in japan , elderly individuals defined as people aged 65  years or more are known to account 
 for more than 25% of the population . as people age , there is a concomitant decrease in 
 muscle strength1 .
this age - related change 
 affects their ability to walk2 . as the 
 ability to walk of the elderly decreases
, there is an associated decrease in their balance 
 ability3 , 4 , and independent daily living activities5 , and they show an increased risk of fall6 . therefore it is necessary to maintain the 
 ability to walk of the elderly .
ground reaction force is an external force involved in walking , which affects the 
 acceleration of the body s center of mass7 .
the vertical ground reaction force is often measured in gait 
 analysis , and is regarded as a representative measure of walking8 .
the two peaks of the vertical ground reaction force during 
 the early stance and late stance phases reflect the support of the body s center of mass . 

the minimum value in the mid - stance indicates that the vertical force applied to the ground 
 decreases , and thus , the reaction force from the ground to the body also decreases9 .
the profile of the vertical ground reaction force correlates with many parameters of gait 
 and functional performance , such as walking speed10 , the timed up and go test time and the functional reach 
 distance11 in the elderly .
it is also 
 known that the vertical ground reaction force during walking is affected by aging .
elderly 
 people exhibit lower first peak and second peak forces , and a higher minimum value at 
 mid - stance than young people12 .
the 
 vertical ground reaction force provides correlative information about the ability to walk of 
 elderly people .
several studies have reported that elderly individuals exhibit age - related 
 alteration in their kinetic profiles of walking .
hip and knee joint moments of the elderly 
 are significantly lower than those of young people13 . moreover , devita and hortobagyi14 reported that elderly people showed different distributions of 
 joint moments and powers compared to young individuals .
support during the stance phase is 
 achieved by a net extensor pattern of moments at the ankle , knee , and hip joint15 .
age - related alterations in lower 
 extremity joint moment suggest there is a difference in the support strategy between the 
 elderly and the young . to date , it is unclear what causes the difference in the profile of the ground reaction 
 force between the elderly and the young .
the results of our previous study16 suggest that both men and women make use 
 of different joint moments for the generation of antero - posterior ground reaction forces as 
 they age , showing an age - related alteration in the motor pattern used to generate the 
 antero - posterior ground reaction force .
moreover , if elderly people exhibit an age - related 
 alteration in support moment of the lower extremity , changes in joint moment may affect the 
 magnitude of the vertical ground reaction force . by analyzing the relationships between 
 vertical ground reaction force and lower extremity joint moments , it would be possible to 
 clarify the difference in the support strategies of elderly and young people .
therefore the 
 purpose of this study was to examine the relationships between the vertical ground reaction 
 force and the hip , knee , and ankle joint moments during walking by the elderly and young 
 individuals .
we hypothesized that the relation between the joint moments and vertical ground 
 reaction force differs between the elderly and the young , similar to our previous study16 , and that there is an age - related change 
 in the support strategy use for walking .
forty community - dwelling elderly people , 65  years old or older ( mean age , 70.1  years ; age 
 range , 6580  years ) , and 40 young people , aged 20 to 29  years ( mean age , 23.2  years ) , 
 participated in this study . there were 20 males and 20 females in each group .
the exclusion criteria included neurologic disorders , osteoarthritis , 
 rheumatic arthritis , joint pain affecting walking , and a history of surgery to the lower 
 extremities or spine .
all procedures were approved by the hiroshima international university 
 human research ethics committee and all participants gave their written , informed consent 
 prior to enrollment . in the walking trials , all the participants walked barefoot to the end of a 7  m walkway at 
 a self - selected preferred walking speed .
a 3d 
 motion analysis system with 8 infrared cameras ( vicon mx ; vicon motion systems ; oxford , uk ) 
 and 8 force plates ( amti ; watertown , ma , usa ) was used to record kinematic and kinetic data 
 at sampling frequencies of 100 and 1,000  hz , respectively .
the force plate layout was 
 designed to measure the ground reaction force of each limb using four force plates arranged 
 in the longitudinal direction .
participants were instructed to walk placing their foot on 
 the left or right force plate , avoiding stepping on two force plates simultaneously .
a total of 30 reflective markers were placed bilaterally , over the following landmarks of 
 each participant : acromion process , olecranon , styloid process of the radius , anterior 
 superior iliac spine , posterior superior iliac spine , greater trochanter , medial and lateral 
 femoral condyles , mid - point between the greater trochanter and the lateral femoral condyles , 
 medial and lateral malleoli , mid - point between the lateral knee joint line and the lateral 
 malleolus , head of the first and fifth metatarsal , and the calcaneal tuberosity .
these 
 anatomical markers were used to construct anatomical coordinate systems for the pelvis , 
 thigh , shank , and foot segments .
the coordinates of each joint center were calculated following the description of a 
 previous study17 .
a 7-link segmental model was developed to calculate the kinematic and kinetic variables of 
 the hip , knee , and ankle joints using the inverse dynamics technique of davis et al.18 and vaughan et al19 .
anthropometric parameters for mass , position of the 
 center of mass , and moment of inertia for the segments were obtained from the report by 
 okada et al20 .
we calculated the hip , 
 knee , and ankle joint moments using a local coordinate system with the origin at the joint 
 center . in this study , each joint moment was normalized to the subject s body mass 
 ( nm / kg ) .
walking speed ( m / s ) was calculated using the center of gravity ( cog ) of the whole body , and 
 averaged over a 3-s data collection period .
stride length ( m ) was measured as the 
 antero - posterior distance between the left calcaneal tuberosity marker at one heel contact 
 and at the next heel contact .
initial contact was assumed to occur when the vertical reaction force exceeded 10 n , and 
 toe off was assumed to occur in the first frame following the initial contact when the 
 vertical reaction force fell below 10 n. to account for different builds , ground reaction 
 force was normalized to each subject s body mass ( n / kg ) .
first and second peaks of vertical 
 ground reaction force , and the minimum value between the two peaks were extracted from the 
 ground reaction force profile ( fig
1.graph representation of the vertical ground reaction forces and the joint moment of 
 the hip , knee , and ankle during a gait cycle ) .
peak values of each joint moment during the stance phase were extracted from the 
 joint moment profile ( fig .
graph representation of the vertical ground reaction forces and the joint moment of 
 the hip , knee , and ankle during a gait cycle two - way analysis of variance was performed to examine the differences between the elderly 
 and the young and the interactions between age and gender , with age , body height , body 
 weight , body mass index ( bmi ) , and spatio - temporal parameters .
two - way analysis of 
 covariance was also performed to examine the difference between the elderly and the young , 
 and the interactions between age and gender with the vertical ground reaction force , and 
 peak values of each joint moment using walking speed as the covariate .
step - wise linear regression was performed to determine the joint moments that predicted the 
 amplitude of vertical ground reaction force in the elderly men , elderly women , young men , 
 and young women .
relationships among the peak values of the hip , knee , and ankle joint 
 moments during the early stance phase and the first peak force , and the minimum value of the 
 vertical ground reaction force , were investigated .
relationships among the peak values of 
 the hip , knee , and ankle joint moments during the late stance phase and the second peak 
 force of the vertical ground reaction force were also investigated .
the stepping - method 
 criteria were : an f - value of 0.05 for inclusion in the segmental model , 
 and 0.10 for removal from the model .
the baseline demographic characteristics of the study subjects are presented in table 1table 1.characteristics of the subjectvariableelderlyyoungmales ( n = 20)females ( n = 20)males ( n = 20)females ( n = 20)age ( years)69.45 ( 4.80)*70.70 ( 4.62)*22.80 ( 2.80)23.50 ( 2.70)height ( m)1.64 ( 0.07)*1.49 ( 0.04)*1.71 ( 0.07)1.57 ( 0.05)weight ( kg)62.28 ( 9.78)52.09 ( 6.20)64.04 ( 8.34)52.91 ( 9.92)bmi ( kg / m)23.06 ( 2.96)*23.25 ( 2.69)*21.74 ( 2.63)21.21 ( 3.30)value : mean ( standard deviation : sd ) . * : significant difference from the young , p 
 < 0.05 .  : bmi : body mass index .
the elderly participants had significantly shorter body height and higher bmi 
 than the younger study participants , but there was no significant difference in their body 
 masses .
value : mean ( standard deviation : sd ) . * : significant difference from the young , p 
 < 0.05 .
 : bmi : body mass index the spatio - temporal parameters and peak values of joint moment are presented in table 2table 2.spatio-temporal parameters and peak values of joint momentvariableelderlyyoungmales ( n = 20)females ( n = 20)males ( n = 20)females ( n = 20)walking speed ( m / s)1.20 ( 0.16)*1.15 ( 0.12)*1.26 ( 0.12)1.29 ( 0.10)stride length ( % body height)73.63 ( 6.79)*73.59 ( 5.48)*77.47 ( 5.64)81.68 ( 5.69)first peak force ( n / kg ) 10.57 ( 0.94)10.71 ( 1.06)10.96 ( 0.75)10.90 ( 0.79)minimum force ( n / kg)7.29 ( 0.58)7.52 ( 0.58)7.20 ( 0.58)7.00 ( 0.60)second peak force ( n / kg ) 10.43 ( 0.50)9.97 ( 0.61)*10.64 ( 0.52)10.74 ( 0.51)hipextension moment ( nm / kg ) 0.76 ( 0.31)0.58 ( 0.29)0.72 ( 0.19)0.64 ( 0.29)flexion moment ( nm / kg ) 0.67 ( 0.17)0.61 ( 0.17)0.71 ( 0.14)0.60 ( 0.13)kneeflexion moment during early stance ( nm / kg)0.23 ( 0.15)0.21 ( 0.11)0.08 ( 0.17)0.15 ( 0.11)extension moment during early stance ( nm / kg)0.58 ( 0.29)0.74 ( 0.26)0.71 ( 0.20)0.76 ( 0.27)flexion moment during terminal stance ( nm / kg)0.21 ( 0.09)0.10 ( 0.14)0.18 ( 0.12)0.19 ( 0.12)extension moment during terminal stance ( nm / kg)0.20 ( 0.07)*0.26 ( 0.12)*0.17 ( 0.08)0.17 ( 0.10)ankledorsiflexsion moment ( nm / kg)0.21 ( 0.10)0.24 ( 0.11)0.25 ( 0.08)0.21 ( 0.11)plantarflexion moment ( nm / kg)1.16 ( 0.22)*1.02 ( 0.16)*1.30 ( 0.11)1.22 ( 0.19)value : mean ( standard deviation : sd ) . * : significant difference between age groups , p 
 < 0.05 .  : significant difference between gender groups , p < 0.05 .
the elderly males and females had significantly slower walking speeds , with 
 shorter stride lengths than the young participants , but there was no significant different 
 in cadence between the two age groups .
the second peak of the vertical ground reaction force 
 exhibited a significant interaction between age and gender , and the elderly females had a 
 significantly lower second peak of vertical ground reaction force than the younger females . 

there was an approximately 7.5 percent difference between the elderly females and the young 
 females groups .
on the other hand , the extension moment during the terminal stance phase of 
 the knee was higher in the elderly participants .
there were approximately 14.5 and 30.0 
 percent differences between the elderly and the young groups of both males and females , 
 respectively .
although the flexion moment during the terminal stance phase of the knee 
 exhibited a significant interaction between age and gender , there was no significant 
 difference between the age groups , neither in malese nor in females .
value : mean ( standard deviation : sd ) . * : significant difference between age groups , p 
 < 0.05 .
 : vertical ground reaction force the results of step - wise linear regression analyses of vertical ground reaction force are 
 presented in table 3table 3.models of the vertical ground reaction force by forward step - wise regression 
 analysis.subjectindependent variableunstandardizedcoefficientstandardizedcoefficientadjusted rfirst peak forceelderly femalehip extension moment2.140.610.34young female knee extension moment during early stance1.350.470.22minimum value at mid - stanceyoung maleknee extension moment during early stance1.950.690.48elderly femaleknee extension moment during early stance1.310.510.26young femaleknee extension moment during early stance1.040.540.29second peak forceyoung maleknee extension moment during late stance2.670.450.21elderly female ankle plantarflexion moment2.220.570.33 * : p < 0.05 .  : vertical ground reaction force , and the joint moments related to the vertical ground reaction force are shown 
 in fig . 2fig .
2.the joint moments that can predict the amplitude of the vertical ground reaction 
 force in each group .
elderly males exhibited no statistically significant factors related to the vertical 
 ground reaction force . * : p < 0.05 .
 : vertical ground reaction force the joint moments that can predict the amplitude of the vertical ground reaction 
 force in each group for the young males and females , and elderly women groups , the knee extension moment during 
 the early stance phase was related to the minimum value of the vertical ground reaction 
 force at mid - stance . for young males ,
the knee extension moment during the late stance phase was related to the 
 second peak of the vertical ground reaction force .
on the other hand , for young females , the 
 knee extension moment during the early stance phase was related to the first peak of the 
 vertical ground reaction force .
in contrast , for the group of elderly females , the hip 
 extension moment was related to the first peak of the vertical ground reaction force , and 
 the ankle plantarflexor moment was related to the second peak of vertical ground reaction 
 force .
this study examined the relationships between the vertical ground reaction force and hip , 
 knee , and ankle joint moments during walking in young and elderly individuals .
the analyses 
 of this study were performed in consideration of gender as well as age .
this was done 
 because a previous report21 indicated 
 that there are significant gender differences in walking speed , stride length , kinematics 
 and kinetics . in this study
, some parameters exhibited a significant gender difference and 
 interaction between age and gender .
thus , in order to maximize the ability to determine the 
 effects of age on walking , the study subjects were divided by gender .
the first peak and the minimum value at mid - stance of the vertical ground reaction force 
 were not significantly different between the elderly and the young . on the other hand ,
the 
 second peak of the vertical ground reaction force of the elderly women was significantly 
 lower than that of the young women .
yamada and maie11 reported that elderly people have lower first peak and second peak 
 forces , and a higher minimum value at mid - stance than younger people . on the other hand , 

larish et al.22 reported that there was 
 no significant difference in the first peak force , but a significant difference in the 
 second peak of the vertical ground reaction force .
because the amplitude of the peak 
 vertical ground reaction force is affected by cadence rather than stride length23 , there may be no difference between the 
 elderly and young people in the first peak of the vertical ground reaction force .
both the 
 first and second peaks of the vertical ground reaction force correspond to the upward 
 acceleration of the body center of mass8 . 

the elderly females in this study had low upward acceleration of body center of mass at 
 push - off .
this may increase the load on the front limb when producing upward acceleration 
 during the double support phase of elderly females .
the plantar flexion moment of the ankle of the elderly was smaller by approximately 14.5 
 percent than that of the young people .
therefore , the plantarflexor muscles of the elderly 
 exhibited an age - related functional decline .
knee extension moment during early stance had a negative relationship with the minimum 
 value at mid - stance , and the knee extension moment during late stance had a positive 
 relation with the second peak of the vertical ground reaction force the young males .
the 
 knee extension moment during early stance had a positive relation with the first peak force , 
 and a negative relation with the minimum value at mid - stance of the ground reaction force of 
 the young females .
these results suggest that the vertical ground reaction force may be 
 controlled by knee extension in young people . in our previous study16 ,
the knee extension moment during late stance was 
 associated with the anterior component of the ground reaction force of young males , and the 
 knee joint moment was associated with the antero - posterior component of the ground reaction 
 force of young females .
the knee extension moment of young people is important for 
 decreasing the reaction force from the ground to the body at mid - stance , and increasing the 
 ground reaction force during weight acceptance and the push off phase .
it is necessary to 
 have muscle strength in order to increase the joint moment during walking .
brown et al.25 reported that there was a relationship 
 between quadriceps strength and walking ability , especially in young people rather than in 
 the elderly .
therefore knee joint function is important for support during walking by young 
 people . in the elderly females of this study , the extension moment of the hip had a positive 
 relationship with the first peak of the ground reaction force . in our previous study16 ,
the extension moment of the hip was 
 associated with the posterior component of ground reaction force in elderly females , and the 
 present result is consistent with the result of that previous study .
the elderly females had 
 a large hip extension moment , and their ground reaction force during the early stance phase 
 was relatively high . although the knee joint moment was associated with the first peak of 
 the vertical ground reaction force in the young females , the hip joint moment was associated 
 with the first peak of the vertical ground reaction force in the elderly females .
the 
 elderly and young females in this study made use of different joints to control of the 
 vertical ground reaction force .
these results are mostly consistent with our hypothesis that 
 there is an age - related change in the support strategy of walking .
support for the body is 
 provided by a combination of knee extensors and hip extensors during the early stance 
 phase26 .
while walking , moreover , elderly people 
 had more forward trunk lean , and a large hip flexion angle during early stance14 . because the function of the muscles 
 surrounding the knee joint reduces with aging in addition to age - related alternation in the 
 attitude of walking ,
weight acceptance during the early stance phase of the elderly is 
 accomplished by the hip joint . for weight acceptance in the initial stance , therefore , it is 
 necessary for elderly females to develop a hip extensor .
the extension moment during the early stance phase of the knee had a negative relationship 
 with the minimum value of the ground reaction force at mid - stance in the elderly females . 

this result was similar to the one found for the young females in this study . in order to 
 reduce the mechanical load exerted on the body at mid - stance
, it may be important for 
 elderly females to strengthen the quadriceps muscles , and generate a knee extension moment 
 during early stance .
moreover the plantarflexion moment of the ankle had a positive 
 relationship with the second peak of the ground reaction force of the elderly females . 

winter15 reported that the ankle 
 plantarflexion moment was the major contributor to support during push - off .
the rate of 
 decline in maximal voluntary isometric force with aging is greatest in the plantar 
 flexors28 .
it is probables that because 
 the elderly females had a lower plantarflexion moment of the ankle than that of young 
 females , the second peak of the vertical ground reaction force was smaller in the elderly 
 females in this study .
a previous study reported the importance of plantarflexor muscles of 
 the ankle during walking by the elderly29 .
therefore it is important to strengthen the plantarflexor muscles of 
 the ankle of elderly females in order to enhance support during late stance .
in contrast , no factors related to the first peak and second peak forces and the minimum 
 value of the vertical ground reaction force of the elderly males were found .
it has been 
 reported that the contribution of joint moment to the antero - posterior ground reaction force 
 is small in elderly males16 . because 
 individual variation was greater among the elderly males , factors related to the vertical 
 ground reaction force were found not to be significant , similar to the antero - posterior 
 ground reaction force .
the changes in physical function with aging are not equal in males 
 and females30 , and physical 
 characteristics , lifestyle , and social background are different between males and females . 

therefore there are gender differences in the age - related changes of walking 
 characteristics , and elderly males make use of various joint moment patterns to the control 
 of the vertical ground reaction force .
the relationship between the vertical ground 
 reaction force and lower extremity joint moments during walking in both elderly and young 
 subjects were investigated .
however , step - wise linear regression analysis did not find a 
 factor related to the vertical ground reaction force in several models , because we analyzed 
 the data divided by age and gender .
using mechanical analysis using equations of motion , 
 kepple et al.26 showed that support of 
 the body was provided by the ankle plantarflexor moment during single limb support , and by a 
 combination of ankle plantarflexors , knee extensors , and hip extensors during double - limb 
 support in younger people . therefore in order to clarify the age - related changes in walking 
 mechanism of elderly people , it will be necessary to perform detailed analyses of dynamic 
 equations of motion .
furthermore , we were unable to identify the causative factor of 
 age - related changes in the support strategy of walking .
further studies are required to 
 determine the causes of changes in strategy with aging .
 [ purpose ] this study examined the relationships between joint moment and the control of 
 the vertical ground reaction force during walking in the elderly and young male and female 
 individuals . [ subjects and methods ] forty elderly people , 65  years old or older ( 20 males 
 and 20 females ) , and 40 young people , 20 to 29  years old ( 20 males and 20 females ) , 
 participated in this study . 
 joint moment and vertical ground reaction force during walking 
 were obtained using a 3d motion analysis system and force plates . 
 stepwise linear 
 regression analysis determined the joint moments that predict the amplitude of the 
 vertical ground reaction force . 
 [ results ] knee extension moment was related to the 
 vertical ground reaction force in the young males and females . on the other hand , in the 
 elderly females , hip , ankle , and knee joint moments 
 were related to the first peak and 
 second peak forces , and the minimum value of vertical ground reaction force , respectively . 
 
 [ conclusion ] our results suggest that the young males and females make use of the knee 
 joint moment to control of the vertical ground reaction force . 
 there were differences 
 between the elderly and the young females with regard to the joints used for the control 
 of the vertical ground reaction force . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION