arthrogryposis ( amc ) is characterised by the presence of multiple joint contractures in multiple body areas that are present at birth , with a reported incidence from 1 per 3,000 to 1 per 5,100 live births [ 1 , 2 ] .
it has been defined as a rare but heterogeneous disorder   with different subtypes , of which amyoplasia is the most common .
the ability of functional ambulation depends on factors such as the severity of lower limb deformities and muscle weakness in the lower limbs , primarily in the hip and knee extensor muscles [ 4 , 5 ] .
independent walking before the age of 2.5  years has been reported , and 85  % of children with amyoplasia were found to be ambulators by the age of 5  years .
the presence of muscle weakness in the lower extremities was assumed to have more influence on walking ability than the severity of contractures .
for efficient mobility , the use of a wheelchair may be required [ 3 , 6 , 8 ] .
orthoses are also often used to enhance or facilitate walking and to compensate for muscle weakness and lower extremity deformities , as well as to maintain the lower extremities in an aligned position [ 4 , 9 ] .
the most commonly used orthoses types in children with amc are ankle  foot orthoses ( afos ) or knee  ankle  foot orthoses ( kafos ) with a knee locking mechanism [ 4 , 9 ] . in children with plantarflexor weakness , orthoses with a carbon fibre spring ankle joint
variations in gait pattern with respect to orthosis use have been observed ; in children using kafos with locked knee joints , more extensive trunk and pelvic movements were observed compared to children using afos and shoes .
it may , thus , be assumed that physical effort during walking is high in children with amc .
a functional exercise capacity test has been developed for use during a 6-min walking period that reflects the functional level for daily physical activities .
the physiological cost index ( pci ) offers a value of the heart rate per metre walked .
although assuming a linear relation between heart rate and oxygen consumption , the pci method has been criticised for use in children with disabilities , due to the children s difficulties in achieving a state of steady heart rate during walking [ 15 , 16 ] . instead , the measurement of oxygen uptake has been recommended , allowing calculation of the oxygen consumption and cost , referred to as physical effort and indicator of the efficiency of walking , respectively . to reduce the variability between participants of different ages ,
the scheme uses the net oxygen utilisation instead of gross oxygen utilisation , the former evaluating solely the amount of energy used to ambulate except resting time . in youths with amc
, the ambulatory and physical activity has been studied using a step activity monitor and was found to be lower compared to that of typically developing ( td ) youths .
to our knowledge , there is no report on energy effort ; thus , the aim of this study was to evaluate energy expenditure and exercise capacity during walking in children with amc .
children with amc born between the years 1993 and 2007 , who were treated at the children s orthopaedic department of karolinska university hospital , stockholm and at the uppsala university children s hospital , were invited to participate in the study .
thirty - one children fulfilled the inclusion criteria of lower limb deformities or contractures , independent ambulation with or without orthoses , and age between 5 and 18  years .
of the 31 children eligible for inclusion , seven families declined participation , thus , 24 children with a mean age of 11.1 ( 4.3 ) years took part in the study between april 2011 and march 2012 .
twenty - five td children with a mean age of 11.4 ( 4.1 ) years constituted the control group .
the study was approved by the regional ethical review board in stockholm , sweden . written informed consent was obtained from the participants and their parents .
the children were divided into three groups based on orthosis use , as has been described in a previous study : amc1 ( n  =  3 ) wore kafos with locked knee joints ( kafo - lk ) ; in amc2 ( n  =  11 ) , 3/11 children wore kafos with open knee joints ( with an extension stop ) ( kafo - o ) , 7/11 wore afos of different types and 1/11 wore kafo - lk with compensation for limb length discrepancy and a foot orthosis ( fo ) ; and amc3 ( n  =  10 ) used shoes .
the distributions of gender , age , height and weight are shown in table  1 .
prescriptions of orthoses were based on the presence of muscle weakness , joint contractures and deformities according to the orthotic programme of karolinska university hospital . in amc1 ,
the locked knee joints were prescribed due to knee extensor weakness . in this group ,
two children used carbon fibre springs ( fig .  1 ) . in amc2 , kafo - os with free flexion
were prescribed in one child with hyperextension and in two children to control foot and thigh alignment , all of them with carbon fibre springs ( fig .  2 ) .
five children had various afo types ( two solid , two hinged and one flexible carbon fibre ) to stabilise the foot and ankle joint based on material criteria due to weight and acceptance of orthoses ( table  2 ) .
all children used their orthoses for 8  h or more daily.table  1distributions of gender , age ,
height and weight mean ( sd ) in the groupsamc1amc2amc3tdgendermale39417female268age ( years)16.8 ( 0.2)10.0 ( 4.1)10.7 ( 4.1)11.4 ( 4.1)height ( cm)169.0 ( 3.5)132.6 ( 22.8)135.8 ( 25.2)148.8 ( 24.1)weight ( kg)70.7 ( 27.0)29.9 ( 12.0)35.3 ( 14.9)43.6 ( 18.0)amc arthrogryposis multiplex congenita , amc1 used knee  ankle 
foot orthoses with open knee joints or ankle  foot orthoses , amc3 used shoes , td typically developing childrenfig
.  1knee  ankle  foot orthosis with locked knee joint and carbon fibre spring ankle joint ( kafo - lk - c)fig . 
2knee  ankle  foot orthosis with open knee joint and extension stop , and carbon fibre spring ankle joint ( kafo - o - c)fig .  3ankle  foot orthosis with carbon fibre spring ankle joint ( afo - c)table  2orthoses types , group , functional ambulation , muscle strength , joint contractures and previous orthopaedic surgery in all participants with amcsubjectorthosesgroupfunctional ambulationmuscle strength ( grade 05)joint contractures ( )orthopaedic surgery procedurehipkneeanklehipkneeplantarbonysoft tissueext l / rabd l / rext l / rdors l / rplant l / rflexion l / rflexion l / rflexion l / r1kafo - lk - camc1ha4/44/43/30/00/020/+1540/50hip bi , ankle bi2kafo - lkamc1ha4/44/43/30/00/0/1020/20knee bi , ankle bi3kafo - lk - camc1ha4/44/43/31/11/130/3025/25knee bi , ankle bi4kafo - o - camc2ha4/44/44/30/00/110//10knee uni , ankle biknee uni5afo - hamc2ca4/44/44/43/44/4+15//5ankle bi6afo - samc2ha4/44/44/43/31/125/20ankle bi7kafo - o - camc2ca4/44/44/40/00/010/1015/5hip uni , ankle bi8afo - hamc2ca4/44/44/44/42/2+20/+20ankle bi9afo - camc2ca5/45/45/44/44/3/10/10knee uni , ankle uni10kafo - o - camc2ca5/55/55/54/44/4+20/+2010/ankle bi11afo - samc2ca4/44/45/55/35/3spine fusionankle uni12afo - camc2ca4/44/45/54/43/3ankle bi13fo / kafo - lkamc2ca5/45/45/05/05/0/15/7510/50knee uni14afo - fcamc2ca4/44/44/42/22/210/ankle bi15amc3ca4/44/44/44/44/410/ankle bi16insolesamc3ca4/44/44/44/44/420/2015/17amc3ca4/44/44/44/34/410/knee bi , ankle uni18amc3ca4/45/55/54/44/410/10/+20ankle bi19amc3ca5/55/55/53/34/420amc3ca4/44/44/44/44/4+20/+15hip bi , ankle uni21insolesamc3ca4/44/44/43/44/4hip bi22amc3ca5/55/54/44/44/4knee biankle uni23amc3ca5/55/54/44/44/4/10spine fusion24amc3ca5/55/54/44/45/5/105/hip bi 
foot orthosis , lk locked knee joint , c carbon fibre ankle joint , o open knee joint with extension stop , afo ankle 
foot orthosis , h hinged , s solid , fo foot orthosis , fc flexible carbon fibre , amc arthrogryposis multiplex congenita , ha household ambulator , ca community ambulator , l left , r right , ext extension , abd abduction , + indicates hyperextension , dors dorsiflexion , plant plantarflexion , bi bilaterally , uni unilaterallyunilaterallycompensation for limb length discrepancy distributions of gender , age , height and weight mean ( sd ) in the groups amc arthrogryposis multiplex congenita , amc1 used knee  ankle  foot orthoses with locked knee joints , amc2 used knee  ankle  foot orthoses with open knee joints or ankle  foot orthoses , amc3 used shoes , td typically developing children knee  ankle  foot orthosis with locked knee joint and carbon fibre spring ankle joint ( kafo - lk - c ) knee  ankle  foot orthosis with open knee joint and extension stop , and carbon fibre spring ankle joint ( kafo - o - c ) ankle  foot orthosis with carbon fibre spring ankle joint ( afo - c ) orthoses types , group , functional ambulation , muscle strength , joint contractures and previous orthopaedic surgery in all participants with amc kafo knee  ankle  foot orthosis , lk locked knee joint , c carbon fibre ankle joint , o open knee joint with extension stop , afo ankle  foot orthosis , h hinged , s solid , fo foot orthosis , fc flexible carbon fibre , amc arthrogryposis multiplex congenita , ha household ambulator , ca community ambulator , l left , r right , ext extension , abd abduction , + indicates hyperextension , dors dorsiflexion , plant plantarflexion , bi bilaterally , uni unilaterally compensation for limb length discrepancy nineteen children were community ambulators , 11 with no need for a wheelchair and eight with the need for a wheelchair for longer distances outdoors , and five children were household ambulators ( table  2 ) .
the strength of the lower limb muscles was tested manually according to a 05 graded scale , with 5 indicating normal strength to grade 0 indicating the absence of muscle strength .
hip extension and hip abduction ranged between grades 4 and 5 , knee extension between grades 0 and 5 , and ankle dorsiflexion and plantarflexion between grades 0 and 5 ( table  2 ) .
hip and knee flexion contractures were defined as 10 from a neutral joint position , and plantarflexion contractures as less than a neutral position .
flexion contractures uni- or bilaterally were seen in the ankle in 12 children , in the knee in nine and in the hip in six children .
knee hyperextension uni- or bilaterally was seen in six children ( table  2 ) .
assessment of muscle strength and joint range of motion were performed by the same physiotherapist ( lv ) .
twenty - two participants had undergone orthopaedic surgery , of which 19 had bony surgery and three soft tissue surgery ( table  2 ) .
energy expenditure was measured as the rate of oxygen uptake during walking at a self - selected speed using the portable telemetric system cosmed k4b ( cosmed srl , rome , italy ) , which is a valid and reliable method to measure energy during walking in children with disabilities .
functional exercise capacity was measured with the 6mwt , which is a valid and reliable method for assessing endurance and exercise tolerance in healthy children   and in children with disabilities .
the heart rate was measured with a polar heart rate monitor ( polar electro oy , kempele , finland ) .
both the o2-walking test and the 6mwt were performed on the same 21-m oval walking track in the gait laboratory . the 6mwt was performed after completion of the o2 measurement .
prior to the o2-walking test , the child was given verbal instructions and the opportunity to become familiar with the equipment .
the test was standardised to 5  min rest , followed by 5  min walking at a self - selected speed and completed by 5  min rest after the test . during the two resting periods ,
the child was asked to sit quietly in a comfortable chair in order to achieve a stable heart rate .
prior to the 6mwt , it was assured that the heart rate was at a stable resting steady - state level .
each child was instructed to cover as long a distance as possible in 6  min , without running .
encouragement such as  keep going  and  you are doing well  , as well as announcement of the time remaining , was given to the child with respect to the guidelines of the american thoracic society ( ats ) .
oxygen consumption was measured in terms of the rate of oxygen consumed while walking , both as normalised by body weight per unit time ( ml / kg / min ) and as oxygen cost per metre walked during the test ( ml / kg / m ) . gross oxygen consumption and gross oxygen cost , including both resting and walking oxygen utilisation ,
were calculated from the last 2  min of the walking period and were normalised by body weight and the weight of orthoses and shoes . to calculate the net oxygen consumption ,
the last 2  min of the first resting period and the last 2  min of the walking period were used to calculate the net non - dimensional consumption ( nnconsumption ) and cost ( nncost ) .
the net non - dimensional values were calculated including parameters of body weight , leg length and the acceleration due to gravity .
the 6mwt was analysed based on the distance walked and the mean walking velocity during the complete test . for both tests ,
the walking velocity was normalised to leg length and is presented as the normalised walking velocity ( n walking velocity ) .
amc1 was excluded from the statistical comparison with the other groups because of the low participant number , with only descriptive statistics reported as mean ( sd ) . a one - way analysis of variance ( anova )
was used to compare values between amc2 , amc3 and td children . to identify significant differences among means between groups ,
a post hoc analysis was performed using fisher s least significant difference ( lsd ) test .
all statistical analyses were carried out using commercially available software ( spss version 19.0 ) .
children with amc born between the years 1993 and 2007 , who were treated at the children s orthopaedic department of karolinska university hospital , stockholm and at the uppsala university children s hospital , were invited to participate in the study .
thirty - one children fulfilled the inclusion criteria of lower limb deformities or contractures , independent ambulation with or without orthoses , and age between 5 and 18  years .
of the 31 children eligible for inclusion , seven families declined participation , thus , 24 children with a mean age of 11.1 ( 4.3 ) years took part in the study between april 2011 and march 2012 .
twenty - five td children with a mean age of 11.4 ( 4.1 ) years constituted the control group .
the study was approved by the regional ethical review board in stockholm , sweden . written informed consent was obtained from the participants and their parents .
the children were divided into three groups based on orthosis use , as has been described in a previous study : amc1 ( n  =  3 ) wore kafos with locked knee joints ( kafo - lk ) ; in amc2 ( n  =  11 ) , 3/11 children wore kafos with open knee joints ( with an extension stop ) ( kafo - o ) , 7/11 wore afos of different types and 1/11 wore kafo - lk with compensation for limb length discrepancy and a foot orthosis ( fo ) ; and amc3 ( n  =  10 ) used shoes .
the distributions of gender , age , height and weight are shown in table  1 .
prescriptions of orthoses were based on the presence of muscle weakness , joint contractures and deformities according to the orthotic programme of karolinska university hospital . in amc1 ,
the locked knee joints were prescribed due to knee extensor weakness . in this group ,
two children used carbon fibre springs ( fig .  1 ) . in amc2 , kafo - os with free flexion
were prescribed in one child with hyperextension and in two children to control foot and thigh alignment , all of them with carbon fibre springs ( fig .  2 ) .
five children had various afo types ( two solid , two hinged and one flexible carbon fibre ) to stabilise the foot and ankle joint based on material criteria due to weight and acceptance of orthoses ( table  2 ) .
all children used their orthoses for 8  h or more daily.table  1distributions of gender , age ,
height and weight mean ( sd ) in the groupsamc1amc2amc3tdgendermale39417female268age ( years)16.8 ( 0.2)10.0 ( 4.1)10.7 ( 4.1)11.4 ( 4.1)height ( cm)169.0 ( 3.5)132.6 ( 22.8)135.8 ( 25.2)148.8 ( 24.1)weight ( kg)70.7 ( 27.0)29.9 ( 12.0)35.3 ( 14.9)43.6 ( 18.0)amc arthrogryposis multiplex congenita , amc1 used knee  ankle 
foot orthoses with open knee joints or ankle  foot orthoses , amc3 used shoes , td typically developing childrenfig
.  1knee  ankle  foot orthosis with locked knee joint and carbon fibre spring ankle joint ( kafo - lk - c)fig . 
2knee  ankle  foot orthosis with open knee joint and extension stop , and carbon fibre spring ankle joint ( kafo - o - c)fig .  3ankle  foot orthosis with carbon fibre spring ankle joint ( afo - c)table  2orthoses types , group , functional ambulation , muscle strength , joint contractures and previous orthopaedic surgery in all participants with amcsubjectorthosesgroupfunctional ambulationmuscle strength ( grade 05)joint contractures ( )orthopaedic surgery procedurehipkneeanklehipkneeplantarbonysoft tissueext l / rabd l / rext l / rdors l / rplant l / rflexion l / rflexion l / rflexion l / r1kafo - lk - camc1ha4/44/43/30/00/020/+1540/50hip bi , ankle bi2kafo - lkamc1ha4/44/43/30/00/0/1020/20knee bi , ankle bi3kafo - lk - camc1ha4/44/43/31/11/130/3025/25knee bi , ankle bi4kafo - o - camc2ha4/44/44/30/00/110//10knee uni , ankle biknee uni5afo - hamc2ca4/44/44/43/44/4+15//5ankle bi6afo - samc2ha4/44/44/43/31/125/20ankle bi7kafo - o - camc2ca4/44/44/40/00/010/1015/5hip uni , ankle bi8afo - hamc2ca4/44/44/44/42/2+20/+20ankle bi9afo - camc2ca5/45/45/44/44/3/10/10knee uni , ankle uni10kafo - o - camc2ca5/55/55/54/44/4+20/+2010/ankle bi11afo - samc2ca4/44/45/55/35/3spine fusionankle uni12afo - camc2ca4/44/45/54/43/3ankle bi13fo / kafo - lkamc2ca5/45/45/05/05/0/15/7510/50knee uni14afo - fcamc2ca4/44/44/42/22/210/ankle bi15amc3ca4/44/44/44/44/410/ankle bi16insolesamc3ca4/44/44/44/44/420/2015/17amc3ca4/44/44/44/34/410/knee bi , ankle uni18amc3ca4/45/55/54/44/410/10/+20ankle bi19amc3ca5/55/55/53/34/420amc3ca4/44/44/44/44/4+20/+15hip bi , ankle uni21insolesamc3ca4/44/44/43/44/4hip bi22amc3ca5/55/54/44/44/4knee biankle uni23amc3ca5/55/54/44/44/4/10spine fusion24amc3ca5/55/54/44/45/5/105/hip bi 
foot orthosis , lk locked knee joint , c carbon fibre ankle joint , o open knee joint with extension stop , afo ankle 
foot orthosis , h hinged , s solid , fo foot orthosis , fc flexible carbon fibre , amc arthrogryposis multiplex congenita , ha household ambulator , ca community ambulator , l left , r right , ext extension , abd abduction , + indicates hyperextension , dors dorsiflexion , plant plantarflexion , bi bilaterally , uni unilaterallyunilaterallycompensation for limb length discrepancy distributions of gender , age , height and weight mean ( sd ) in the groups amc arthrogryposis multiplex congenita , amc1 used knee  ankle  foot orthoses with locked knee joints , amc2 used knee  ankle  foot orthoses with open knee joints or ankle  foot orthoses , amc3 used shoes , td typically developing children knee  ankle  foot orthosis with locked knee joint and carbon fibre spring ankle joint ( kafo - lk - c ) knee  ankle  foot orthosis with open knee joint and extension stop , and carbon fibre spring ankle joint ( kafo - o - c ) ankle  foot orthosis with carbon fibre spring ankle joint ( afo - c ) orthoses types , group , functional ambulation , muscle strength , joint contractures and previous orthopaedic surgery in all participants with amc kafo knee  ankle  foot orthosis , lk locked knee joint , c carbon fibre ankle joint , o open knee joint with extension stop , afo ankle  foot orthosis , h hinged , s solid , fo foot orthosis , fc flexible carbon fibre , amc arthrogryposis multiplex congenita , ha household ambulator , ca community ambulator , l left , r right , ext extension , abd abduction , + indicates hyperextension , dors dorsiflexion , plant plantarflexion , bi bilaterally , uni unilaterally compensation for limb length discrepancy nineteen children were community ambulators , 11 with no need for a wheelchair and eight with the need for a wheelchair for longer distances outdoors , and five children were household ambulators ( table  2 ) .
the strength of the lower limb muscles was tested manually according to a 05 graded scale , with 5 indicating normal strength to grade 0 indicating the absence of muscle strength .
hip extension and hip abduction ranged between grades 4 and 5 , knee extension between grades 0 and 5 , and ankle dorsiflexion and plantarflexion between grades 0 and 5 ( table  2 ) .
hip and knee flexion contractures were defined as 10 from a neutral joint position , and plantarflexion contractures as less than a neutral position .
flexion contractures uni- or bilaterally were seen in the ankle in 12 children , in the knee in nine and in the hip in six children .
knee hyperextension uni- or bilaterally was seen in six children ( table  2 ) .
assessment of muscle strength and joint range of motion were performed by the same physiotherapist ( lv ) .
twenty - two participants had undergone orthopaedic surgery , of which 19 had bony surgery and three soft tissue surgery ( table  2 ) .
energy expenditure was measured as the rate of oxygen uptake during walking at a self - selected speed using the portable telemetric system cosmed k4b ( cosmed srl , rome , italy ) , which is a valid and reliable method to measure energy during walking in children with disabilities .
functional exercise capacity was measured with the 6mwt , which is a valid and reliable method for assessing endurance and exercise tolerance in healthy children   and in children with disabilities .
the heart rate was measured with a polar heart rate monitor ( polar electro oy , kempele , finland ) .
both the o2-walking test and the 6mwt were performed on the same 21-m oval walking track in the gait laboratory . the 6mwt was performed after completion of the o2 measurement .
prior to the o2-walking test , the child was given verbal instructions and the opportunity to become familiar with the equipment .
the test was standardised to 5  min rest , followed by 5  min walking at a self - selected speed and completed by 5  min rest after the test . during the two resting periods ,
the child was asked to sit quietly in a comfortable chair in order to achieve a stable heart rate .
prior to the 6mwt , it was assured that the heart rate was at a stable resting steady - state level .
each child was instructed to cover as long a distance as possible in 6  min , without running .
encouragement such as  keep going  and  you are doing well  , as well as announcement of the time remaining , was given to the child with respect to the guidelines of the american thoracic society ( ats ) .
oxygen consumption was measured in terms of the rate of oxygen consumed while walking , both as normalised by body weight per unit time ( ml / kg / min ) and as oxygen cost per metre walked during the test ( ml / kg / m ) .
gross oxygen consumption and gross oxygen cost , including both resting and walking oxygen utilisation , were calculated from the last 2  min of the walking period and were normalised by body weight and the weight of orthoses and shoes . to calculate the net oxygen consumption ,
the last 2  min of the first resting period and the last 2  min of the walking period were used to calculate the net non - dimensional consumption ( nnconsumption ) and cost ( nncost ) .
the net non - dimensional values were calculated including parameters of body weight , leg length and the acceleration due to gravity .
the 6mwt was analysed based on the distance walked and the mean walking velocity during the complete test .
for both tests , the walking velocity was normalised to leg length and is presented as the normalised walking velocity ( n walking velocity ) .
amc1 was excluded from the statistical comparison with the other groups because of the low participant number , with only descriptive statistics reported as mean ( sd ) . a one - way analysis of variance ( anova )
was used to compare values between amc2 , amc3 and td children . to identify significant differences among means between groups , a post hoc analysis
all statistical analyses were carried out using commercially available software ( spss version 19.0 ) .
the gross oxygen consumption ( ml / kg / min ) ranged in amc1 between 10.15 and 14.56 , in amc2 between 16.38 and 29.20 , in amc3 between 12.92 and 25.69 , and in td children between 11.51 and 24.94 .
the gross oxygen cost ( ml / kg / m ) ranged in amc1 between 0.28 and 0.40 , in amc2 between 0.21 and 0.71 , in amc3 between 0.19 and 0.41 , and in td children between 0.16 and 0.43 . the walking velocity ( m / s ) was in amc1 0.61 , ranged in amc2 between 0.60 and 1.31 , in amc3 between 0.55 and 1.29 , and in td children between 0.75 and 1.45 .
nnconsumption was higher in amc2 and amc3 than in the td children , but not significantly .
nncost was higher in amc2 and amc3 than in the td children . as analysed with fisher s lsd post hoc test , amc2 had significantly higher nncost than td children .
n walking velocity was lower in amc2 than in amc3 and the td children , but not significantly ( table  3).table  3normalised oxygen consumption ( nnconsumption ) , normalised oxygen cost ( nncost ) and normalised walking velocity in amc2 , amc3 and td children during the o2-walking testamc2 ( n  =  10 ) ( mean , sd)amc3 ( n  =  9 ) ( mean , sd)td ( n  =  25 ) ( mean , sd)p - valueamc2amc3 p - valueamc2td p - valueamc3td p - valuennconsumption0.156 ( 0.053)0.159 ( 0.070)0.123 ( 0.028)0.055nncost0.455 ( 0.193)0.398 ( 0.146)0.308 ( 0.069 ) 
 0.007 
 0.319 
 0.002 
 0.067n walking velocity0.361 ( 0.072)0.401 ( 0.093)0.403 ( 0.050)0.223n normalised , amc arthrogryposis multiplex congenita , td typically developing children normalised oxygen consumption ( nnconsumption ) , normalised oxygen cost ( nncost ) and normalised walking velocity in amc2 , amc3 and td children during the o2-walking test n normalised , amc arthrogryposis multiplex congenita , td typically developing children in amc1 , the nnconsumption mean ( sd ) was 0.082 ( 0.008 ) , the nncost mean ( sd ) was 0.385 ( 0.031 ) and the n walking velocity mean ( sd ) was 0.214 ( 0.005 ) . the walking velocity ( m / s ) ranged in amc1 between 0.82 and 0.93 , in amc2 between 0.66 and 1.53 , in amc3 between 0.81 and 1.88 , and in td children between 0.96 and 1.98 .
as analysed with fisher s lsd post hoc test , td children walked a significantly longer distance than amc2 and amc3 .
n walking velocity differed significantly between amc2 , amc3 and the td children . as analysed with fisher s lsd post hoc test , amc2 had significantly lower n walking velocity than td children ( table  4).table  4walked distance and normalised walking velocity in amc2 , amc3 and td children during the 6-minute walk test ( 6mwt)amc2 ( n  =  11 ) ( mean , sd)amc3 ( n  =  10 ) ( mean , sd)td ( n  =  25 ) ( mean , sd)p - valueamc2amc3 p - valueamc2td p - valueamc3td p - valuedistance ( m)402.7 ( 108.0)476.8 ( 126.1)565.1 ( 110.0 ) 
 0.001 
 0.141<0.001 
 0.043 
 n walking velocity0.435 ( 0.030)0.501 ( 0.100)0.564 ( 0.073 ) 
 0.001 
 0.088 
 < 0.001 
 0.055 m metres , n normalised , amc arthrogryposis multiplex congenita , td typically developing children walked distance and normalised walking velocity in amc2 , amc3 and td children during the 6-minute walk test ( 6mwt ) m metres , n normalised , amc arthrogryposis multiplex congenita , td typically developing children amc1 walked a mean ( sd ) distance of 312.0 ( 19.7 ) m and had a n walking velocity mean ( sd ) of 0.304 ( 0.015 ) .
the gross oxygen consumption ( ml / kg / min ) ranged in amc1 between 10.15 and 14.56 , in amc2 between 16.38 and 29.20 , in amc3 between 12.92 and 25.69 , and in td children between 11.51 and 24.94 .
the gross oxygen cost ( ml / kg / m ) ranged in amc1 between 0.28 and 0.40 , in amc2 between 0.21 and 0.71 , in amc3 between 0.19 and 0.41 , and in td children between 0.16 and 0.43 . the walking velocity ( m / s ) was in amc1 0.61 , ranged in amc2 between 0.60 and 1.31 , in amc3 between 0.55 and 1.29 , and in td children between 0.75 and 1.45 .
nnconsumption was higher in amc2 and amc3 than in the td children , but not significantly .
nncost was higher in amc2 and amc3 than in the td children . as analysed with fisher s lsd post hoc test , amc2 had significantly higher nncost than td children .
n walking velocity was lower in amc2 than in amc3 and the td children , but not significantly ( table  3).table  3normalised oxygen consumption ( nnconsumption ) , normalised oxygen cost ( nncost ) and normalised walking velocity in amc2 , amc3 and td children during the o2-walking testamc2 ( n  =  10 ) ( mean , sd)amc3 ( n  =  9 ) ( mean , sd)td ( n  =  25 ) ( mean , sd)p - valueamc2amc3 p - valueamc2td p - valueamc3td p - valuennconsumption0.156 ( 0.053)0.159 ( 0.070)0.123 ( 0.028)0.055nncost0.455 ( 0.193)0.398 ( 0.146)0.308 ( 0.069 ) 
 0.007 
 0.319 
 0.002 
 0.067n walking velocity0.361 ( 0.072)0.401 ( 0.093)0.403 ( 0.050)0.223n normalised , amc arthrogryposis multiplex congenita , td typically developing children normalised oxygen consumption ( nnconsumption ) , normalised oxygen cost ( nncost ) and normalised walking velocity in amc2 , amc3 and td children during the o2-walking test n normalised , amc arthrogryposis multiplex congenita , td typically developing children in amc1 , the nnconsumption mean ( sd ) was 0.082 ( 0.008 ) , the nncost mean ( sd ) was 0.385 ( 0.031 ) and the n walking velocity mean ( sd ) was 0.214 ( 0.005 ) .
the walking velocity ( m / s ) ranged in amc1 between 0.82 and 0.93 , in amc2 between 0.66 and 1.53 , in amc3 between 0.81 and 1.88 , and in td children between 0.96 and 1.98 .
as analysed with fisher s lsd post hoc test , td children walked a significantly longer distance than amc2 and amc3 .
as analysed with fisher s lsd post hoc test , amc2 had significantly lower n walking velocity than td children ( table  4).table  4walked distance and normalised walking velocity in amc2 , amc3 and td children during the 6-minute walk test ( 6mwt)amc2 ( n  =  11 ) ( mean , sd)amc3 ( n  =  10 ) ( mean , sd)td ( n  =  25 ) ( mean , sd)p - valueamc2amc3 p - valueamc2td p - valueamc3td p - valuedistance ( m)402.7 ( 108.0)476.8 ( 126.1)565.1 ( 110.0 ) 
 0.001 
 0.141<0.001 
 0.043 
 n walking velocity0.435 ( 0.030)0.501 ( 0.100)0.564 ( 0.073 ) 
 0.001 
 0.088 
 < 0.001 
 0.055 m metres , n normalised , amc arthrogryposis multiplex congenita , td typically developing children walked distance and normalised walking velocity in amc2 , amc3 and td children during the 6-minute walk test ( 6mwt ) m metres ,
n normalised , amc arthrogryposis multiplex congenita , td typically developing children amc1 walked a mean ( sd ) distance of 312.0 ( 19.7 ) m and had a n walking velocity mean ( sd ) of 0.304 ( 0.015 ) .
the results in this study confirm that the energy effort during walking at a self - selected speed is increased in children with amc .
this was seen as higher nnconsumption in amc2 and amc3 than in the td children , as well as a higher nncost in amc2 and amc3 compared to td children , which indicates a less efficient gait in children with amc .
higher n walking velocity was found in all groups during the 6mwt compared to the o2-walking test , which was expected , as the instructions on walking speed differed between the two tests .
another explanation is that wearing the mask during the o2-walking test may have reduced the visual field , possibly influencing walking velocity . in this study , we included children aged 517  years , and , therefore , we chose to normalise the data with the net non - dimensional scheme , which has been recommended when including children of different ages . since orthoses aim to stabilise joints , they influence both gait pattern and walking function .
we , therefore , divided the participants into groups according to their orthoses , as has been described in a previous study . among the amc groups ,
we could not , however , confirm this observation statistically due to the low participant number .
children in amc1 required above - knee orthotic solutions with locked knee joints and were totally dependent on orthoses to achieve walking ability .
this group had the lowest nnconsumption of all groups , including td children , which is probably related to their low walking velocity to decrease physical exertion .
in normal gait , the knee flexes during the swing phase , which is not possible in a locked kafo .
compensatory movements such as pelvic hiking or circumduction of the leg   are , therefore , necessary . in persons with knees immobilised with a cast ,
, it was possible to maintain the oxygen consumption at an acceptable level by reducing walking velocity , which contributes to an increase in oxygen cost .
the highest nncost was found in amc2 , of whom all needed orthoses and all except two children were community ambulators .
joint contractures , which are the main characteristics in amc , were frequently found in the ankle joint in amc1 and in amc2 , but only in two children in amc3 .
the children in amc3 , who use only shoes , reached walking speeds most similar to the td children during the o2-walking test .
this may be due to not only fewer joint contractures than the other amc groups , but also to the good plantarflexor strength in all children in amc3 .
all participants had good muscle strength in hip extensors and hip abductors , and only one child had a hip flexion contracture of 20. the presence of hip flexion contracture < 20 and active hip motion were reported to be important factors to maintain community or independent walking . walking distance is the main outcome measure in the 6mwt .
the children with amc walked shorter distances than the td children , wherein children in amc1 walked the shortest distance , which reflects the difficulties in keeping up with peers and , therefore , making wheelchair use necessary and useful . the large variety in walked distance within the amc2 , amc3 and td groups , however , may partly be explained by age variations , as this parameter was not normalised .
the adolescents in amc1 ambulated indoors using orthoses with locked knee joints and used powered wheelchairs for transportation .
this is  in accordance with murray and fixsen , who reported that adolescents felt more integrated into the community in a wheelchair than walking with crutches and orthoses . to meet each individual s requirements
, we recommend a combination of wheelchair and orthoses use for children who use orthoses for walking function .
the requirement of walkers has been described in children walking with kafos , and often with forearm support instead of standard walker grips . furthermore , limited ambulation due to poor protective responses of the upper extremity has been reported .
fifteen of 24 children in our study had upper limb involvement , of which two had locked kafos bilaterally , though none used a walking aid .
this is believed to be due to the stable orthosis construction offering the children sufficient stability to walk without an external walking aid .
another important treatment goal is to improve biomechanical alignment with adequate orthoses and footwear combinations . in our participants ,
heel height was adjusted in the orthoses and/or the shoes , when required , for each child s contracture .
while lower ambulatory and physical activity has been reported in youths with amc compared to td youths , this is , to our knowledge , the first study on physical effort in children with amc .
simultaneously , they reflect the rarity and complexity of this condition . a walking test of longer duration or walking at higher or fixed speeds may have discriminated even more between the groups .
this , however , may have further reduced the study population due to limited endurance , which has been reported in youths with amc .
nevertheless , the self - selected speed which was chosen in this study has been suggested to be the best simulation of how children with disabilities function in the community .
energy expenditure and functional exercise capacity were evaluated in children with amc during walking in their habitual orthoses , ranging from kafo with locked knee joints , to afo , to shoes only .
the energy effort during walking at a self - selected speed was higher in children with amc than in td children the children walking with open kafos or afos , most of whom were community ambulators , walked with almost the same n walking velocity as amc children not using orthoses and td children .
their nncost , however , was higher , which indicates a less efficient gait in this group .
the children walking with kafo with locked knee joints were totally dependent on orthoses to achieve walking ability .
they had the lowest n walking velocity of all groups , and reducing walking velocity is interpreted as a strategy to minimise exertion . during the exercise capacity test in the 6mwt , the children with amc walked a shorter distance than the td children , which reflects their difficulties in keeping up with peers .
 purposeexcessive movements during walking have been observed by gait analysis in children with arthrogryposis ( amc ) using orthoses compared to children using only shoes . 
 the aim of this study was to evaluate energy expenditure and functional exercise capacity in children with amc.methodstwenty-four children with amc and 25 typically developing ( td ) children underwent oxygen measurement and the 6-minute walk test ( 6mwt ) . 
 children were divided into amc1 using knee  ankle  
 foot orthoses with locked knee joints ( kafo - lk ) ; amc2 kafos with open knee joints ( kafo - o ) or ankle  foot orthoses ( afo ) ; and amc3 using shoes.resultsthe net non - dimensional oxygen cost ( nncost ) was lower in td ( 0.308 ) than in amc2 ( 0.455 , n  =  10 ) ( p  =  0.002 ) . 
 there were no differences in the net non - dimensional consumption ( nnconsumption ) or normalised walking velocity . 
 the lowest nnconsumption ( 0.082 ) , nncost ( 0.385 ) and normalised walking velocity ( 0.214 ) were found in amc1 ( n  =  3 ) , but no statistical calculation was performed . in the 6mwt , both amc2 ( 402.7 , n  =  11 ) and amc3 ( 476.8 , 
 n  =  10 ) walked shorter distances ( m ) than td ( 565.1 ) ( p  <  0.001 and p  =  0.043 , respectively ) . 
 amc2 ( 0.435 ) had lower normalised walking velocity than td ( 0.564 ) ( p  <  0.001).conclusionschildren with amc using open kafos or afos ( amc2 ) had higher energy effort represented by significantly higher nncost than td , whereas amc children requiring only shoes ( amc3 ) did not differ significantly from td . to maintain the nnconsumption at an acceptable level , children using locked kafos ( amc1 ) slowed down their walking velocity . compared to td , the exercise capacity was lower in children with amc using open kafos or afos and shoes , represented by lower walking velocity and shorter distance walked during the 6mwt . 
Introduction
Methods
Participants
O
Data analysis
Statistical analysis
Results
O
6MWT
Discussion
Conclusion