Assessment of cardiorespiratory and neuromotor fitness in children with developmental coordination disorder

Research in Developmental Disabilities 35 (2014) 3554–3561

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Research in Developmental Disabilities

Assessment of cardiorespiratory and neuromotor fitness in children with developmental coordination disorder Faic¸al Farhat a,*, Kaouthar Masmoudi b, John Cairney c, Ines Hsairi d, Chahinez Triki d, Wassim Moalla a a

Research Unit EM2S: Education, Motricite´, Sport et Sante´’’ ISSEP, Sfax, Tunisia Service d’Explorations Fonctionnelles, Unite´ d’Effort Cardio-pulmonaire, Hoˆpital Habib Bourguiba, Sfax, Tunisia c Departments of Family Medicine and Kinesiology, The Infant Child Health (INCH) Research Lab, and The CanChild Centre for Studies in Childhood Disability, McMaster University, Hamilton, Ontario, Canada d Research Unit Neuropediatry UR.0805, Hedi Chaker Hosıˆtal Faculty of Medicine, Sfax, Tunisia b

A R T I C L E I N F O

A B S T R A C T

Article history: Received 23 May 2014 Received in revised form 14 August 2014 Accepted 20 August 2014 Available online 19 September 2014

The decreased participation in physical activity by children with probable developmental coordination disorder (pDCD) has raised concerns about their aerobic fitness and lung function levels. The purpose of the present study was to examine assessment of cardiorespiratory and neuromotor fitness, using laboratory-based tests during an incremental treadmill protocol in healthy children with and without pDCD. Twenty sex children ages 6–9 years took part in this study. Motor coordination was assessed using the Movement Assessment Battery for Children (MABC). All participants performed a cardiopulmonary exercise test (CPET) on a cycle ergometer. Pulmonary function was assessed by spirometric measurements (forced vital capacity: FVC, forced expiratory volume in 1 s: FEV1) and walking distance (6MWD) was assessed using the 6-min walking test. The children with pDCD had lower VO2 max than children without pDCD (p < 0.01). Moreover, FVC and FEV1 were significantly higher in children without pDCD than in children with the disorder (p < 0.05, p < 0.01 respectively). Likewise, children with pDCD had poorer performance on the 6MWD than children without pDCD (p < 0.01). A significant correlation between the absolute value for FEV1 and 6MWD (r = 0.637, p < 0.05) in pDCD group was observed. We found a significant correlation between VO2 max and MABC score (r = 0.612, p < .001) and between VO2 max and 6MWD (r = 0.502, p < .001) for all children. Moreover, a significant correlation between VO2 max and FEV1 (r = 0.668, p < .05) was found in children with pDCD. Overall, the reduced aerobic capacity of DCD was associated with decreased of lung function, as well as an alteration of peripheral muscle responses. ß 2014 Elsevier Ltd. All rights reserved.

Keywords: Developmental coordination disorder Pulmonary function Aerobic fitness 6-Min walking test

1. Introduction The diagnosis of developmental coordination disorder (DCD) is based on performance level of movement skills substantially below that expected for a child’s chronological age and measured intelligence (American Psychiatric Association, 1994). The decreased participation in physical activity by children with DCD has raised concerns regarding their

* Corresponding author. Tel.: +216 24374148; fax: +216 74 444 014. E-mail address: [email protected] (F. Farhat). http://dx.doi.org/10.1016/j.ridd.2014.08.028 0891-4222/ß 2014 Elsevier Ltd. All rights reserved.

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aerobic fitness levels. In fact, children with DCD are frequently found to have lower fitness levels when compared to their typically developing peers (Cairney, Hay, Faught, Flouris, & Klentrou, 2007; Haga, 2007). Children with DCD experience difficulty performing a variety of motor tasks, which significantly effects their involvement in activities of daily living as walking, running, climbing and jumping (Wu, Lin, & Li, 2010). As a result of their coordination difficulties, many children with DCD are less physically active (Bouffard, Watkinson, Thompson, Causgrove Dunn, & Romanow, 1996; Cairney et al., 2005), and much more likely to be sedentary than children without DCD (Cairney, Hay, Veldhuizen, & Faught, 2011). Previously, it has been reported that children with DCD may, in addition to their motor problems, develop lower levels of health-related physical fitness (Rivilis et al., 2011). However, much of this research examined differences between children with low motor coordination and typically developing children in non-clinical settings. To date, the level of physical fitness for children who are clinically diagnosed with DCD and referred to treatment is unclear, even though DCD can cause decreased participation in physical activity and below average performance on different components of physical fitness (Haga, 2009). One particular component of poor fitness strongly correlated with health is aerobic power, which is consistently found to be lower in children with DCD (Cairney et al., 2011; Wu et al., 2010). The gold standard for assessing cardiopulmonary fitness is maximal oxygen uptake (VO2 max) measured during cardiopulmonary exercise test (CPET) until exhaustion according to standardized protocols (Chia, Guelfi, & Licari, 2009). The CPET can be performed in field or in laboratory-based settings. Recent research has shown that estimates of peakVO2 obtained in both settings show that children with DCD have significantly lower aerobic fitness than typically developing children regardless of the test used (Cairney, Hay, Veldhuizen, & Faught, 2010). DCD has been identified as a chronic health condition, which is associated with a decrease in cardiopulmonary function, as well as an alteration of peripheral muscle responses (Van der Hoek et al., 2012). Therefore, sub-maximal exercise testing may provide a safe, practical means of evaluating functional status, monitoring treatment effectiveness and establishing prognosis. Recently, the development of field tests, such as walking tests, can be used to measure the functional exercise capacities of healthy or unfit subjects. The 6-min walking test (6MWT) has emerged as a common sub-maximal test in clinical settings, and recent normative data have extended its application (Li et al., 2005). The 6MWT has shown good validity and reliability, and is considered to be a clinically relevant test because it closely resembles common physical activities of daily living (walking), and cab be used to estimate cardiopulmonary fitness in healthy children (Lesser et al., 2010) and in children with various diseases (Solway, Brooks, Lacasse, & Thomas, 2001). The 6MWT may be a particularly useful submaximal test for children with DCD because the motor coordination demands of the test are minimal. Children with DCD, especially those with balance problems, may find treadmill running difficult. Pedaling (cycle ergometer) and line contact pivots on shuttle run tests may also be difficult for children with DCD. Compared to these other tasks, walking may be easier for children with motor coordination difficulties. Previous studies have shown that lung function is directly related to cardiopulmonary fitness (Eisenmann et al., 1999). For example, Jakes et al. (2002) showed that adults without regular physical activities had generally poorer lung function. Like sub-maximal testing, assessing lung function in children with DCD might provide a safe and efficient means of assessing aerobic fitness. Interestingly, only one study has examined indices of lung function and VO2 max in children with DCD (Wu, Cairney, Lin, Li, & Song, 2011). To the best of our knowledge, the possibility of a relationship between 6MWT, FVC, FEV1 and VO2 max has never been verified. Therefore, the purpose of the present study was to examine, through a cross-sectional study, cardiorespiratory and neuromotor fitness using laboratory-based tests during an incremental treadmill protocol in children with and without DCD. 2. Materials and methods 2.1. Participants The study has been approved by the Ethical local Committee of the University Hospital of Sfax. To recruit participants for the lab-based component of the current study, we received permission from 3 of 5 primary schools (60%) from a middle-class region in Tunisia; After being screened with the Movement ABC test (Henderson & Sugden, 1992), children were categorized into one of two groups: DCD (17 boys and 4 girls) and Without DCD (21 boys and 7 girls). All children with DCD were asked to participate in the laboratory test. Four children (2 boys and 2 girls) were excluded because they have intellectual impairment. In addition, four children (2 boys and 2 girls) did not attend the assessment session. However, thirteen boys without DCD ranging between 6 and 9 years were randomly selected from 28 children. Finally, 26 boys (13 DCD and 13 without DCD) attended laboratory test to assess their maximal cardiopulmonary function. Prior to testing, the protocol was explained in detail to the subjects and their parents. After this, all subjects signed a written informed consent in accordance with the principals outlined in the Declaration of Helsinki in 1975. The subjects were examined by a physician to ensure their health was sufficient to participate in testing and to rule out the presence of chronic diseases. Children were assigned to the DCD group if they had difficulties with daily living skills as assessed using parent questionnaires and clinical interviews. Inclusion criteria included no intellectual impairment; no diagnosed emotional, neurological, or motor disorder, and no intervention during the past 3 months that affected leisure participation patterns. Children were excluded if a history of learning difficulties or any behavioral or orthopedics problems were reported. Because the pulmonary function test was conducted in this study, children with any cardio-respiratory diseases (cystic fibrosis, asthma) or acute respiratory infection prior to 1 month of data collection were also excluded. As controls,

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children with normal development were recruited from the community on a volunteer basis. They were subject to the same inclusion and exclusion criteria set for the DCD group, except that they did not have any history of DCD. The subjects were examined by a physician to ensure their health was sufficient to participate in testing and to rule out the presence of chronic diseases that might impact physical activity. All participants reported no problems with hearing or vision. 2.2. Procedure We started by collecting socio-demographic and anthropometric data. Subjects completed testing over three consecutive sessions on different days. During the first session, each subject was tested on the MABC (Henderson & Sugden, 1992), a standardized test for children aged between 4 and 12 years, which have been validated on children with and without coordination problems (Wright & Sugden, 1996). We have elected to use the term probable DCD (pDCD) to identify children scoring below the 15th percentile on the MABC (Poulsen, Ziviani, Cuskelly, & Smith, 2007). During the second session, the Differential Scales of Intellectual Efficiency Test (EDEI-A) is a reliable measure of intelligence that can be administered by health professionals for Tunisian children (Ben Rejeb, 2001). It is a cognitive screening tool that measures two areas of cognitive functioning (verbal and non-verbal). It has been recommended that children with an estimated intelligence quotient below 70 should be excluded from a diagnosis of DCD as their movement may be discrepant with this cognitive ability (Sugden, 2006). Next, all subjects completed the 6MWT test. In the third session, pulmonary function test was assessed using a body plethysmograph and the CPET using cycle ergometer was conducted. 2.3. Measurements 2.3.1. Demographic questionnaire A demographic questionnaire, including questions about family socio-demographic status, child’s health status, medications, treatments and para-medical therapies was completed by parents. 2.3.2. Anthropometry Height and weight were measured in the laboratory with the child dressed in light clothing and without shoes. Height was measured to the nearest 0.5 cm using a fixed stadiometer (Seca, Hultafors, Sweden) and body mass (weight) was measured to the nearest 0.5 kg with a electronic device (Avery Berkel model HL 120; Avery Weigh-Tronix Inc., Fairmont, MN, USA). Body mass index (BMI) was calculated as body mass in kilograms divided by height in meters squared (kg m 2). 2.3.3. The Movement Assessment Battery for Children The MABC is an extended version of the Test of Motor Impairment (Stott, Moyes, & Henderson, 1985). It was designed to better identify children with movement difficulties. We used the MABC (Henderson & Sugden, 1992) as the criterion test of motor competence, assessing both gross and fine motor coordination in children aged between 4 and 12 years. It is a formal standardized test that provides both a quantitative and a qualitative evaluation of the child’s motor competence in daily life. The psychometric research performed on the MABC reported that is a reliable and useful assessment (Crawford, Wilson, & Dewey, 2001). Recently it has been found to identify more children than other motor tests and appears to identify more readily those children who have additional learning or attention problems (Crawford et al., 2001). The MABC gives an estimate of motor competence in terms of speed and/or accuracy (outcome of movement). The MABC test contains 8 items in any one of four age bands (4–6, 7–8, 9–10, and 11–12 years of age). The eight items that are used to assess children ages 7–8 years include: placing pegs in a peg board, threading a lace, and drawing a line into a trail (manual dexterity); bouncing and catching a ball with one hand and throwing a bean bag into a box (ball skills); standing on one leg (static balance), jumping in squares, and heel-to-toe walking on a line (dynamic balance). The total impairment score (TIS), ranging from 0 to 40, is the sum of converted raw scores on the eight items that each child attempts during a formal assessment. A lower score represents a better task performance. The total impairment score can be converted into a percentile score: scores at or below the 5th percentile indicates a definite motor problem and a score at or below 15th percentile indicates a clinical risk range that should be monitored (Henderson & Sugden, 1992). The MABC has sufficient reliability, with a minimum test–retest at any age of 0.75 and inter-rater reliability of 0.70 (Henderson & Sugden, 1992). Studies on validity have shown 80% agreement between the MABC and Bruininks–Oseresky Test of Motor Performance (Crawford et al., 2001). 2.3.4. 6-Min walking test (6MWT) The 6MWT was conducted at the hospital on a flat surface in a 30-m-long covered corridor marked every 2 m. The test was conducted according to the recommendations of the American Thoracic Society (American Thoracic Society, 2002). To eliminate the learning effect, we conducted the 6MWT twice. A practice test was conducted first, then after a period of at least 1 h, the second test was conducted. We report the higher distance of the two tests. Participants were instructed to walk the longest distance possible at their own pace during the allotted time. Individuals were allowed to stop and rest during the test, but were instructed to resume walking as soon as they felt capable of doing so. Standardized encouragements (for example, ‘keep going’, ‘you are doing well’) and announcement of time remaining were given to all children. Before

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beginning the test, the participants sat on a chair located near the starting position for at least 10 min to determine the rest heart rate (HR) values and oxy-hemoglobin saturation (finger pulse oxymeter; Nonin Medical, Inc., Minneapolis, MN, USA). These parameters were also recorded every minute during and 5 min after the test. Dyspnea scores were measured on a Borg scale at the end of the test (Borg, 1982). The subject indicated the number corresponding to the perception of his effort and his feeling of breathlessness. 2.3.5. Pulmonary function test Measurements of pulmonary function were performed on a spirometer (SensorMedics V6200 Autobox; SensorMedics Co., CA, USA). The forced expiratory volume in 1 s (FEV1) and the forced vital capacity (FVC) were obtained for each child. Results were expressed as absolute values and as percentage of predicted reference values (% predicted) according to Quanjer et al. (1993). The FEV1/FVC ratio was then calculated. 2.3.6. Exercise testing Maximal exercise testing was conducted using an electromagnetic ergocycle (SensorMedics: Vmax encore 29c; SensorMedics Co., CA, USA), with an incremental protocol under physician supervision. Prior to each experiment, the volume and the gas analyzers were calibrated. The theoretical VO2 max was estimated at 40 ml min 1 kg 1. From this value we deduced the maximal theoretical using the following formula: Power = (VO2 max VO2 rest)/10.3 (Mercier, Grosbois, & Prefaut, 1997). Subjects wore a mask for gas exchange analysis using breath-by-breath collection through a rubber mouth-piece attached to a one-way valve with low resistance and small dead space. The 3-min warm-up was conducted at 20% of estimated maximal workload. The workload was then increased every minute. The rate of increase was defined as 8% of maximal estimated workload, in order to obtain the maximal workload in about 10 min. Heart rate (HR) and systolic/ diastolic blood pressure (BP) were continuously checked by electrocardiography (Corina; CardioSoft; Version 0.3). To ensure that peak oxygen uptake (VO2 peak) was reached, at least three of the following criteria had to be observed: An increase in VO2 < 5 ml with the last increase in work rate; achievement of age predicted maximal cardiac frequency; a respiratory exchange ratio >1.0; and an inability to maintain the required pedaling frequency (60 revolutions per min, rpm) despite maximal effort and verbal standardized encouragement. VO2 peak was used to represent maximum VO2. At the end of the test, each patient had 3-min of active recovery and 3-min of passive recovery. The anaerobic threshold (AT) was determined by the V-slope method and confirmed by the traditional gas exchange (Beaver, Wasserman, & Whipp, 1986). 2.4. Statistical procedure All statistical analyses were conducted using the statistical package for the social sciences software (SPSS, version 17.0, SPSS Inc., Chicago, IL, USA). All variables were examined to determine their distribution. The 95% confidence interval (CI) was used to measure precision of estimates. Data are presented as means (M) and standard deviations (SD). Independent t-test was utilized to assess differences between the DCD and the control groups. Pearson’s correlations were conducted to assess the relationships among VO2 max, FVC, FEV1, MABC scores and the 6-min walk distance (6MWD). The significance level was set at (p < 0.05), and corrected using an appropriate Bonferroni adjustment for the Univariate tests in order to maintain the overall type one error at 5% (i.e., alpha = 0.01 for comparisons of the three outcomes among groups). 3. Result 3.1. Participants Demographic characteristics of the DCD group (n = 13) and control group (n = 13) are outlined in Table 1. In this study, the mean age of the two groups did not differ (8.23  0.91 vs. 8.53  0.96; p = 0.423). Moreover, no significant differences between both groups were observed with regard to height, weight, sitting height, lower limb length, body mass index and IQ (p > 0.05). However, as expected, a significant difference in total impairment scores in the MABC test between groups was observed (t = 9.295, p < 0.001; Table 2). Table 1 Comparison of demographic data between children with and without DCD.

Age (years) Height (cm) Weight (kg) Sitting height (cm) Lower limb length (cm) Body mass index (kg m 2) QI

DCD Children (n = 13)

Typical Children (n = 13)

8.23  0.91 135.3  6.61 31.23  6.16 69.61  3.92 63.76  6.43 16.9  1.87 100  14.47

8.53  0.96 132.84  4.46 28.15  3.48 69.3  3.22 67.69  2.86 15.91  1.31 106.61  14.25

Values are means  SD; QI, intellectual quotient.

t

p 0.816 1.112 1.566 0.218 2.009 1.57 0.816

0.423 0.277 0.13 0.829 0.056 0.13 0.423

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Table 2 Comparison of pulmonary function, cardiorespiratory parameters, 6MWD and MABC test between children with and without DCD.

1

VO2 rest (ml min kg ) VO2 AT (ml min kg 1) VO2 max (ml min kg 1) VE AT (l min 1) VE max (l min 1) HR AT HR max FVC (%) FEV1 (%) FEV1: FVC 6MWD (m) MABC score

DCD children (n = 13)

Typical children (n = 13)

7.09  1.75 29.39  6.26 40.08  6.07 30.71  10.25 43.23  11.61 147  13.55 174.84  11.46 101.25  11.47 102.15  10.29 0.88  0.077 567.8  83.9 16.84  4.99

9.67  3.02 39.14  7.6 48.46  7.18 32.98  4.43 45.33  7.22 141  20.28 177.69  19.75 115.3  17.98 118.61  16.08 0.88  0.055 669  39.75 3.5  1.98

t

p 2.657 3.483 3.21 0.883 0.551 0.824 0.449 2.307 3.109 0.097 3.928 9.295

0.014* 0.002** 0.004** 0.387NS 0.586NS 0.419NS 0.657NS 0.030* 0.005** 0.924NS 0.001** 0.0001***

AT, anaerobic threshold; FVC, forced vital capacity; FEV, forced expiratory volume; VE, ventilation; HR, heart rate; VO2 max, maximal oxygen uptake; 6MWD, 6-min walking distance; MABC, Movement Assessment Battery for Children. NS No significant. * Significant difference between groups (p < 0.05). ** Significant difference between groups (p < 0.01). *** Significant difference between groups (p < 0.001).

3.2. Pulmonary function tests Pulmonary function was normal and no restrictive or obstructive syndrome was observed. There were no significant differences between the groups on FEV1: FVC. However, a significant difference in predictive values between the groups was found for FVC (p < 0.05) and FEV1 (p < 0.01; see Table 2). 3.3. Exercise capacity Table 2 reports differences between subjects with pDCD and controls on cardiorespiratory parameters, pulmonary function and 6MWD. The children with DCD showed significant lower VO2 rest (p < 0.05), VO2 max (p < 0.01) and VO2AT (p < 0.01) than their typically developing peers. No significant correlations between VE and HR at AT and VO2 max were found between groups. 3.4. 6MWT 6MWT was completed by all the subjects without premature end or breaks. No symptoms or clinical complications occurred (arrhythmia, cyanosis, etc.) during testing. The mean distance walked during 6MWT was significantly longer in children without DCD than pDCD group (669  39.8 vs. 567.8  83.9 m; see Table 2). Table 3 Correlation between MABC scores, FVC, FEV1, 6MWD, VO2 AT and VO2 max between children with and without DCD. VO2 AT (ml min kg

1

)

VO2 max (ml min kg

1

)

MABC score

6MWD (m)

Total children (n = 26) MABC score 6MWD (m) FVC (%) FEV1 (%)

0.519** 0.35 0.408** 0.555**

0.615** 0.502** 0.368 0.516**

0.641*** 0.359 0.432*

0.128 0.224

Children with DCD (n = 13) MABC score 6MWD (m) FVC (%) FEV1 (%)

0.143 0.224 0.324 0.659*

0.498 0.284 0.14 0.668**

0.181 0.221 0.186

0.016 0.114

Children without DCD (n = 13) MABC score 6MWD (m) FVC (%) FEV1 (%)

0.008 0.004 0.363 0.351

0.12 0.24 0.192 0.128

0.547 0.487 0.624*

0.614* 0.622*

AT, anaerobic threshold; FVC, forced vital capacity; FEV, forced expiratory volume; VE, ventilation; HR, heart rate; VO2 max, maximal oxygen uptake; 6MWD, 6-min walking distance; MABC, Movement Assessment Battery for Children. NS No significant. * Significant difference between groups (p < 0.05). ** Significant difference between groups (p < 0.01). *** Significant difference between groups (p < 0.001).

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3.5. Correlation between motor competences, sub-maximal and aerobic capacity A significant negative correlation (r = 0.432, p < 0.05) between MABC score and FEV1 and a negative correlation between MABC score and 6MWD (r = 0.641, p < 0.001) were observed for all children (Table 3). However, a positive significant correlation between absolute value of FEV1 and 6MWD (r = 0.438, p < 0.05) was found for all participants also. The relationships among FVC, FEV1, MABC scores and 6MWD were conducted separately for each group. MABC scores and FEV1 (r = 0.624, p < 0.05) was found to be correlated in children without DCD. In children with DCD, a significant correlation between the absolute value for FEV1 and 6MWD (r = 0.637, p < 0.05) was observed. A significant negative correlation between FVC and 6MWD (r = 0.614, p < 0.05), and between FEV1 and 6MWD (r = 0.622, p < 0.05) were found in the typically developing group (see Table 3). Likewise, the relationships between VO2 max and pulmonary function, and walking capacity were analyzed. We found a significant correlation between VO2 max and MABC score (r = 0.612, p < 0.001) and between VO2 max and 6MWD (r = 0.502, p < 0.001) for all children. A significant correlation between VO2 max and FEV1 (r = 0.668, p < 0.05) was observed in children with pDCD. No significant correlations between VO2 max and all measures were found in typical group.

4. Discussion The purpose of the present study was, firstly to examine pulmonary function, cardiorespiratory and neuromotor fitness, and secondly, to analyze correlations among these measures, using a laboratory-based test in children with and without pDCD. Our study confirmed that children with pDCD show lower cardiorespiratory fitness than healthy children due to weaker pulmonary and cardiac adaptation. The results of this study show that cardiorespiratory capacity (VE and HR) measured at maximal effort and at anaerobic threshold is not decreased in pDCD children as compared with children without DCD. This finding confirms that pulmonary or cardiac disease does not explain decreased VO2 max of pDCD at maximal and submaximal levels of effort. Our measure of cardiorespiratory fitness is based on a laboratory assessment to determine VO2 max. The results of our study, showed that children with a clinical diagnosis of DCD had significantly and clinically relevant lower VO2 max, compared to children without DCD. Children with pDCD showed significantly lower values in percent predicted of FVC and FEV1 than children without DCD. To the best of our knowledge, the present study was the first to assess performance on timed walking distance in children with pDCD. Compared to the healthy controls, pDCD children had significantly less walking distance (15% less distance covered) during 6MWT. The results of previous studies suggest that compromised lung and cardiac function in this population are the likely mechanisms for the poor cardiorespiratory fitness that is commonly observed in the population based on either laboratory or field-based tests (Cairney, Hay, Wade, Faught, & Flouris, 2006; Hands, 2008). Our results are in accord with those reported by Wu et al. (2010), but confirm this result with lab-based assessments of peak VO2. Cairney et al. (2010) did find deficits in respiratory function and particularly in VO2 max value in the pDCD group compared with the comparison group in both testing protocols; the Leger shuttle test compared against the cycle ergometer test. Similarly, Wu et al. (2010) suggest that both boys and girls with DCD had poor lung function and field test results than when compared to typically developing peers of the same gender. Moreover, in a related study by Cantell, Crawford, and Doyle-Barker (2008), no such differences were observed between children with high and low motor competence. Likewise, the exercise intolerance was also reported by Wu et al. (2010) who demonstrated a significantly lower performance in the 800 m run in children with pDCD compared to children without DCD. Our results found a significant correlation between VO2 max and percent predicted of FEV1 in all children and pDCD group separately. We showed a negative significant correlation between MABC score and FEV1 (p < 0.05) and a significant correlation between lung function and 6MWD in both group. Indeed, Wu et al. (2011) reported a significant correlation between MABC score and FVC. In pulmonary function tests, children were instructed to take a full breath in and blow out as hard and fast as possible. Inspiration should be full and unhurried, and expiration tested should be continued without pause. This technique requires coordination. Similarly, we found a significant correlation between MABC score and the 6MWD (p < 0.001). However, from clinical experience, it is apparent that problems with coordination may have effected the testing for both the lung function and the 6MWT (Lammers, Hislop, Flynn, & Haworth, 2008). Our findings are contrary to previous studies (Burns et al., 2009; Wu et al., 2010). Wu et al. (2010) showed no significant correlations between FVC and peak VO2 for children with DCD. Burns et al. (2009) also found no significant correlations between VO2 max and any measure of respiratory function between extremely low birth weight children and typically developing children. Contrary to our findings, Wu et al. (2011) did not report any significant correlations between FVC and the completion time in the 800-m run. It is well known that the 6MWT is a valid submaximal test for evaluating exercise capacity (Lammers et al., 2008). In our study, the distance walked during the test was not correlated with maximum VO2 in pDCD children. Although the distance walked is usually considered an index of sub-maximal exercise capacity, we found no correlation with the anaerobic threshold. The literature, however, is contradictory as to whether physical activity and VO2 in children are related (Armstrong & van Mechelen, 2008).

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The reduced aerobic capacity of DCD was associated with decreased of lung function in our data. Faught et al. (2013) demonstrated that differences between levels of motor coordination in children with and without DCD affect oxygen cost. In fact, children with DCD are likely to experience earlier fatigue than their well coordinated peers as a result of VO2 at AT. The results in our study confirm that children with pDCD have a lower VO2 at AT than children without DCD. There is some evidence to show that perceived adequacy toward physical activity may contribute to the difference in VO2 between children with and without pDCD (Silman et al., 2011). An explanation for the lower VO2 max may be that children with pDCD, owing to poor motor skills and lower self-efficacy, participate less in physical activities, which, in turn, may have a negative effect on physical fitness (Van der Hoek et al., 2012). The interaction between DCD and physical inactivity can limit air exchange and VO2 max during exercise. In this context, our study found significant correlations among VE, absolute values of FVC and FEV1 (r = 0.642, p < 0.05; r = 0.762, p < 0.01 respectively) only in pDCD children. Moreover to compensate for airway obstruction and an increased dead space, children with pDCD often employ a large minute ventilation to meet the increased demands of exercise (Godfrey & Mearns, 1971). On the another hand, the longer distance in the 6MWT was accompanied by a height value in lung function for children without DCD. Our results could be explained by intolerance and/or due to motivational issues in pDCD children during the 6MWT. Moreover, another possible mediating variable of the relationship between aerobic fitness and DCD could be associated to the psychological state. In fact, Chia et al. (2009) suggests that these children withdrew because they perceived the task to be difficult. In this context, as Lammers et al. (2008) note, attributing differences in walk distances to physiological mechanisms in children must be done with caution. Quantitatively, the largest difference in 6MWD between children with and without DCD could be explained, in part, by the increase in muscle strength and consequently for exercise tolerance such as walking distance (Frontera, Meredith, O’Reilly, Knuttgen, & Evans, 1988) in children with pDCD than in children without DCD, as dyspnea did not appear to be a concern in either group. 4.1. Limitations The limitations of the current study are the sample size and evaluation procedures of children in laboratory. However, future study needs to recruit more children with pDCD including girls and boys to investigate further in greater depth. The reduced aerobic capacity of DCD children is associated with decreased maximal muscle strength. Muscle fatigue can be more accurately quantified by spectral analysis applied to surface electromyography. Also, behavioral/attentional problems could have interfered with test performance. In another hand, our results suggest that greater attention should be paid to a possible relationship between DCD, inactivity and overweight/obesity in children. From this perspective, it is important to understand which factors were dominant in limiting VO2 max: the peripheral or the central component of exercise intolerance in pDCD children. Peripheral muscle deconditioning is dominated by alteration of muscle oxidative metabolism. The deconditioning is often worsened by a sedentary life style but also by pDCD children limiting their physical activity. 5. Conclusion Based on the results of this study, children with DCD were characterized by exercise intolerance as objective by poorer cardiorespiratory fitness and pulmonary function when compared to children without DCD. It is strongly recommended that longitudinal studies analyze and compare changes in pulmonary function in children with and without pDCD to have a better understanding of the relationships between motor competence and factors related to cardiopulmonary fitness. Future research should assess the factors that influence aerobic fitness and how patterns of physical activity and physical fitness are created in children with poor motor competence to provide information critical for the design of effective interventions. Conflict of interest We have no conflict of interest to disclose. Acknowledgements We thank the Department of Child Neurology, He´di Chaker Hospital (Sfax, Tunisia), Service d’Explorations Fonctionnelles, Unite´ d’Effort Cardio-pulmonaire, Hoˆpital Habib Bourguiba (Sfax, Tunisia), Service de pseudo psychiatry, Hoˆpital Habib Bourguiba (Sfax, Tunisia) El Rahma school (Sfax, Tunisia), El Kamel school (Sfax, Tunisia) and the participating children and their parents, for their contributions. References American Psychiatric Association (1994). Diagnostic and statistical manual of mental disorders (4th ed.). Washington, DC: American Psychiatric Association. American Thoracic Society (ATS) (2002). ATS statement: Guidelines for the six-minute walk test. American Journal of Respiratory and Critical Care Medicine, 166, 111–117.

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