized and sampled at a rate of 1,000  hz .
missing data 
 in the marker trajectories were interpolated using the evart 5.04 software ( motion analysis 
 corporation , santa rosa , ca , usa ) .
the raw three - dimensional coordinate data were filtered 
 using a zero - lag fourth - order low - pass butterworth filter with a cut - off frequency of 6 hz 
 using with visual 3d v5.02 software ( c - motion , inc .
the joint angles and moments were calculated using the euler angle method for 
 flexion / extension , abduction / adduction , and internal / external rotation sequences according 
 to the recommendations of the international society of biomechanics25,26,27 .
the side hop movement was divided into two phases : contact phase and jumping phase .
contact 
 phase was defined as the duration from when the vertical ground reaction force exceeded 10 n 
 to when it decreased to less than 10n before toe - off .
the side hop represents repetition of 
 jumping and landing motions in two opposite ( lateral - to - medial and medial - to - lateral ) 
 directions , and kinematic and kinetic features of the lower extremity at the touchdown phase 
 are thought to be different between the lateral and medial touchdown phases ( fig .
top : 
 lateral touch - down phase for the right leg ( ltdp ) .
a )  b ) : illustration shows 
 kinematic sequence at touching down with the right foot on a force - plate toward the 
 right .
b )  c ) : illustration shows kinematic sequence until the moment of taking off 
 with the right foot toward the left .
bottom : medial touch - down phase for the right leg 
 ( mtdp ) d )  e ) : illustration shows kinematic sequence at touching down with the right 
 foot on the other force - plate toward the left .
e )  f ) : illustration shows kinematic 
 sequence until the moment of taking off with the right foot toward the right . ) .
therefore , motion analysis was separately performed for the two contact phases : the 
 lateral touchdown phase ( ltdp ) and the medial touchdown phase ( mtdp ) .
top : 
 lateral touch - down phase for the right leg ( ltdp ) .
a )  b ) : illustration shows 
 kinematic sequence at touching down with the right foot on a force - plate toward the 
 right .
b )  c ) : illustration shows kinematic sequence until the moment of taking off 
 with the right foot toward the left .
bottom : medial touch - down phase for the right leg 
 ( mtdp ) d )  e ) : illustration shows kinematic sequence at touching down with the right 
 foot on the other force - plate toward the left .
e )  f ) : illustration shows kinematic 
 sequence until the moment of taking off with the right foot toward the right . in the second session for clinical evaluation ,
rom measurements for extension in supine position 
 and internal / external rotation ( ir / er ) in prone position were performed using this 
 device28 , 29 .
each measurement was performed three times and the maximum value 
 of the three measurements was adopted .
when the tests were performed using the inclinometer 
 in previous study , it was assured that the intra - class correlation coefficients ranged from 
 0.94 to 0.99 .
ny , usa ) was used for isokinetic strength measurement of the 
 hip extensor .
the test was performed twice in three repetitions with maximum voluntary 
 effort at 60/sec , and the peak torque was estimated .
hip external rotator strength was 
 evaluated using a hand - held dynamometer ( hhd ) , microfet2 ( hoggan scientific , llc , ut , usa ) , 
 for static force of hip external rotation with hip abducted in the half - squat posture .
use 
 of the hhd has been shown to provide excellent intra - class correlation coefficients for 
 measurement of maximal isometric strength of hip muscles19 , 30 .
the participants were 
 told to stabilize themselves by crossing their arms in front of their chest .
strength 
 measurement was performed while the subject was holding a half - squat posture with neutral 
 position of the knee and hip in coronal and horizontal planes .
the force pad of the hhd was 
 placed on the lateral epicondyle of the femur , and the examiner manually applied resistance 
 to the subject s motion .
the subject being tested exerted a submaximal voluntary 
 contraction , and the examiner confirmed whether the action was appropriately conducted by 
 the subject as instructed . following this confirmation , the subject exerted a 5-second 
 maximum isometric contraction against the hhd .
the test was performed twice for each leg . 
 between the 2 test trials , a rest period of one minute
the highest force 
 value during the two consecutive measurements for each leg was used for data analysis . for data analysis of the kinematic and kinetic parameters of the knee , angle and moment 
 values for internal / external rotation and adduction / abduction
therefore , each individual s data were time - normalized in 
 each phase and averaged across their limbs .
the parameters adopted for the analysis were the 
 highest angle and moment values during each phase .
analytical parameters derived from clinical measurement of the hip were 
 internal / external rotation and extension angles for rom , and the weight - adjusted extensor 
 torque and external rotator force values for strength .
statistical analyses were performed using the statistical package for the social sciences 
 ( spss ) version 20.0 for windows ( ibm , chicago , il , usa ) .
the paired t - test was used for the 
 comparison of joint angle and moment of the knee between ltdp and mtdp .
pearson correlation 
 coefficients were calculated to assess the relationship between kinematic / kinetic variables 
 of the knee and the clinical parameters of the hip . for all statistical analyses ,
in the motion analysis of the knee during side hop , kinematic and kinetic sequences during 
 the touchdown phase showed distinctly different features between the ltdp and mtdp . during 
 ltdp
, the knee was aligned in an abducted and externally rotated position throughout the 
 period of contact . regarding the moment , the kinetic sequence in this phase started with 
 small adduction moment followed by increasing abduction moment until the mid - phase and 
 subsequent return to small adduction moment in the late phase .
2.mean time sequence data of knee internal ( + ) /external (  ) rotation and adduction 
 ( + ) /abduction (  ) angles and moments during ltdp from initial contact ( 0% ) to toe - off 
 ( 100% ) .
ir : internal rotation ; er : external rotation ; add : adduction ; abd : 
 abduction ) .
mean time sequence data of knee internal ( + ) /external (  ) rotation and adduction 
 ( + ) /abduction (  ) angles and moments during ltdp from initial contact ( 0% ) to toe - off 
 ( 100% ) .
ir : internal rotation ; er : external rotation ; add : adduction ; abd : 
 abduction during mtdp , the knee was initially aligned in an abducted and externally rotated position . 
 by the mid - phase , a gradual increase in abduction and a decrease in external rotation
subsequently , the knee position returned to the initial condition . sequential 
 change in moment during mtdp exhibited the reverse pattern compared to that in ltdp . in the 
 initial phase ,
subsequently , adduction 
 moment applied to the knee was gradually increased until the mid - phase followed by 
 subsequent return to abduction moment in the late phase .
the internal / external rotation 
 moments were smaller compared to the adduction / abduction moments , while a small increase in 
 external rotation moment was observed in the mid - phase ( fig . 3fig .
3.mean time sequence data of knee internal ( + ) /external (  ) rotation and adduction 
 ( + ) /abduction (  ) angles and moments during mtdp from initial contact ( 0% ) to toe - off 
 ( 100% ) .
ir : internal rotation ; er : external rotation ; add : adduction ; abd : 
 abduction ) .
mean time sequence data of knee internal ( + ) /external (  ) rotation and adduction 
 ( + ) /abduction (  ) angles and moments during mtdp from initial contact ( 0% ) to toe - off 
 ( 100% ) .
ir : internal rotation ; er : external rotation ; add : adduction ; abd : 
 abduction on comparing kinematic parameters of the knee between ltdp and mtdp , ltdp was characterized 
 by a significantly larger abduction angle than mtdp , while a significantly larger external 
 rotation angle was observed for mtdp compared to ltdp ( table 1table 1.comparison of knee joint angle and moment of external / internal rotation and 
 abduction / adduction between ltdp and mtdp during side hopltdpmtdpangle ( deg)min er3.3  6.76.2  6.9**max er6.3  6.511.7  5.7**min abd5.0  3.40.1  2.2**max abd9.5  4.07.2  4.1**moment ( 10 ) max ir0.8  0.70.0  0.2**max er1.2  1.01.5
 1.0*the value was calculated as moment ( nm ) divided by weight ( n ) and height 
 ( m)*p<0.05 , * * p<0.01 .
ir : internal rotation ; er : external rotation ; add : 
 adduction ; abd : abduction ; ltdp : lateral touchdown phase ; mtdp : medial touchdown 
 phase ) .
regarding the moment values , internal rotation moment and abduction moment 
 in ltdp were significantly larger than those in mtdp ( table 1 ) .
moreover , manifestations of maximal abduction moment in ltdp and maximal 
 adduction moment in mtdp in the mid - phase represented a distinct contrast between the two 
 phases ( figs . 2 and 3 ) .
the value was calculated as moment ( nm ) divided by weight ( n ) and height 
 ( m)*p<0.05 , * * p<0.01 .
ir : internal rotation ; er : external rotation ; add : 
 adduction ; abd : abduction ; ltdp : lateral touchdown phase ; mtdp : medial touchdown 
 phase in this study population , the rom values were relatively small for hip extension and 
 internal rotation ( table 2table 2.clinical measurements for hip range of motion and strength ( n=16 limbs)hip range of motionextension ( deg)11.4  3.3(5.017.0)internal rotation ( deg)32.5  12.8(5.050.0)external rotation ( deg)42.3  10.3(25.060.0)muscle strengthhip extension ( nm / kg)4.8  1.2(2.87.2)hip external rotation ( % ) 27.9  9.3(13.940.9)minimum and maximum values are shown in parentheses .
significant 
 difference between internal and external rotation at p - value less than 0.05 . 

the value was calculated as torque ( nm / kg ) divided by weight ( kg ) . 

the value was calculated as force ( % ) divided by weight ( kg ) . ) .
muscle strength measurement showed substantial variability despite 
 the fairly uniform subject demographics ( male rugby football players in their twenties ) .
significant 
 difference between internal and external rotation at p - value less than 0.05 . 

the value was calculated as torque ( nm / kg ) divided by weight ( kg ) . 

the value was calculated as force ( % ) divided by weight ( kg ) .
the relationship between the kinematic and kinetic variables of the knee during ltdp and 
 the clinical parameters of the hip is presented in table 3table 3.pearson correlation coefficients between clinical measurements of hip and 
 kinematic / kinetic data of the knee joint during lateral touch - down phaseknee joint angleknee joint momentmin er max ermin abdmax abdmax irmax ermax addmax abdhip range of motionextension ( deg)0.060.120.350.340.260.230.060.01internal rotation ( deg)0.470.51*0.080.090.150.52 * 0.030.53*external rotation ( deg)0.020.020.040.100.210.230.040.03muscle strengthhip extension ( nm / kg)0.040.000.090.190.040.070.140.07hip external rotation ( % ) 0.000.000.360.390.410.490.180.42the value was calculated as torque ( nm / kg ) divided by weight ( kg ) . 

the value was calculated as force ( % ) divided by weight ( kg ) . 

ir : internal rotation ; er : external rotation ; add : adduction ; abd : 
 abduction .
a statistically significant correlation was demonstrated between the internal 
 rotation rom of the hip and the maximal external rotation angle of the knee during this 
 phase . regarding the moment , maximal external rotation and maximal abduction moments during 
 ltdp
relevant analysis for 
 the parameters during mtdp showed no significant correlation between the kinematic / kinetic 
 results of the knee and the clinical parameters of the hip ( table 4table 4.pearson correlation coefficients between clinical measurements of hip and 
 kinematic / kinetic data of the knee joint during medial touch - down phaseknee joint angleknee joint momentmin ermax ermin abdmax abdmax irmax ermax addmax abdhip range of motionextension ( deg)0.070.000.280.460.320.04 0.140.20internal rotation ( deg)0.350.430.090.050.03 0.100.000.39external rotation ( deg)0.050.110.080.020.18 0.320.080.45muscle strengthhip extension ( nm / kg)0.140.150.060.380.18 0.480.010.22hip external rotation ( % ) 0.100.170.340.450.13 0.200.480.45the value was calculated as torque ( nm / kg ) divided by weight ( kg ) . 

the value was calculated as force ( % ) divided by weight ( kg ) .
ir : 
 internal rotation ; er : external rotation ; add : adduction ; abd : abduction ) .
the value was calculated as torque ( nm / kg ) divided by weight ( kg ) . 

the value was calculated as force ( % ) divided by weight ( kg ) . 

ir : internal rotation ; er : external rotation ; add : adduction ; abd : 
 abduction the value was calculated as torque ( nm / kg ) divided by weight ( kg ) . 

the value was calculated as force ( % ) divided by weight ( kg ) .
ir : 
 internal rotation ; er : external rotation ; add : adduction ; abd : abduction
in the first part of this study , a three - dimensional motion analysis of the knee during 
 side - hopping motion was performed to investigate how the knee is positioned and loaded . 

consequently , significant differences in kinematic and kinetic parameters were noted between 
 motions during ltdp and mtdp . during ltdp ,
a significantly larger abduction angle was 
 observed , which corresponded to primary knee kinematics at the time of non - contact acl 
 injury11,12,13 .
indeed , dynamic knee 
 valgus observed in the frontal plane during common athletic screening tasks is regarded as a 
 motion pattern associated with a high risk for knee injuries31 .
in contrast , the load applied to the knee during mtdp was 
 generally smaller than that applied during ltdp , and kinematically the knee was 
 characterized by a larger external rotation angle in positioning . in the analysis of hip / knee interaction , internal rotation rom of the hip
was shown to 
 influence knee kinematics and kinetics during ltdp in side hop motion .
side hop motion 
 involves complex trunk and limb motions , and thus the evaluation should include motion 
 components other than the knee .
a preliminary study showed that quadriceps and hamstrings 
 strength had little correlation with side hop performance32 .
previous relevant studies reported a negative correlation between 
 extensor and abductor strengths of the hip and valgus angle of the knee during various 
 tasks16,17,18 , 33 .
the present study has shown that hip rom can be a factor that 
 influences knee motion during side hop .
the hop test has been used for assessment of patients following acl reconstruction to 
 evaluate postoperative functional recovery and the risk for reinjures .
patients who undergo 
 acl reconstruction exhibit varying degrees of functional limitations4 . during postoperative rehabilitation , accomplishment of an 
 effective training protocol based on accurate evaluation of functional recovery is essential 
 to enable uneventful return to sports activity .
proposed that strength 
 training with correction of faulty movement biomechanics and balance training are the most 
 effective methods for preventing acl injuries34 .
the single - leg squatting test has been tested for efficacy and 
 reliability in evaluation of hip function35 , 36 .
the hop test can be regarded as a dynamic 
 single - leg squatting test , although a small body of work is available in lending support to 
 the test performance . to date , 4 types of hop tests have been reported in literature : 
 single - leg , cross - over , triple , and 6  m timed hop tests4 , 37 .
previous studies on 
 these tests have correlated the test results with knee function measured with only a 
 self - reported questionnaire37 . however
since there is a paucity of data available in 
 literature regarding motion characteristics during side hopping , the results of this study 
 can provide valuable information that can help understand three - dimensional knee motion and 
 hip / knee interaction during side hop motion . based on the results of this study ,
it is 
 proposed that internal rotation rom of the hip should be included as a factor for evaluation 
 in hop test results . for subjects with increased internal rotation rom of the hip ,
exercises 
 that strengthen the hip external rotator and abductor may effectively reduce the risk for 
 acl injury or re - injury after acl reconstruction .
first , the sample size was small , and the 
 obtained results are not robust enough to draw definitive conclusion . moreover ,
inclusion of 
 the results obtained from either bilateral or unilateral limbs among the subjects may result 
 in an uneven data distribution and bias in data acquisition .
second , the subject population 
 was limited to male rugby players whereas previous studies have shown gender differences in 
 dynamic knee joint motion during various functional performance tests5 , 8,9,10 , 17 , 18 .
third , this study did 
 not include the assessment of hip adduction / abduction rom , which would also be potential 
 factors related to the knee valgus motion . in future studies
, the assessment of hip 
 adduction / abduction rom should be included in the evaluation of the hip to clarify the 
 relationship of hip function and knee kinematics / kinetics during the hop test .
 [ purpose ] kinematic and kinetic characteristics of the limb during side - hopping and 
 hip / knee interaction during this motion have not been clarified . 
 the purposes of this 
 study were to examine the biomechanical parameters of the knee during side hop and analyze 
 its relationship with clinical measurements of hip function . 
 [ subjects and methods ] eleven 
 male college rugby players were included . 
 a three - dimensional motion analysis system was 
 used to assess motion characteristics of the knee during side hop . 
 in addition , hip range 
 of motion and muscle strength were evaluated . 
 subsequently , the relationship between knee 
 motion and the clinical parameters of the hip was analyzed . 
 [ results ] in the lateral 
 touchdown phase , the knee was positioned in an abducted and externally rotated position , 
 and increasing abduction moment was applied to the knee . 
 an analysis of the interaction 
 between knee motion and hip function showed that range of motion for hip internal rotation 
 was significantly correlated with external rotation angle and external rotation / abduction 
 moments of the knee during the lateral touchdown phase . 
 [ conclusion ] range of motion for 
 hip internal rotation should be taken into consideration for identifying the biomechanical 
 characteristics in the side hop test results . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION