the time - series waveforms of mechanical energy generation , absorption , and transfer through 
 the joints indicate how movements are produced and controlled .
previous studies have used 
 these waveforms to evaluate and describe the efficiency of human movement1,2,3 .
these waveforms can be calculated by 
 combining joint reaction forces and joint moments with segmental and joint kinematics4 .
above all , it is important to investigate 
 the work that is done with joint moments , because the energy generated by this work is used 
 to move body segments .
williams and cavanagh stated that mechanical energy flow is generated 
 in intersegments , and the energy is stored in muscles , thereby reducing the generation of 
 physiological energy by the muscle and enhancing the efficiency of the movement5 .
previous studies have suggested that age - related physical deformities have a variety of 
 influences on walking efficiency6,7,8 . 

a distinctive feature of kyphosis is excessive hip and knee 
 flexion in a standing position .
this compensation may be necessary to maintain the 
 gravitational line within the base of support10 , but increases the load on the lower extremities during standing 
 and walking according to kinematic and kinetic studies11,12,13 .
however , there is little insight into the strategy used for gait 
 with this compensation . to our knowledge , there are no studies that have investigated the influence of kyphosis 
 posture on mechanical energy flow in the lower extremities during gait .
the influence of 
 kyphosis on the lower extremities during gait should be investigated to prevent secondary 
 impairments .
therefore , in this study , these influences were examined in healthy subjects as 
 a preliminary step towards understanding the biomechanical demands on the lower extremities 
 during the gait of elderly people with kyphosis .
eight healthy young males ( mean age , 21.8  1.3  years , mean height , 170.5  6.8  cm , and 
 mean weight , 60.2  6.8  kg ) participated in our study . the subjects had no self - reported 
 musculoskeletal and/or neurological disorders that affected their gait .
this study was 
 approved by the institutional review board of hiroshima university , and each subject was 
 informed of the objectives of the study and provided written informed consent prior to 
 participation .
the subjects walked along a 10  m walkway at a self - selected gait speed under 2 conditions : 
 a normal gait , in which no brace was attached ( condition n ) , and gait with trunk flexion 
 formed with a brace which was attached to simulate spinal curvature ( condition tf ) .
trials 
 under each condition were conducted in a random order , and the measurement of each task was 
 repeated 5 times . to quantify the degree of spinal curvature as a scale for kyphosis ( k ) , 
 the method of milne and lauder was used14 
 ( fig .
1.the method of quantifying the degree of spinal curvature as a scale for kyphosisc7 : spinous process of seventh cervical vertebra ; th7 : spinous process of seventh 
 thoracic vertebra ; l4 : spinous process of fourth lumber vertebra ; k : the degree of 
 spinal curvature ) .
under condition tf , spinal curvature was simulated by attaching a brace to increase 
 the kyphosis index to greater than 13 during standing14 .
the method of quantifying the degree of spinal curvature as a scale for kyphosis c7 : spinous process of seventh cervical vertebra ; th7 : spinous process of seventh 
 thoracic vertebra ; l4 : spinous process of fourth lumber vertebra ; k : the degree of 
 spinal curvature fourteen - millimeter diameter reflecting markers were attached to the following 36 bilateral 
 anatomical landmarks : the temples , lateral ends of the superior nuchal line , tragus , 
 acromion , olecranon , styloid process of the ulna , inferior edge of the last rib , great 
 trochanter , lateral and medial epicondyles of the femur , lateral and medial condyles of the 
 tibia , lateral and medial malleoli , first and fifth metatarsal heads , and calcaneal 
 tuberosity .
the 3-dimensional ( 3d ) coordinates of these points were recorded using a 3d 
 motion analysis system ( vicon motion system , oxford , uk ) with 6 cameras at a sampling rate 
 of 100  hz . at the same time , ground reaction force was recorded using 8 force plates ( tec 
 gihan , uji , japan ) at a sampling frequency of 1,000  hz . the kinematic data were filtered with a cut - off frequency of 6  hz , and the kinetic data 
 were filtered with a frequency of 20  hz using a fourth - order low - pass butterworth filter . an 
 eight - part rigid - body linked model was constructed consisting of the thorax , pelvis , thighs , 
 shanks , and feet , using the recorded marker coordinates .
spatial coordinates were described 
 using a right - handed coordinate system , and the medio - lateral direction was labeled the 
 ml - axis , the anteroposterior direction as the ap - axis , and the vertical direction as the 
 v - axis when standing .
the local coordinate system was similarly defined as the absolute 
 coordinate system with the ml - axis , ap - axis , and v - axis .
the com coordinates of the whole 
 body , the velocities of com , the joint and segment angle were calculated .
the initial contact of the left leg was defined as the time when the vertical ground 
 reaction force reached 10 n. the vertical coordinates of com were labeled 
 comv and the anteroposterior component of the velocity of 
 com was used as the gait velocity .
the stance phase of walking was defined as the time 
 between initial contact and toe - off of the left leg , and the time between the initial 
 contact of the left leg and the moment when comv reached its 
 maximum was defined as the com rising phase .
the means of the backward leaning angles of the thorax and thigh in the standing position , 
 the means of gait velocity during one walking cycle and the displacement of 
 comv ( comv , the value of the 
 maximum over the minimum ) , and the motion time during the com rising phase were calculated 
 the data of the markers 3d coordinates .
the joint moments were calculated using the data of 
 the marker s 3d coordinates and the ground reaction force data during the com rising phase 
 by integrating the joints moments with respect to time .
the rate of work acting on the joint 
 ( power ) by the joint moment ( i.e. , joint power ) was calculated as 
 follows : p(j ) = m(j)(j ) ( eq .
a)where 
 p(j ) is the mechanical power acting on the joint , m(j ) 
 is the joint moment acting on the joint , and (j ) is the angular velocity of the joint . 

similarly , the power generated by the joint moment acting on the segment at the joint was 
 calculated as follows : p(s ) = m(s)(s ) ( eq . 

b)where p(s ) is the mechanical power transferred to or 
 from segment at its joint , m(s ) is the joint moment acting on the segment 
 at the joint and (s ) is the angular velocity of the segment .
a positive power indicates 
 mechanical energy generated by the muscle of the joint , while a negative power indicates 
 mechanical energy absorbed by the muscle of the joint15,16,17 .
additionally , in terms of joint power , a positive or a negative 
 value indicates whether the muscles of a joint were contracting concentrically or 
 eccentrically , respectively . the mechanical work done by the joint moment during the com 
 rising phase was calculated by integrating the power at the joints and at the endpoints of 
 adjoining segments with respect to time .
the positional relationship with the pelvis was 
 used to determine the expression of the proximal or distal portions in segments .
statistical analyses were performed using statistical software spss 22.0 for windows ( ibm , 
 tokyo , japan ) .
the shapiro - wilk test was used to test the normality of the data . the paired 
 t - test or
wilcoxon signed rank tests in cases of non - normality , to test statistical 
 significance of differences between the conditions .
the thorax backward leaning angle under condition tf was significantly lower than under 
 condition n , and the angle of the thigh under condition tf was significantly higher than 
 under condition n during standing ( p<0.01 , table 
 1table 1.spatiotemporal data of the two conditionscondition ncondition tfcomv [ % bh]1.76  0.271.48  0.38*com - velap [ m / s]1.15  0.101.06  0.11*motion time [ s]0.28 ( 0.270.31)0.32 ( 0.280.34)backward leaning angles [ deg]thorax5.03  5.1828.37  27.12**thigh9.29 
2.910.91  7.10**joint moments [ nms / kg]hip extension0.02 ( 0.010.04 ) 0.14 ( 0.020.24)*knee extension0.07  0.020.08  0.04mean  standard deviation , * p<0.05 , * * p<0.01 .
comv ( the displacement of the vertical coordinates of 
 com ) and com - velap ( gait speed ) during one walking cycle , 
 motion time and joints moments during the com rising phase , and backward leaning 
 angles in the standing position were calculated ) .
comv during the walking cycle under condition 
 tf was significantly lower than that under condition n ( p<0.05 ) .
the mean of gait 
 velocity during the walking cycle under condition tf was significantly lower than that under 
 condition n ( p<0.05 ) .
there was no significant difference in the motion time of the com 
 rising phase between the two conditions ( table 
 1 ) .
the moment of the hip extension during the com rising phase under condition tf 
 was significantly higher than under condition n ( p<0.05 ) .
there were no significant 
 differences in the moments of the knee extension during the com rising phase between the two 
 conditions ( table 1 ) .
the positive mechanical 
 work in the proximal portion of the thigh during the com rising phase under condition tf was 
 significantly higher than that under condition n ( p<0.05 ) .
the positive mechanical work 
 of the knee joint during the com rising phase under condition tf was significantly lower 
 than that under condition n ( p<0.05 ) .
there were no significant differences in the 
 positive mechanical work in the distal portion of the thigh , the negative mechanical work in 
 the proximal portion of the shank , or the negative mechanical work of the knee joint of the 
 com rising phase between the two conditions ( table 
 2table 2.mechanical work performed under the two conditionscondition ncondition tfmechanical works [ j / kg]thigh proximal pp0.02 ( 0.010.05)0.13 ( 0.020.21)*thigh distal pp0.08 ( 0.050.13)0.09 ( 0.040.15)shank proximal np0.10  0.050.13  0.09knee pp0.02  0.010.01  0.01*knee np0.04  0.020.06  0.03mean  standard deviation or median ( iqr ) , * p<0.05 . thigh proximal pp : positive power in the proximal portion 
 of the thigh
. thigh distal pp : positive power in the distal portion of 
 the thigh .
2.the time - series waveforms of mechanical energy in the knee during gait lines 
 represent mean powers normalized to body mass ( w / kg ) .
no significant difference was 
 seen in the mechanical energy flow of the knee between the two conditions ( the power 
 of the proximal portion of the shank and the distal portion of the thigh did not 
 change ) ; however , the positive mechanical work of the knee joint under condition tf 
 was significantly lower than that under condition n during the com rising phase 
 ( shaded area ) . ) .
comv ( the displacement of the vertical coordinates of 
 com ) and com - velap ( gait speed ) during one walking cycle , 
 motion time and joints moments during the com rising phase , and backward leaning 
 angles in the standing position were calculated mean  standard deviation or median ( iqr ) , * p<0.05 .
knee np : negative power in the knee joint the time - series waveforms of mechanical energy in the knee during gait lines 
 represent mean powers normalized to body mass ( w / kg ) .
no significant difference was 
 seen in the mechanical energy flow of the knee between the two conditions ( the power 
 of the proximal portion of the shank and the distal portion of the thigh did not 
 change ) ; however , the positive mechanical work of the knee joint under condition tf 
 was significantly lower than that under condition n during the com rising phase 
 ( shaded area ) .
a human efficiently moves the com forward and walks by acquiring potential energy during 
 the com rising phase and converting it into kinetic energy18 , 19 . in this study , 

comv and the mean of gait velocity were significantly lower 
 under condition tf than those under condition n. potential and kinetic energies are 
 proportional to the position and square of velocity , respectively ; thus , our results show 
 that the amount of potential energy that can be converted to kinetic energy was reduced 
 under condition tf , and , as a result , the gait speed was also reduced . to determine the 
 cause , change in the activity of the lower extremities
was investigated focusing on the 
 phase when com was moving upward and forward . here , how mechanical energy flow was generated in the knee joint under condition n during 
 the com rising phase is discussed .
the power showed a negative value in the proximal portion 
 of the shank and a positive value in the distal portion of the thigh , and there was a 
 transition from a negative to positive value at the knee joint .
the knee extensor muscles 
 absorbed mechanical energy as a result of the forward leaning movement of the shank , and 
 then they contracted concentrically . eventually , as mechanical energy was generated , the 
 knee extensor muscles caused forward leaning movement of the thigh .
thus , knee extension was 
 performed efficiently using mechanical energy during this phase and utilization of 
 physiological energy by the muscles was suppressed under condition n5 .
previous studies have demonstrated that humans perform knee 
 extension most efficiently during the phase from the loading response to mid - stance by 
 contracting the quadriceps first eccentrically and then concentrically , similar to a 
 spring19 , 20 .
thus , under condition n , the movement was performed efficiently 
 using mechanical energy and moved com upward during the com rising phase .
the power showed a negative value in the proximal portion of the shank and a positive value 
 in the distal portion of the thigh under condition tf , and no significant difference found 
 between the two conditions in the mechanical work performed by either of them , or the 
 negative work of the knee joint .
however , the positive mechanical work of the joint under 
 condition tf was significantly lower than that under condition n. these results indicate 
 there was little difference in mechanical energy flow between the two conditions , but the 
 amount of work done by knee extension under condition tf was significantly lower than that 
 under condition n. stretching the lower extremities by extending the knee moves com upward 
 during the com rising phase .
a previous study reported that the rate of utilization of 
 gravity during walking is related to the knee extension angle and the positive work of the 
 knee joint during the com rising phase21 . 

thus , comv would decrease because knee extension could not be 
 performed sufficiently to move com higher .
regarding the hip joint during the com rising phase , the hip extension moment and the 
 positive mechanical work in the proximal portion of the thigh under condition tf were 
 significantly higher than those under condition n. these results indicate that modifying the 
 demands of the muscular control of the hip extensor muscles increased the mechanical energy 
 generated by the proximal portion of the thigh under condition tf .
previous research has 
 shown that standing or walking with the trunk flexed increases the muscular demands on the 
 hip extensor muscles to maintain the posture22 .
therefore , our results may show that the demands on the hip 
 extensors were increased by sustained hip flexion because the knee could not be sufficiently 
 extended .
consequently , there were no significant differences in mechanical energy flow in the knee 
 between the two conditions during the com rising phase .
the results of this study also 
 suggest that a strategy with less mechanical energy was chosen , because the knee could not 
 be sufficiently extended , and upward movement of com decreased in the trunk flexed posture . 

finally , the mechanical demand on the hip extensor muscles may increase because the knee 
 can not be extended sufficiently , and the demands may cause secondary disabilities .
first , the sample size was small and the subjects were 
 healthy young people ; therefore results can not be generalized to elderly people with 
 kyphosis .
therefore , it is unclear whether the exact same 
 conditions were replicated for each subject .
however , by observing the tendencies in our 
 data the general influence of the posture on the lower extremities was determined .
third , 
 the kinetics data provide only limited insight into muscle activities using an inverse 
 dynamics approach .
finally , the values of mechanical energy were interpreted without 
 observing the joint movement in detail , relying on the values and waveforms of the power and 
 joint moments .
therefore , it will also be necessary to determine the relationship between 
 the joint movement and the kinetic aspects .
 [ purpose ] the time - series waveforms of mechanical energy generation , absorption , and 
 transfer through the joints indicate how movements are produced and controlled . 
 previous 
 studies have used these waveforms to evaluate and describe the efficiency of human 
 movements . 
 the purpose of this study was to examine the influence of trunk flexion on 
 mechanical energy flow in the lower extremities during gait . 
 [ subjects and methods ] the 
 subjects were 8 healthy young males ( mean age , 21.8  1.3  years , mean height , 170.5  
 6.8  cm , and mean weight , 60.2  6.8  kg ) . 
 subjects walked at a self - selected gait speed 
 under 2 conditions : normal gait ( condition n ) , and gait with trunk flexion formed with a 
 brace to simulate spinal curvature ( condition tf ) . 
 the data collected from initial contact 
 to the mid - stance of gait was analyzed . 
 [ results ] there were no significant differences 
 between the 2 conditions in the mechanical energy flow in the knee joint and negative 
 mechanical work in the knee joint . however , the positive mechanical work of the knee joint 
 under condition tf was significantly less than that under condition n. [ conclusion ] trunk 
 flexion led to knee flexion in a standing posture . 
 thus , a strategy of moving of center of 
 mass upward by knee extension using less mechanical energy was selected during gait in the 
 trunk flexed posture . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION