in sports like basketball and soccer , fatigue inevitably occurs during a game or practice 
 session .
certain lower extremity injuries , such as non - contact anterior cruciate ligament 
 ( acl ) injuries , have a higher risk of occurrence in the final phase of games , e.g. , between 
 76 and 90  min of a soccer game1,2,3 . altered movement 
 patterns caused by fatigue may explain non - contact acl injuries , as well as other injuries 
 that occur during sporting activities .
in contrast , to explain its effect on the lower extremities , evidence indicates that 
 fatigue induces malalignment of the lower extremities and knee joints stability during 
 sporting activities7,8,9 . furthermore , while athletes 
 run in fatigue conditions , fatigue causes an increase in impact acceleration , which is 
 regarded as the rapid deceleration of the tibia , during landings10 .
moreover , lower extremity angles and the ground reaction 
 force during landings are affected by fatigue11 , 
 12 .
these findings indicate that 
 fatigue alters lower extremity landing strategies and the ability to attenuate impact during 
 landing .
thus , the influence of fatigue during landing , an important movement , requires 
 appropriate evaluation to consider measures to prevent lower extremity injuries .
therefore , most 
 clinicians use video analysis to evaluate lower extremity angles and alignments at specific 
 times13,14,15 . however , lower extremity 
 injuries may cause changes to angles and velocities over time , such as the direction change 
 phase in sidestepping movements or the landing phase of a jump16 , 17 .
it is also 
 known that knee joint angular velocity variation during the landing phase is higher in 
 females than in males18,19,20 and some studies 
 have measured knee angular velocity to evaluate capacity to attenuate impact , which may be 
 associated with knee injury risk6 , 18 , 19 .
accordingly , lower extremity angular velocity during landing 
 reflects the ability to attenuate impact because it results from rapid angular variations in 
 the lower extremities during landing .
therefore , lower extremity angles and angular 
 velocities should be evaluated to identify biomechanical factors associated with lower 
 extremity injuries .
evidence indicates that fatigue causes changes in lower extremity kinematics , including 
 angles and alignments during landing4 , 7 , 8 . 

fatigue may also alter lower extremity angular velocities and the ability to attenuate 
 impact during landing . in our previous study
, it has reported that fatigue might decrease 
 the ability to attenuate impact by increasing the peak angular velocity in the direction of 
 knee flexion during a single - leg jump landing21 .
however , no studies to date have addressed the angular velocity 
 variables of lower extremities , including hip and ankle joints , during fatigue conditions . 

thus , the purpose of this study was to evaluate the influence of fatigue on the lower 
 extremity alignments and angular velocities during landing from a single - leg drop vertical 
 jump .
the research hypothesis was that landings during fatigue condition decrease the 
 ability to attenuate impact in the lower extremity , i.e. , angular velocities in the lower 
 extremity would increase significantly in landings during fatigue condition compared with 
 non - fatigue condition .
thirty - four females ( age : 20.7  1.8  years old ; height : 159.9  5.6  cm ; weight : 52.7  
 5.9  kg ) without a history of orthopedic hip , knee and ankle surgery participated in this 
 study .
the dominant foot was right in 30 subjects and left in 4 subjects ; dominance was 
 determined as preferred side to kick a ball22 .
all participants granted written informed consent for 
 participation before testing and were randomly assigned to either the fatigue group ( n=17 ; 
 age : 21.2  2.1  years old ; height : 160.5  5.0  cm ; weight : 52.9  7.1  kg ) or control group 
 ( n=17 ; age : 20.2  1.4  years old ; height : 159.3  6.3  cm ; weight : 52.6  4.5  kg ) .
this study 
 followed the declaration of helsinki and was approved by the ethics committee at the saitama 
 medical university , saitama , japan ( m-54 ) .
a three - dimensional motion analysis system employing eight cameras ( vicon mx system , vicon 
 motion systems , oxford , uk ) was used to record lower extremity kinematic data and the center 
 of mass during single - leg drop vertical jumps .
kinematic data were sampled at 240  hz with a 
 16  hz low - pass filter and a fourth - order zero lag butterworth filter .
thirty - five reflective 
 markers were placed on specific anatomical landmarks according to the plug - in - gait full body 
 model , whisch is widely employed by researchers who use vicon23 .
two force plates ( msa-6 mini amp , amti , ma , usa ) recorded ground 
 reaction forces during landing , from each single - leg drop vertical jump , at a 1,200  hz 
 sampling rate .
they performed a 
 single - leg drop vertical jump on their dominant foot ( referred to as the  pre - trial  ) , which 
 involves a first landing after dropping from a 40  cm .
a single - leg drop vertical jump 
 consists of 1st landing after dropping down from a 40  cm box and then a second landing after 
 a maximum vertical jump rebounding from the drop . all participants were shown the testing 
 sequence by assistant researchers , and several practice trials were conducted to enable them 
 to correctly perform the required task .
subsequent trials were repeated until data from five 
 successful trials were achieved ; these were excluded if the person lost their balance during 
 the landing .
after the pre - trial , all participants were required to use a bike ergometer 
 ( combi , japan ) , with participants in the fatigue group pedaling at 100  w per minute for 5 
 minutes or until they exceeded 17 ( very hard ) on the borg scale24 , 25 .
a poster 
 showing the borg scale was placed in front of the bike ergometer , and level of fatigue was 
 recorded at 30 second intervals . in contrast , all participants in the control group pedaled 
 a bike ergometer without load ( less than 10  w per minute ) for 5 minutes , with fatigue level 
 was verified in the same way . after using the bike ergometer
, all participants repeated the 
 second single - leg drop vertical jump trial ( referred to as the  post - trial  ) according to 
 the pre - trial procedure .
landing from a single - leg drop vertical jump was defined as the period from initial ground 
 contact to takeoff during the first landing .
two force plates were used to determine initial 
 ground contact and takeoff , with the sampling rate set at 1,200  hz .
initial ground contact 
 was defined as the moment when the force plate data indicated that vertical ground reaction 
 force exceeded 10 n , whereas takeoff was defined as the moment when the force plate data 
 indicated that vertical ground reaction force was < 10 n. all outcome measures were 
 analyzed and computed as the average of three pre - trials and post - trials .
the angles and 
 angular velocities of hip flexion , knee flexion , and ankle dorsiflexion analyzed during all 
 first landings .
hip flexion , knee flexion , and ankle dorsiflexion angles were calculated as 
 positive values from filtered three - dimensional coordinate data ; these parameters were 
 defined as angular displacements from static anatomical positions . to decrease data 

fluctuation , hip flexion , as well as knee flexion , and ankle dorsiflexion angular 
 velocities , were calculated by differentiating respective mean angles over five frames using 
 a moving average .
peak hip flexion , knee flexion , and ankle dorsiflexion angles as well as 
 angular velocities were determined from each maximum value during the first landings of the 
 pre - trials and post - trial .
the vertical position of the center of mass was calculated from 
 filtered 3d coordinate data during all first landings ; to decrease data fluctuation , 
 vertical position of the center of mass was calculated by differentiating mean vertical 
 position of the center of mass over five frames using a moving average .
minimum vertical 
 position and vertical velocity , which were defined as the lowest position and peak velocity 
 in the direction of the ground , were determined from each minimum value during all first 
 landings .
the ratios ( % ) of all kinematic variables in the post - trials to those in the 
 pre - trials were calculated .
hip flexion , knee flexion , ankle dorsiflexion angle , and angular velocity at specific times 
 were analyzed the moment of the initial ground contact , 40 milliseconds after the initial 
 ground contact , and the peak vertical ground reaction .
the moment of 40 milliseconds after 
 the initial ground contact was chosen because some reports had shown acl strain reached its 
 peak value within approximately 40 milliseconds after initial ground contact26 , 27 .
data were analyzed using the spss software ( version 19.0 ) , and unpaired t - tests were used 
 to compare changes in lower - extremity kinematics and center of mass between the fatigue and 
 control groups .
ratios of the vertical position of the center of mass during the post - trials to each 
 variable during the pre - trials did not differ significantly between the fatigue group ( 99.5 
  2.4% ; pre - trial 831.3  40.1  mm , post - trial 827.3  47.2  mm ) and the control group ( 100.4 
  1.4% ; pre - trial 816.7  45.5  mm , post - trial 819.8  46.2  mm ) . ratios of the vertical 
 velocity of the center of mass during the post - trials to each variable during the pre - trials 
 did not differ between the fatigue group ( 100.8  5.5% ; pre - trial 1,966.8  228.9  mm / s , 
 post - trial 1,978.4  229.5  mm / s ) and the control group ( 99.0  4.2% ; pre - trial 1,969.0  
 175.0  mm / s , post - trial 1,945.7  161.9 
findings were similar for the ratios of 
 post - trial of the vertical position and velocity center of mass to their pre - trial 
 counterparts .
the ratios of hip flexion , knee flexion , ankle dorsiflexion angles , and angular velocities 
 during the post - trials to each variable during the pre - trials are shown in table 1table 1.peak angles and angular velocities of lower extremity , and ratios of each 
 variable in the post - trials to those in the pre - trialsfatigue group ( n=17)control group ( n=17)pre - trialpost - trial the ratio ( % ) pre - trialpost - trial the ratio ( % ) hip flexionpeak angle41.1  12.641.8  9.341.2  13.3100.1  6.740.6  10.396.6  8.6peak angular velocity216.7  44.6238.9  51.8231.6  42.5107.8  9.7 * 238.4  56.199.6  9.2knee flexionpeak angle60.8  8.260.0  9.960.6  8.199.8  4.960.1  8.7100.7  6.2peak angular velocity419.2  53.4450.6  57.6443.5  52.4106.2  7.3 * 452.2  49.9100.8
 6.6ankle dorsiflexionpeak angle30.1  6.929.2  4.930.5  7.2101.2  5.931.0  4.7107.0  13.5peak angular velocity674.6  22.3715.3  42.0655.8  88.898.0 
7.6692.0  142.297.2  9.8the ratios ( % ) of each kinematic variable in the post - trials to those in 
 the pre - trials were presented alongside .
angles and angular velocities of hip , knee flexion , and ankle 
 dorsiflexion are shown in each ( deg ) and ( deg / s ) .
* significant difference between the ratios of fatigue and control groups 
 ( p<0.05 ) .. the ratio of hip flexion angular velocity during the post - trials to that 
 during the pre - trials increased significantly in the fatigue group .
furthermore , the ratio 
 of knee flexion angular velocity during the post - trials to that during the pre - trial also 
 increased significantly in the fatigue group ( p<0.05 ) ; however , the ratio of ankle 
 dorsiflexion angular velocity did not differ between groups during either set of trials . in 
 contrast ,
peak hip flexion , as well as knee flexion and ankle dorsiflexion angles did not 
 alter after the fatigue protocol ; thus , hip and knee flexion angular velocities increased 
 significantly in the fatigue group compared to the control group , whereas all lower 
 extremity joint angles were unchanged .
the ratios ( % ) of each kinematic variable in the post - trials to those in 
 the pre - trials were presented alongside .
angles and angular velocities of hip , knee flexion , and ankle 
 dorsiflexion are shown in each ( deg ) and ( deg / s ) . * significant difference between the ratios of fatigue and control groups 
 ( p<0.05 ) . the ratios of hip flexion , knee flexion , ankle dorsiflexion angles , and angular velocities 
 during the post - trials to each variable during the pre - trials at the moment of initial 
 ground contact , 40 milliseconds after the initial ground contact , and the peak ground 
 reaction force , are shown in tables 2table 2.angles of lower extremity and ratios of each variable in the post - trials to those 
 in the pre - trials at specific timesfatigue group ( n=17)control group ( n=17)pre - trial ( deg)post - trial ( deg)the ratio ( % ) pre - trial ( deg)post - trial ( deg)the ratio ( % ) initial ground contacthip flexion angle21.7  8.421.4  6.421.0  8.297.8  16.121.0  6.799.2  12.2knee flexion angle18.2  6.318.5  7.117.1  6.094.5  11.818.4  7.4101.0
 16.2ankle dorsiflexion angle16.3  6.115.1  4.915.1  5.893.5  15.213.3  4.390.9  20.140 ms after initial ground contacthip flexion angle25.8  9.025.8  7.025.3  8.899.2  13.125.3  7.4109.1
 7.4ankle dorsiflexion angle2.7  5.45.4  3.13.5  5.336.1  409.66.3  4.1130.9  75.9the moment of peak ground reaction 
 forcehip flexion angle30.5  8.930.6  7.529.5  9.496.1  7.229.1  7.795.3
5.194.0  23.7the ratios ( % ) of each kinematic variable in the post - trials to those in 
 the pre - trials were presented alongside and 3table 3.angular velocities of lower extremity and ratios of each variable in the 
 post - trials to those in the pre - trials at specific timesfatigue group ( n=17)control group ( n=17)pre - trial ( deg / s)post - trial ( deg / s)the ratio ( % ) pre - trial ( deg / s)post - trial ( deg / s)the ratio ( % ) initial ground contacthip flexion angular velocity95.8  55.9104.6  67.398.0  54.299.7  37.099.8  56.4118.4
 13.4ankle dorsiflexion angular velocity600.4  136.1672.5  139.2589.8  106.499.2  6.6642.4  162.195.7  13.140 ms after initial ground contacthip flexion angular velocity186.7  47.8205.0  62.6201.0  48.0109.1  37.0 * 205.9 
61.8100.8  8.2knee flexion angular velocity408.3  56.3429.0  65.3425.9  62.1104.4  7.0434.1  57.6101.7  6.3ankle dorsiflexion angular velocity601.9  102.2600.6  99.5583.9  75.197.7  7.5579.3  65.097.5  9.3the moment of peak ground reaction 
 forcehip flexion angular velocity207.9  47.2216.9  63.9228.1  41.8111.4  12.7233.6  53.6114.0
 39.6knee flexion angular velocity389.8  59.4387.8  77.6427.7  55.5110.6  10.7432.0  47.9115.5 
 50.5the ratios ( % ) of each kinematic variable in the post - trials to those in 
 the pre - trials were presented alongside . * significant difference between the ratios of fatigue and control groups 
 ( p<0.05 ) .
first , results show that angles , and angular velocities of lower extremity 
 joints upon initial ground contact were not altered by the fatigue protocol .
second , 40 
 milliseconds after the initial ground contact , the ratio of hip flexion angular velocity 
 during the post - trials to each variable during the pre - trials increased significantly in the 
 fatigue group in comparison with the control group ( p<0.05 ) , whereas knee flexion and 
 ankle dorsiflexion angular velocities were unchanged following the fatigue protocol . 
 finally , at the peak vertical ground reaction force , lower extremity joint angles and 
 angular velocities were also unchanged following the fatigue protocol . the ratios ( % ) of each kinematic variable in the post - trials to those in 
 the pre - trials
were presented alongside the ratios ( % ) of each kinematic variable in the post - trials to those in 
 the pre - trials were presented alongside . * significant difference between the ratios of fatigue and control groups 
 ( p<0.05 )
the purpose of this study was to reveal influences of fatigue on the lower extremity 
 alignments , and angular velocities , during landing from a single - leg drop vertical jump .
one 
 of the key findings of this study is that it is important to select appropriate evaluation 
 parameters and timing during landing for assessing effects of fatigue .
lower extremity kinematics and ground reaction force during landings alter under fatigue 
 conditions5 , 6 , 11 , and reveal hip or knee 
 angles , moments , and alignments of lower extremities during landing during fatigue 
 conditions .
moreover , modification of these parameters by fatigue may a biomechanical factor 
 leading to lower extremity injuries , including acl injuries .
although evidence indicates 
 that fatigue alters lower extremity kinematics , and that fatigue is a risk factor for 
 incurring such injuries , most studies measured hip and knee angles6 , 11 , 19 , or moments5 , during landings . however , lower extremity injuries , including acl 
 injuries , may change lower extremity angles or velocities over time , including the phase of 
 changing direction in sidestepping , or landing from a jump .
furthermore , rapid changes in 
 lower extremity alignment occur frequently in many situations during sporting activities , 
 and these findings indicate that measuring angle and angular velocity during fatigue is 
 useful for preventing lower extremity injuries .
the results of this study show that peak hip flexion and knee flexion angular velocities 
 increased significantly after the fatigue protocol , suggesting that fatigue decreased 
 capacity to perform deceleration movements in the hip and knee joints during landings from 
 jumps .
other investigators have suggested that knee kinematics play an important role in 
 attenuating impact during running or jump landings6 , 
 28 , 29 ; indeed , these results , indicate that fatigue may induce the knee 
 joint to decrease impact attenuation during landings because knee flexion angular velocity 
 increased significantly after the fatigue protocol .
moreover , we showed that hip flexion 
 angular velocity increased significantly during landings after the fatigue protocol , and 
 that fatigue might induce the hip and knee joints to decrease impact attenuation during 
 landings .
in contrast , peak ankle dorsiflexion angular velocities did not change after the 
 fatigue protocol ; thus , on the basis of these findings , angular velocity of knee and hip 
 flexion during fatigue conditions should be analyzed to evaluate their capacity for impact 
 attenuation by the lower extremities during landings .
hip and knee angular velocities 
 altered by fatigue may be important parameters to measure to prevent knee injuries . in this study , we measured hip flexion , knee flexion , and ankle dorsiflexion angles as well 
 as angular velocities , at the moment of initial ground contact , 40 milliseconds after 
 initial ground contact , and peak ground reaction force .
the moment of initial ground contact 
 represents the initial phase when contraction of the lower extremity muscles occurs during 
 the landing phase .
furthermore , during the phase between initial ground contact and the peak 
 ground reaction force , the ground reaction force in the vertical direction rapidly increased 
 from the moment of initial ground contact during landings .
subsequently , it is likely that 
 the ground reaction force continues to decrease until the takeoff from a vertical jump . for 
 these reasons , fatigue may alter lower extremity kinematics upon initial ground contact with 
 increasing muscle contractions of the lower extremity and the vertical ground reaction 
 force . however , lower extremity kinematic values were unchanged by fatigue condition at the 
 moment of initial ground contact and peak ground reaction force .
used video analysis to show that most acl injuries occur approximately 
 2550 milliseconds after initial ground contact during landings17 , and koga et al .
have suggested that it is likely that the 
 majority of acl injuries occur within 40 milliseconds after initial ground contact30 .
indeed , other evidence shows that acl is 
 strained during the initial phase of landings , approximately 40 milliseconds after initial 
 ground contact , during landings26 , 27 . according to the results of this study , 

40 milliseconds after initial ground contact , hip flexion angular velocity increased 
 significantly after the fatigue protocol , whereas other kinematics variables were unchanged . 

thus , we conclude that fatigue does not affect knee kinematics , 40 milliseconds after 
 initial ground contact , the point at which most acl injuries are considered to occur . in 
 contract , hip flexion angular velocity was significantly affected by fatigue condition 40 
 milliseconds after initial ground contact .
evidence indicates that increased hip flexion 
 angle during landings is a biomechanical risk factor for acl injuries17 .
however , we were unable to demonstrate that fatigue 
 affect hip flexion angle or the position of the center of mass during landings , considering 
 that hip flexion angle and the position of the center of mass did not change after fatigue 
 in this study .
furthermore , acl injuries are caused by the combined movement of lower 
 extremity angles in the sagittal plane and their variables in the frontal or horizontal 
 planes , including hip adduction or knee abduction angles15 .
therefore , it may be necessary to consider the influence of hip 
 flexion velocity on acl injuries by measuring knee abduction and hip adduction angles or 
 knee external or internal angles , or both .
this study shows that fatigue decreased the ability to attenuate impact by increasing 
 angular velocity in the direction of hip and knee flexion during a single - leg jump landing . 

moreover , 40 milliseconds after initial ground contact , hip flexion angular velocity 
 increased during fatigue , whereas knee and ankle kinematics remained unchanged .
these 
 results indicate a requirement to evaluate movement quality over time by measuring peak hip 
 and knee flexion angular velocities during the landing phase during fatigue conditions . in 
 addition
, it may be necessary to evaluate these parameters at each peak variable and at 
 specific times such as 40 milliseconds after initial ground contact during the landing phase 
 during fatigue conditions .
these findings further suggest that measuring hip and knee 
 angular velocity during landings might be useful for efforts to prevent knee injuries during 
 fatigue conditions .
the results of this study indicate that fatigue decreases the ability to attenuate impact 
 by increasing angular velocity in the direction of hip and knee flexion during single - leg 
 jump landing .
furthermore , it may be necessary to evaluate knee and hip angular velocities 
 at their peak values and a specific time such as 40 milliseconds after initial ground 
 contact , during the landing phase under fatigue conditions .
these findings suggest that 
 measuring hip and knee flexion angular velocities during fatigue conditions may serve as 
 important evaluation parameters to prevent knee injuries , including acl injuries .
 [ purpose ] fatigue alters lower extremity landing strategies and decreases the ability to 
 attenuate impact during landing . 
 the purpose of this study was to reveal the influence of 
 fatigue on dynamic alignment and joint angular velocities in the lower extremities during 
 a single leg landing . 
 [ subjects and methods ] the 34 female college students were randomly 
 assigned to either the fatigue or control group . 
 the fatigue group performed single - leg 
 drop vertical jumps before , and after , the fatigue protocol , which was performed using a 
 bike ergometer . 
 lower extremity kinematic data were acquired using a three - dimensional 
 motion analysis system . 
 the ratio of each variable ( % ) , for the pre - fatigue to 
 post - fatigue protocols , were calculated to compare differences between each group . 
 
 [ results ] peak hip and knee flexion angular velocities increased significantly in the 
 fatigue group compared with the control group . 
 furthermore , hip flexion angular velocity 
 increased significantly between each group at 40 milliseconds after initial ground 
 contact . 
 [ conclusion ] fatigue reduced the ability to attenuate impact by increasing 
 angular velocities in the direction of hip and knee flexion during landings . 
 these 
 findings indicate a requirement to evaluate movement quality over time by measuring hip 
 and knee flexion angular velocities in landings during fatigue conditions . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION