Children and adolescents with Down syndrome,physical fitness and physical activity

Ken Pitetti,Tray Baynard,Stamatis Agiovlasitis

aDepartment of Physical Therapy,College of Health Professions,Wichita State University,Wichita,KS 67260-0043,USA

bDepartment of Kinesiology&Nutrition,University of Illinois at Chicago,Chicago,IL 60612,USA

cDepartment of Kinesiology,Mississippi State University,Mississippi State,MS 39762,USA

Children and adolescents with Down syndrome,physical fitness and physical activity

Ken Pitettia,*,Tracy Baynardb,Stamatis Agiovlasitisc

aDepartment of Physical Therapy,College of Health Professions,Wichita State University,Wichita,KS 67260-0043,USA

bDepartment of Kinesiology&Nutrition,University of Illinois at Chicago,Chicago,IL 60612,USA

cDepartment of Kinesiology,Mississippi State University,Mississippi State,MS 39762,USA

Children(5—12 years)and adolescents(13—19 years)with Down syndrome(DS)possess a set of health,anatomical,physiological, cognitive,and psycho-social attributes predisposing them to limitations on their physical fitness and physical activity(PA)capacities.The paucity of studies and their conflicting findings prevent a clear understanding and/or substantiation of these limitations.The purpose of this article was to review the measurement,determinants and promotion of physical fitness and PA for youth(i.e.,children and adolescents)with DS. The existing body of research indicates that youth with DS:1)have low cardiovascular and muscular fitness/exercise capacity;2)demonstrate a greater prevalence of overweight and obesity;3)a large proportion do not meet the recommended amount of daily aerobic activity;and 4)their PA likely declines through childhood and into adolescence.Future research should focus on:1)strength testing and training protocols;2) methodologies to determine PA levels;and 3)practical interventions to increase PA.

Copyright?2012,Shanghai University of Sport.Production and hosting by Elsevier B.V.All rights reserved.

Activity;Adolescents;Children;Down syndrome;Fitness

1.Introduction

1.1.Background and genetics

TheconditionofDownsyndrome(DS)wasfirstdescribedin a clinical lecture report delivered in 1866 by the British physician,John Langdon Down,entitled“Observation on the ethnic classification of Mongoloid idiots”.1Dr.Down used the derogatory term “Mongoloid”because children with DS displayed facial features(e.g.,epicanthal fold)that shared similarities with the prevailing erroneous ethnic theory established by the German anatomist,Johann Friedrich Blumenbach.2Catalyzed by editorials inThe Lancet,together with political pressure by advocacy groups and parental organizations in the 1960s,the term“mongolism”was dropped from medical references by the World Health Organization and replaced by“Down syndrome”during the 1970s.3The cause of DS remained unknown until the 1950s when it became possible to identify chromosomal abnormalities with the discovery of karyotypetechniques.In1959theFrenchgeneticist,Dr.Jerome Lejeune,reported that DS was caused by an extra 21st chromosome,an aneuploidy condition known as trisomy 21.4The vast majority of cases of DS are due to full trisomy 21,with everysomaticcellinthebodyhavinganextra21stchromosome (Table 1).A smaller percentage of persons with DS are due to mosaic trisomy(2%—4%)and Robertsonian translocation (3%—4%).5Reports have suggested that children with mosaicDS have similar health and developmental difficulties as children with full trisomy 21,but to a lesser degree6and those with DS caused by Robertsonian translocation have increased frequency of psychiatric disorders.7

1.2.Demographics

In the United States the incidence of DS at birth from 1979 to 2003 increased from 9.0 to 11.8 per 10,000 births (Table 1).8It is suggested that the probable cause for the increase in DS is related to the trend toward later child bearing,with women over 35 being five times more likely than younger women to have children with DS.5

1.3.The intellectual and health profiles of persons with Down syndrome

Although about one-third of the causes of intellectual disabilities(ID)remain unknown,the three major known causes of ID are DS,Fetal Alcohol Spectrum Disorder,and Fragile X syndrome.6As is the case for all persons with ID, the degree of ID in persons with DS is determined by limitations in both cognitive function and in adaptive behavior (e.g.,conceptual,social,and practical adaptive skills).7-9The degree of cognitive impairment is variable,with the majority classified as mild(IQ:50—55—69)or moderate(30—35 to 50—55)while a smaller percentage(＜10%)are severe (IQ＜20—25 to 30—35).10,11Adaptive behavior encompasses conceptual skills(language,reading and writing,self directions),social skills(self esteem,gullibility,naivete′,avoids victimization)and practical adaptive skills(dressing,toileting, preparing meals,using transportation,occupational skills)that allow individuals to be functional in their everyday life.10,11Adaptive behavior may improve with early intervention but the level of function varies according to innate capacities and social support systems(Table 1).10,11

Table 1 Description,demographics,and health profiles of persons with Down syndrome.

Although ID affects every day functioning,the health profiles of children and adolescents with ID without DS are unremarkable.In contrast,the effects of DS involve a range of medical conditions in addition to ID that may be identified at birth or develop during the life span(Table 1).These conditions include increased risk of congenital heart disease(50%), hearing loss(75%),eye disease(60%),obstructive sleep apnea (75%),gastrointestinal conditions(10%),thyroid(hypothyroidism)disease(15%),and atlanto-axial/atlanto-occipital instability(10%—30%).8,9Excluding birth,nearly half of children with DS are hospitalized before age 3,with the principle causes for hospitalization being congenital heart disease(e.g.,septal and valvular anomalies)and respiratory illnesses(pneumonia,acute,and chronic bronchitis).10

These medical conditions increase demands on the health care system that can be as much as 13 times higher than for children(0—4 years)without DS.11Health care costs have been reported to decline with age,and by adolescence the cost of health care services have been reported to decrease to 1.1—1.7 times of those without disabilities.12,13However, diagnosis of congenital heart disease and level of independence influence total health care cost,independent of age.12These health care estimates do not address overall cost to families affected by DS,such as special education,individual transport needs,housing modifications,and related cost to family members(e.g.,employment opportunities).

Survival for persons with DS has improved over the past few decades with life expectancy nearing 60 years paralleling the improvement in social and medical support systems.14—16However,mortality rates of persons with DS remain higher overall than those of the general population with the leading causes of death being congenital anomalies(e.g.,heart defects),Alzheimer’s disease,pneumonia,leukemia,and all circulatory diseases(e.g.,ischemic heart disease,cerebrovascular disease).14,17,18Death rates for persons with DS increase significantly after age 40.19The main cause for the upsurge in mortality in middle-aged adults with DS is the deterioration in functional abilities and rise in behavioral problems due to Alzheimer’s disease.20—23Because of early onset of menopause,women with DS are four times more likely to develop Alzheimer’s disease than their male counterparts.24Other disorders of aging in DS are pulmonary disorders(recurrent pneumonia caused by recurrent aspiration),sensory impairments(hearing and vision),and musculoskeletal disorders (e.g.,osteoporosis and orthopedic complications caused by joint instability).23

2.Physical fitness:cardiovascular,muscle,and body composition

2.1.Cardiovascular fitness

2.1.1.Cross-sectional studies

Uniformly,peak aerobic capacity(VO2peak)in both youth and adults with DS is reduced in comparison to their peers without disabilities(WOD)and with ID but without DS.25—27This is accompanied by lower peak work rates as well asa faster time to exhaustion(Table 2).27Such findings were first reported by Eberhard et al.28in 1989,whereby they demonstrated a 15%lower VO2peakusing bicycle ergometry compared to age-matched children WOD.In 1990,Fernhall et al.,26using a validated treadmill protocol,reported lower VO2peakin adolescents with DS when compared to adolescents WOD and that lack of motivation or understanding protocol instructions in participants with DS did not contribute to their findings.

Following these initial contributions,the literature has consistently demonstrated lower VO2peakvalues when individuals with DS are compared to individuals WOD and with ID but without DS.In the largest sample to date(totaln=635;n=133 with DS,n=180 with ID,n=322 WOD),Baynard et al.25reported that VO2peak(absolute and relative values)is lower in children and adolescents with DS through young adulthood and into middle age,regardless of the comparison group.Furthermore,the authors also demonstrated VO2peakdoes not significantly change after the age of～16 years.25Interestingly,the average VO2peakat this age in individuals with DS is similar to normative values found in middle-aged to older persons WOD,yet individuals with DS do not appear to experience the age-related decline in VO2peakas is expected in those WOD(Table 2).25

Three physiologic factors that potentially contribute to low VO2peakvalues in persons with DS are autonomic dysfunction, reduced ventilatory capacity,and metabolic dysfunction. When considering autonomic dysfunction,lower peak heart rate(HR)responses have been consistently reported across all age groups for persons with DS.25The cause of lower peak HR in persons with DS is still not completely understood at this time,but Fernhall et al.29demonstrated in a group of youngadults with DS(mean age 24 years)a blunted to non-existent catecholamine response to peak exercise.This suggests that some portion of the autonomic pathway is dysfunctional and contributes to the lower HR response.Although diminished sympathoexcitation in adults with DS has been reported,30,31to date no studies have included youth with DS(Table 2). Further,with higher obesity levels generally observed in persons with DS,the interaction between autonomic control and obesity may be an important avenue to investigate in this population.

Table 2 Physical fitness in youth with Down syndrome(DS).

It was also postulated that due to the relatively large tongues(macroglossia)compared to the bony confines of the oral cavity in persons with DS,ventilation would be restricted at high work levels and thus limit peak performance.Indeed, adolescents and young adults with DS have been reported to have low peak ventilation.26However,Fernhall and Pitetti27concluded that peak ventilatory parameters(e.g.,ventilatory equivalents,and peak minute ventialation)were appropriate for a given VO2peak.In addition,imaging work has established that children with DS do not have true macroglossia,although they have relatively large tongues compared to their oral cavity volume.32Therefore,ventilation capacity does not appear to restrict the peak performance of youth with DS. However,gaps do exist in the literature with little to no data available for additional ventilatory parameters,such as peak minute ventilation/maximal ventilatory volume or peak tidal volume/ventilatory capacity.This type of data would broaden our understanding of potential limitations to exercise in persons with DS.

Metabolic limitations have also been suggested to constrain the physical capacity of youth and young adults with DS. However,only two studies actually measured ventilatory threshold in adolescents and young adults with DS.33,34Collectively,these two studies reported that ventilatory threshold was difficult to detect(e.g.,V-slope method); however,in those participants where it was detectable,normal ventilatory thresholds were observed when expressed as a percentage of VO2peak.Given that the respiratory exchange ratio is related to substrate utilization and exercise intensity, collection of blood lactate would assist in determining the role that metabolic function may have in limiting exercise performance in youth with DS.However,submaximal and maximal exercise blood lactate levels have not been reported in the literature for youth with DS.In line with this,supramaximal testing(e.g.,Wingate testing)is not reliable enough to date to help complete a metabolic profile in youth with DS.35

It is important to note that several studies using field tests also report lower predicted VO2peakand/or exercise performance in youth and young adults with DS compared to peers WOD(Table 2).36,37The largest field study to date to measure run performance(20-m shuttle run)had 119 youth(11—18 years)with DS,394 youth with ID but without DS,and 80 youth WOD.38Children and adolescents with ID but without DS had better run performance than their peers with DS independent of age,sex,and body mass index(BMI).38Furthermore,those WOD had better run performance than their peers with ID,again independent of age,sex,and BMI.38Thus,poor running performance of children and adolescents with ID,with and without DS,is not a consequence of age, sex,or BMI.

Furthermore,Fernhall et al.39,40produced a regression equation from the results of the 20-m shuttle run to predict VO2peakin children and adolescents with ID,with and without DS.This equation was cross-validated in 2003 by Guerra et al.,41however,this study consisted of a greater percentage of children and adolescents with DS and they were not able to validate Fernhall’s39,40original equation designed for persons with ID to a sample with DS alone.The prediction of VO2peak, from a 20-m shuttle run test,was further examined by Agiovlasitis et al.,42whereby they reported that while shuttle performance was a predictor of VO2peakin youth with DS,the equation had low predictability at the individual level. Therefore,the use of the regression equation developed by Fernhall et al.39,40to predict VO2peakin persons with ID does not appear to be specific for youth with DS.

2.1.2.Training studies

There are relatively very few longitudinal training studies in persons with DS,particularly in youth.For this reason, training studies in young adults with DS will be utilized here with the understanding that it would be likely that youth with DS would experience similar findings.A meta-analysis was conducted by Dodd and Shields43that included four papers, which met their inclusion criteria.44—47They reported that aerobic training programs,which conformed to the American College of Sports Medicine guidelines,were effective for improving VO2peak,peak ventilation,time to exhaustion,and/ or maximum workrate.43

One of the first training studies in adolescents with DS was conducted by Millar et al.,45with 14 individuals participating in a 10-week walking/jogging program,3 days/week at 65%—75%peak HR,which resulted in no change in VO2peak,but did improve treadmill test time by 9%.45Training studies that have measured VO2peaksince the report by Millar et al.45have supported those initial findings,in that improvements have been seen in exercise/work capacity without significant improvements in VO2peakwith training interventions lasting 12—16 weeks(Table 2).9,48Further evidence of positive training effects stems from an often overlooked study, whereby HR recovery following a 3-min step test and resting HR were decreased in a group of 10 individuals with DS (8—18 years)after a 13-week training program.49Lastly,an interesting case study reported that a young 10.5-year-old female with DS following 6 weeks of training did not alter her estimated VO2peak,but her submaximal HR was lower during the treadmill test,as was her respiratory rate,indicating improved work capacity and capacity for change.50

While aerobic training studies are limited in persons with DS,let alone youth,previous work suggests exercise training is beneficial in youth with DS even if VO2peakitself is not necessarily an expected outcome.It is especially important to consider training studies in youth with DS in that early intervention to improve their work/exercise capacity could impact the health anomalies specific to the condition of DS.

2.2.Muscular strength

Few studies exist that have employed strength testing and/ or training in youth with DS.Strength decrements were first characterized in youth with DS in 1994 by Cioni et al.,51in which children and adolescents with DS(n=25)exhibited weak knee extensor strength compared to a WOD control group.Furthermore,these authors concluded that adolescents with DS generally do not show improvements in strength beyond the age of 14 years.51These findings can be extended to the hip abductors as well.52Interestingly,Pitetti and Fernhall53explored the relationship between VO2peakand strength in youths aged 10—17 years with ID(n=29),including eight children with DS,and found a significant relationship between knee flexion/extension strength and aerobic capacity (r=0.56—0.62).Their data suggest that leg strength may be a limiting factor in both work and aerobic capacity in persons with DS,which is also supported by similar work in adults (Table 2).54

There is a small body of literature in young adults with DS (mean age～24—25 years)that further extends the limited findings in youth.Several studies have demonstrated lower muscle strength in young adults with DS for both knee,and/or elbow extensors/flexors compared to participants WOD.55—58In those studies that distinguished between individuals with ID with or without DS,no general differences in strength were observed between these two groups,whereas the control group WOD was always stronger(65%—225%stronger).However,it should be noted that while not different,Croce et al.57have reported lower hamstring to quadriceps ratios in adults with DS(20%)vs.those WOD and 10%lower compared to adults with ID without DS.This may suggest reduced muscular performance and knee joint stability.

The number of strength training intervention studies that included individuals with DS is even smaller,with two studies known to have specifically trained youth with DS.59,60Weber and French59found that resistance training improved overall muscular strength in adolescents with DS(e.g.,10 muscular tests performed),yet this study lacked a control group. Recently,Shields and Taylor60demonstrated that a 10-week community-based progressive resistance training intervention was successful in increasing lower limb strength(as well as muscle endurance)in 23 adolescents with DS,with no changes in upper limb strength.Unfortunately,no measure of work and/or aerobic capacity was performed in these studies.59,60Lastly,a case study demonstrated that a young girl with DS improved general muscle strength(e.g.,trunk,hip,knee,and shoulder)following a 10-week combined aerobic and resistance program.50These data collectively suggest that strength training in youth is beneficial.Little to no data exist on the relationship between strength training and improving work capacity in youth with DS(Table 2).A recent study by Cowley et al.61reported a small,but significant increase in relative VO2peakfollowing a 10-week strength training program in young adults with DS,yet no difference in absolute VO2was observed,suggesting this improvement was mediated by weight loss(～5 kg).

A group of progressive resistance exercise training studies clearly demonstrate that strength training is a safe and important component of exercise prescription in young adults with DS(24—29 years).61—64These studies lasted 10—12 weeks and all employed a 2—3 days/week training of large muscle groups,with improvements found in both upper-and/ or lower-body strength.Interestingly,Cowley etal.61concomitantly measured aerobic capacity and reported no change in absolute VO2peakand a small but significant decrease in relative VO2peak(Table 2).

2.3.Body composition

Equivocal evidence exists regarding body composition in persons with DS.The lack of consistency may involve methodological issues for measuring body composition(e.g., skinfoldsvs.dual energy X-ray absorptiometry)or perhaps comparing weight status using different vehicles,such as a laboratory measures versus BMI.Another possible contributor may be ethnic and/or environmental issues as well. Caution is urged when interpreting global statements on body composition in DS for these reasons.

Numerous investigations have reported that the prevalence of overweight and obesity are substantially higher in individuals with DS compared to their age-matched peers WOD and with ID but without DS.65,66Prasher67reported that～48%of adults with DS were obese,with～27%being overweight (both sexes averaged)and that being overweight and/or obese was associated with living at home compared to group-home situations.This is consistent with Rubin et al’s65report of～48%for men and～56%of women being overweight/ obese.Interestingly,BMI was found to decrease throughout the lifespan in males and females with DS,whereas BMI increases through the lifespan in the general population.67This is in contrast to Baynard et al.,25whereby they reported BMI increases not only in the control group,but also in groups of ID,with and without DS,up through middle-age.Prasher67was unable to adequately explain why BMI would decrease from young adulthood(16—19 years)through the 20 s into the 60 s.Perhaps the Alzheimer’s issues that individuals with DS often experience could provide a possible explanation.

Early work by Sharav and Bowman68demonstrated that young children with DS(～4—5 years)were not different from their siblingsWODinBMI.Additionally,Luke et al.69reported non-significant but higher BMI and lower fat-free mass in children with DS(9 years old)versus a control group WOD, whereas other investigators have observed higher BMIs and/or percentage of body fat in youth with DS compared to peers WOD.52,70However,these studies also included adults in their samples.52,70

In contrast to Luke et al.,69others have observed lower total muscle mass in persons with DS versus individuals WOD.70This has been further substantiated recently in Spanish youth with DS that did exhibit higher body fat content and lower lean mass than peers WOD.71In Greece,researchers observed that 22%of adolescents in their sample were obese and that percent body fat,BMI,fat mass and fat-free mass were also greater in adolescents with DS(10—18 years)than in children with DS (2—9 years).72It is important to note that several studies do suggest that children and adults with developmental disabilities in the United States and Australia have greater prevalence rates of overweight and obesity compared to non-disabled agematched controls.Yet these studies only have a small portion of individuals with DS included.65,66However,the prevalence of overweight/obesity has recently been questioned and it may not be as high as previously thought.73It is also possible international differences exist that need further examination.More work will need to be conducted not only at the physiological level,but work at the environmental,community,societal and psychological levels to fully delineate the numerous factors involved in obesity(Table 2).

3.Physical activity(PA)in youth with DS

Apart from physical fitness,PA has the potential to improve health in youth with DS.Although the relationship of PA and health outcomes has not been directly examined in youth with DS,it is reasonable to assume that the findings in the general population of youth also apply to those with DS.PA may improve the cardiovascular,metabolic,musculo-skeletal,and psychosocial health profiles of all youth.74—76

For this reason,the 2008 Physical Activity Guidelines for Americans devoted a section to children and adolescents.76Specifically,it is recommended that youth older than 6 years perform 60 min of PA daily.Most of aerobic activity should be of either moderate or vigorous intensity,defined as 3.0—5.9 and＞5.9 metabolic equivalent units(METs),respectively; however,vigorous-intensity activity should be performed at least 3 days/week.Muscle-and bone-strengthening activities are important components of the recommended 60 min of daily activity and each should be performed at least 3 days/ week.Importantly,the Guidelines call for the promotion of physical activities that are age-appropriate,enjoyable,and offer variety.Recent evidence,however,suggests that most American youth—especially adolescents—do not meet the required amount of daily PA and that their activity levels decline as they grow.77The issues of interest therefore are:(a) whether the PA levels of youth with DS differ from those of youth WOD,and(b)whether youth with DS meet the current recommendations for PA.

Answering these questions is difficult because research on PA in youth with DS is limited.In a review of research published prior to 2008,78the authors could not conclude whether the PA levels of youth with ID,including those of youth with DS,are different from those of children WOD.As the authors explained,this was due to methodological problems of previous research such as insufficient description of samples and questionable applicability of objective or subjective measurements of PA to youth with ID.

3.1.Measurement of PA in youth with DS

Subjective PA assessments using questionnaires are not as accurate as objective ones.77This difficulty is magnified inyouth with DS by the need to use parental or caregiver/teacher proxy reports.78One study demonstrated questionable accuracy of PA measurement by proxy reports for adults with ID.79It is possible that subjective assessment of PA in youth with DS may benefit if questionnaires are obtained from both parents and teachers or caregivers;such practice may offer a more accurate representation of the activities youths with DS engage in throughout the day.Nevertheless,to our knowledge, the psychometric properties of subjective measures have not been examined in youth with DS.Finally,in-depth interviews—typically with parents—do not allow for a quantification of the amount of PA performed by youth with DS.

Objective PA measurementswith accelerometersor pedometers are preferable,but they should be used carefully in youth with DS.Pedometers appear valid and reliable during locomotion in youth with intellectual or developmental disabilities,80,81but their accuracy may be compromised during games and sport activities conducted in physical education.82Additionally,spring-levered pedometers are less accurate than piezoelectric in people with DS.83Furthermore, using step-rate cut-points to estimate moderate-and vigorousintensity activity in youth with DS may be problematic.Adults with DS have lower step-rate cut-points for activity intensity even when accounting for their shorter height compared to adults without DS84—this may potentially apply to youth with DS.Similarly,accelerometry-determined cut-points for moderate-and vigorous-intensity activity developed for youth WOD may not be applicable to youth with DS.This has been demonstrated for adults with DS85and it has been suggested to be partially due to an altered gait pattern that increases the energy expenditure.86It is possible that this also applies to youth with DS who also exhibit altered gait patterns,87,88potentially due—at least in part—to deficits of the cerebellum.89Additionally,cut-offs developed in youth WOD may be inappropriate for those with DS because the latter have very low cardiovascular fitness.Finally,there have been some reports of compliance issues with accelerometers secured at the hip in youth with DS.90,91Although these difficulties are not extensive,they highlight the need for adequate familiarization and parental supervision.Alternately,researchers could consider other placement sites,such as the wrist,where the accelerometer could be secured with an irremovable strap. These measurement issues should be collectively considered when evaluating previous reports of PA in youth with DS.

3.2.What are the PA levels of youth with DS?

In infants with DS,movement—especially of the legs—may be important for motor development.Children with DS appear to reach stages in motor development with some delay.92A notable example is the onset of independent walking which occurs about a year later(～age 2)in children with DS than children with typical development.93,94It is possible that low levels of motor activity during early infancy may contribute to this phenomenon.One study found that, during the first 6 months of life,infants with DS show lower levels of general movements as measured by observation than infants with typical development.95In contrast,analyzing video recordings,others reported no differences between infants with and without DS in the total amount of leg movements93which are presumably more relevant for the development of walking.Subsequently,infants with DS were found to show more low-intensity and less high-intensity leg motor activity than infants with typical development.96,97Furthermore,greater amount of high-intensity activity at about 1 year of age was associated with earlier walking onset in infants with DS.96,97Finally,a randomized controlled study showed that infants with DS,who underwent home-based stepping training while supported over customized treadmills, achieved independent walking by an average of 101 days earlier than infants with DS not undertaking this intervention; the intervention was initiated once infants with DS could sit independently and it was conducted 5 days/week,8 min/day, until the onset of independent walking.94Taken together,these data suggest that alterations in PA patterns in infants with DS are associated with their motor development,thus supporting the need for early interventions.96

The objective data on PA levels of children and adolescents with DS are limited and provide somewhat conflicting results. Non-resting total energy expenditure measured with doublylabeled water did not differ between 5-and 11-year-old children with DS from the U.S.and peers WOD of similar age and BMI.98In another U.S.study,the amounts of total,lowintensity,and moderate—intensity activity,as well as time spent sedentary measured with hip-accelerometry over 7 days did not differ between children with DS aged 3—10 years and similarly-aged siblings WOD;90however,the children with DS participated in less and shorter bouts of vigorous-intensity activity than their siblings.Notably,children both with and without DS in that study exceeded the recommendations for totaland vigorous-intensity aerobic activity.Similarly, Australian children and adolescents with DS met on average the recommended amounts of moderate-and vigorous-intensity activity as measured by hip-accelerometry91and also appeared more active than U.S.youth.77But there was large between-person variability in activity levels and 58%of the children with DS did not meet the recommended amount.In the same study,youths with DS aged 13—17 years showed lower total,moderate-,and vigorous-intensity activity than those aged 7—12 years,indicating a possible age-associated decline in PA.Recent acceleromety-derived data from the U.S. demonstrated that,on average,children and adolescents with DS did not engage in moderate-to-vigorous PA for 60 min/ day.99Furthermore,PAwas lower and sedentary behavior was higher in older youths with DS compared to younger ones. Another U.S.study using accelerometry100showed that,prior to an intervention,youth with DS aged 8—15 years were active at moderate-to-vigorous levels for an average of about 43 min, although there was significant between-people variation in this value.Finally,cross-sectional accelerometry-determined data from England demonstrated lower amount and faster ageassociated decline of moderate-to-vigorous activity in people with DS than people with ID but without DS across the lifespan(Table 3).101

Table 3 Physical activity in youth with Down syndrome(DS).

Some subjective data—obtained with parental reports or indepth interviews—supplement the literature on PA of youth with DS.Canadian children with DS aged 2—14 years were rated by their parents as less active and as spending more time indoors compared to siblings without DS,68although all youth in this study with DS participated in organized sports at least once per week.69Among 38 Australian youth with DS aged 11—18 years,only two engaged in PA more than 3 days/ week—a finding suggestive of low PA levels.102In a second Australian study,less than one third of 208 youth with DS aged 5—18 years were found to be active at moderate-tovigorous intensity for 60 min/day,although most participated in sports.103Finally,during in-depth interviews,parents of U.S.preschoolers with DS perceived their children as naturally active.In contrast,parents of elementary-school students perceived a gradual decrease in their children’s interest for PA, supporting the possibility for an age-associated decline among youth with DS.104

The existing objective and subjective data do not allow us to conclude with confidence whether youth with DS have lower PA levels than youth WOD,but this appears likely. Additionally,it is not known if youth with DS meet the required amounts of muscle-and bone-strengthening activities.Although there are no longitudinal data on the developmental course of PA in youth with DS,it is likely that PA declines with age in this population.Furthermore,a large proportion of children and adolescents with DS may not meet the recommended amount of daily aerobic activity.

3.3.Determinants of PA

The study of the determinants of PA in youth with DS is still in its infancy.Following the approach of the International Classification of Functioning,Disability and Health,105PA may be influenced by a child’s health status,functional profile, participation in life activities,and contextual factors—either within the person or in the environment.DS is associated with many health conditions,but there is great variability in the number and extent of conditions that youth with DS exhibit.In one study,medical issues were not associated with function among youth with DS.106It is reasonable,however,to assume that acute health problems might present barriers to PA for youth with DS and that,when medical issues are efficiently overcome,PA may be facilitated.

The functional profiles of youth with DS may also be related to their PA levels.Quantitative and qualitative data suggest that lower cognitive,behavioral,and motor skills may present barriers to PA of youth with DS.103,104,107,108Some theorists also argue that people with DS may have a tendency for slower,safer,and more accurate movement patterns due to interactions between neurological,environmental,and taskassociated constraints.109Additionally,the very low aerobic and muscular fitness of youth with DS may potentially affect their involvement in physical activities,especially of higher intensities,but this has not been directly examined(Table 3).

Contextual factors within the person may also be at play.In support of this argument,the aforementioned lower participation in vigorous-intensity activities of children with DS compared to their siblings90was collectively explained by age, sex,race,ethnicity,income,maternal education,and BMI.It is difficult,however,to determine the individual contributions of these factors to PA of the children in that study.As previously discussed,PA in youth with DS may decline with age,91,101,104although one study did not find differences in sport participation between children and adolescents with DS.103Furthermore,whether PA differs between sexes in youth with DS,as in youth WOD,74,77is not known.Data from Taiwan,China showed no difference in recreational and sport participation between boys and girls with DS.108In contrast, another study conducted in Taiwan,China demonstrated that, among youth with ID including DS,boys and those who had positive attitudes towards exercise were more likely to be regularly active after school.110

It is also very likely that the social and physical environment contributes to the PA levels of children and adolescents with DS(Table 3).This is supported by a wide variation in types of leisure participation among youth with DS around the world.68,102,103,108,110,111Furthermore,it has been argued that inactivity among youth with DS may be a learned behavior partially resulting from exclusionary practices.112Importantly, lack of accessible,inclusive,and appropriately-designed programs,as well as negative attitudes,transportation problems,competing family responsibilities,parental educationand income,and lack of friends,all reportedly present barriers to PA for youth with DS.90,104,107

3.4.PA promotion

Youth with DS reportedly participate in a wide range of recreational activities.Examples include walking,swimming, bowling,dancing,and team sports.68,102,103,108,110,111The types of leisure participation among youth with DS likely vary around the world;for example,swimming appears common in Australia,102,103ice-skating in Canada,68and bicycling in Taiwan,China.108Notably,walking is one of the most commonly performed activities among youth with DS,108,110,111suggesting that their altered gait pattern87,88may not present a barrier to PA.Supporting this proposition,children and adolescents with DS exhibit functional independence in locomotion tasks.106Therefore,youth with DS have the potential to participate in all types of culturally-relevant physical activities.

Unfortunately,very little data exist on interventions to promote PA in youth with DS.A randomized-controlled study found that youth with DS who learned to ride a bicycle increased their PA levels about 1 year after the intervention,100suggesting that motor skill development may improve longterm PA.Furthermore,well-designed school programs allow students with ID including those with DS to meet the PA recommendations.113It should also be considered that 12 weeks of exercise training,combined with health education, improves the attitudes towards exercise and psycho-social well being of adults with DS,114but,to our knowledge,similar studies in youth with DS have not been conducted(Table 3).

It has been recommended that PA promotion in all youth aimed at ameliorating the age-associated decline in PA,suppressing the development of pathological processes,and creating life-long PA habits.74To achieve these goals in youth with DS,PA promotion should be multi-factorial.Welldesigned programs should take into account the physical, cognitive,and psycho-social health profiles of children with DS as well as their need for enjoyment and participation.But at the same time,the environmental contexts in which PA, exercise,and recreation occur must be appropriately designed. Parents tell us that the PA of their children with DS may be facilitated by positive familial and social attitudes,and structured programs—ones that are adapted to the abilities of their children,offer youth with DS knowledge on PA and health, utilize their determination to succeed,and promote social interactions and enjoyment.104,107,111

4.Summary and future directions

4.1.Physical fitness

Youth with DS have low peak aerobic capacity coupled with low peak HR,suggesting autonomic dysfunction may be a primary reason explaining low work performance in this population.Furthermore,persons with DS also have low muscular strength compared to individuals WOD.Exercise training utilizing endurance and/or resistance exercise appears beneficial for youth with DS.The role of overweight/obesity in youth with DS does not currently explain lower aerobic capacities in persons with DS,yet more work needs to be conducted to completely understand its role with regards to aerobic capacity and PA in this population.Studies involving multi-factorial issues(e.g.,physiological,environmental,etc.) common to youth with DS are necessary to understand how to optimize quality of life in this population.

4.2.PA

Cross-sectional data suggest that PA likely declines with growth in youth with DS and that a large proportion of these youths do not meet the recommended amount of daily aerobic activity.Effective PA promotion should be multi-factorial—it should be adapted to the physiological,cognitive,and psychosocial profiles of youth with DS,but should also consider environmental modifications.There is a need to improve the assessment of PA with objective and subjective approaches. Furthermore,longitudinal research is needed to examine how PA levels change and what factors contribute to them during growth in youth with DS.Finally,it is also important to examine the effectiveness of specific interventions aimed at increasing PA.

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Received 8 July 2012;revised 7 September 2012;accepted 26 September 2012

*Corresponding author.

E-mail address:ken.pitetti@wichita.edu(K.Pitetti)

Peer review under responsibility of Shanghai University of Sport

2095-2546/$-see front matter Copyright?2012,Shanghai University of Sport.Production and hosting by Elsevier B.V.All rights reserved. http://dx.doi.org/10.1016/j.jshs.2012.10.004