Developmental coordination disorder (DCD) is a neurodevelopmental disorder characterized by difficulties with fine and/or gross motor coordination that significantly affect daily functioning including leisure, academic, and self-care pursuits (1). Although it is a fairly prevalent disorder, affecting approximately 5%–6% of children, it remains widely underrecognized and underdiagnosed (2). Children with DCD have lower levels of cardiorespiratory and musculoskeletal fitness (3), and higher rates of overweight/obesity (4). In longitudinal studies, the differences in cardiorespiratory fitness, body mass index (BMI), and waist circumference between children with and without DCD tend to either persist or widen over time across childhood (5–7). This is highly concerning because children with DCD are, therefore, at a greater risk for poor health-related fitness (HRF) as they age, which increases their risk of cardiovascular disease and other negative health outcomes later in life (8).
The majority of literature examining HRF in DCD has focused on children older than 7 yr, a time in which fitness and body composition differences between children with and without DCD are already present (3,4). Therefore, it is still unclear how or when these deficits emerge. To date, only three studies have comprehensively examined HRF in children with DCD, including children under the age of 6 yr. Aertssen and colleagues (9) found poor anaerobic capacity in young children with DCD as measured by the muscle power sprint test and functional strength measures. Similarly, Schott and colleagues (10) found that 4- to 6-yr-old children with DCD had deficits in anaerobic power as measured by 20-m sprint time, but found no differences in aerobic fitness, upper or lower body muscular strength, flexibility, or BMI. Conversely, Hands and Larkin (11) found more global HRF deficits in young children (ages 5–8 yr) with motor learning difficulties, reporting poorer cardiorespiratory endurance, flexibility, abdominal strength, running speed, long jump distance, and a higher BMI. These conflicting findings may be due to the different ages of the samples, whereby some HRF differences may be more pronounced over age 6 yr, or the reliance on field-based fitness assessments that may be more heavily influenced by motivational and/or environmental factors. It is, therefore, still uncertain which components of HRF are affected by motor coordination difficulties in the preschool years.
In addition to lower HRF, children with DCD are also less active compared with children with typical motor development (3). It is hypothesized that the HRF deficits observed in children with DCD are due in part to hypoactivity, whereby poor motor coordination leads to reductions in physical activity and subsequent declines in HRF (12). Although this activity deficit is thought to explain some of the differences in HRF between children with and without DCD, there is limited evidence testing this mediation pathway and it is still uncertain if these differences in physical activity and fitness are present in the preschool years. Vigorous physical activity (VPA), in comparison to light, moderate, or moderate-to-vigorous physical activity, has been shown to be most strongly associated with HRF outcomes in preschool children (13). Therefore, the purposes of this study were to examine differences in HRF (cardiorespiratory fitness, musculoskeletal fitness, body composition, and flexibility) using objective, laboratory assessments in a large sample of young children (age 4–5 yr) with and without DCD, and to determine if HRF differences are mediated by objectively measured VPA levels.
Children were recruited from 2013 to 2017 as part of the Coordination and Activity Tracking in CHildren (CATCH) study, a prospective, longitudinal cohort of children with and without DCD from Southern Ontario, Canada. The target sample size was 300 typically developing (TD) children and 300 children at risk for DCD (rDCD) age 4 and 5 yr (48–71 months). To recruit equal samples of children with and without DCD, multiple screening procedures were undertaken and modified at different phases of recruitment to maximize the probability of meeting the target sample size. Initially, children were screened for motor difficulties over the telephone using the Developmental Coordination Disorder Questionnaire and some were selected for further screening in the laboratory with the Movement Assessment Battery for Children, Second Edition (MABC-2). All children who scored ≤16th percentile on the MABC-2 were invited into the longitudinal cohort, along with a random sample of TD children (>16th percentile). Telephone screening using the Developmental Coordination Disorder Questionnaire was removed (February 2015) from the screening procedures. Random selection of TD children was also removed (May 2015) such that all children attending the laboratory for MABC-2 testing were invited into the longitudinal cohort. Once the TD cohort was full, only children scoring ≤16th percentile on the MABC-2 were invited into the cohort (August 2016). Recruitment began in October 2013 and ended in June 2017. Details of the study design and recruitment procedure can be found in a previous publication (14). Children were eligible if they could speak/understand English, weighed more than 1500 g at birth, and did not have a diagnosed physical disability or medical condition (e.g., cerebral palsy or muscular dystrophy) that significantly impacted motor coordination. This study was approved by the Hamilton Integrated Research Ethics Board. Parents of all participating children provided informed, written consent. This study is a cross-sectional examination of the baseline CATCH cohort.
Assessment of DCD
All children completed the MABC-2 (15), the criterion standard for assessing risk of DCD in childhood (2). This assessment consists of eight items across three areas of coordination: manual dexterity, aiming and catching, and balance (static and dynamic). Raw scores are converted into standard scores and an overall percentile based on the child’s chronological age. Children scoring at or below the 5th percentile were considered to have DCD, those in the 6th–16th percentile were considered to be at rDCD, and children scoring above the 16th percentile were considered TD. All children also completed the Kauffman Brief Intelligence Test—Second Edition (16), and parents filled out a detailed medical history to confirm that motor deficits were not better explained by an intellectual delay or diagnosed medical condition.
Assessment of HRF
HRF comprises body composition, cardiorespiratory fitness, muscular strength and endurance, and flexibility, all of which are considered the key components related to one’s health and ability to engage in physical activity (17).
Height and weight were measured in duplicate without shoes and in light clothing using a stadiometer (SECA 264, Chino, CA) and digital scale (SECA 869). Measures were repeated if the two measurements were >0.1 cm or >0.1 kg apart. The average of the two closest measures was used to determine height and weight, and calculate BMI (kg·m−2). BMI percentiles were then determined based on the US Centers for Disease Control and Prevention growth charts (18). Presence of overweight/obesity was defined as children whose BMI was greater than or equal to the 85th percentile for their age and sex.
Waist circumference was measured to examine central adiposity at two locations: top of the iliac crest (recommended by the National Institutes of Health) and midway between the top of the iliac crest and the lowest rib (recommended by the World Health Organization) (19). Measurements were taken in duplicate and repeated if differences between measurements were >0.5 cm. Measures were taken against the skin, where possible, during normal exhalation.
Body fat percentage was determined using bioelectric impedance analysis (RJL Quantum IV, Clinton Township, MI) while children were lying supine. Fat-free mass was first determined using an equation that has been validated for children (20). Percent body fat was then calculated as [(body weight − fat-free mass)/body weight] × 100.
Cardiorespiratory (Aerobic) Fitness
Aerobic fitness was assessed using the Bruce Protocol, a progressive treadmill test that increases in speed and grade every 3 min (21). The test was initially created to assess aerobic capacity in adults but has now been adapted and used extensively with children as young as preschool age (22,23). All children started at stage 1 and were required to hold onto the handrails throughout the duration of the test to assist with balance, with a research assistant placed behind the child to ensure safety. Heart rate was measured continuously throughout the test using a heart rate monitor (Polar H7, Kempele, Finland). The test was terminated when the child reached exhaustion, was no longer able to keep up with the speed of the treadmill, or refused to continue despite verbal encouragement. Time to exhaustion was used as an indicator of aerobic fitness. Only children who reached a maximum heart rate ≥180 bpm were included in the analyses.
Short-term muscle power was examined using a Wingate protocol (Bar-Or 1987) on a pediatric cycle ergometer (Lode pediatric; Lode BV, Gronigen, the Netherlands). Children were first required to sprint as fast as possible (~20 s) against the internal resistance of the ergometer only. After a short rest, the Wingate test began: children were instructed to pedal as fast as they could and a resistance relative to their body weight (0.55 N·m·kg−1) was applied once they reached 80% of their maximal pedaling cadence. The children then pedaled against this resistance for 30 s. Peak power (W) was determined as the highest instantaneous power achieved during the test. Mean power (W) was the average power output over the 30-s test. Because young children tend to accumulate their activity in bouts shorter than 10 s (24), the mean power from the first 10 s of the test was also calculated to reduce the potential of confounding motivational factors on performance. This modified 10-s Wingate has been found to be reliable in preschoolers (25). Fatigue was calculated as the percentage drop-off in power over the course of the first 10-s and entire 30-s test. All power outputs were calculated using the LODE Wingate software package (Lode BV). Only children who could pedal >25 rpm were included in the analysis because this is the minimum cadence in which a resistance can be applied to the cycle ergometer.
Long jump, or standing broad jump, is a common, field-based measure of lower body muscular strength/power (26). It has been validated against peak power as measured using the modified 10-s Wingate in preschool children (27). Children were required to stand with their feet behind a marked line and instructed to jump as far as they could and land on two feet. Distance was measured from the line to the back of the closest heel. Children were given three trials, with additional trials conducted only if a child fell or did not perform a successful two-footed takeoff or landing. The best of the three trials was used as an indicator of lower body musculoskeletal fitness.
Flexibility was assessed using the sit-and-reach test. Children were instructed to keep one hand on top of the other and reach forward as far as they could using a slow and controlled motion keeping both legs fully extended. A sit-and-reach box (Novel products, Rockton, IL) was used, and measurements were taken in centimeters, with 23 cm corresponding to the position of the feet against the box. Trials were repeated if the children’s hands came apart or their knees bent.
After completion of the laboratory visit, all children were asked to wear an accelerometer (Actigraph wGT3X, Pensacola, FL) over their right hip for the following 7 d. Children were instructed to wear the accelerometer during all waking hours, only removing it for sleep and/or prolonged water activities. Parents were given a logbook to record the times the accelerometer was put on and removed. Nonwear periods were defined as any time the parent indicated the accelerometer was off and/or ≥60 min of consecutive zero counts. Only children who wore the accelerometer for at least 3 valid days (≥10 h) were included in the analyses. Data were analyzed in 3-s epochs, and Evenson cut points were applied to determine average daily minutes spent in VPA (28). Because Evenson and colleagues used 15-s epochs, their VPA cut point was divided by 5 and applied to each 3-s epoch. The total time spent in VPA was then calculated for each valid day and then averaged across all valid days of wear. All accelerometer data were cleaned and processed using Actilife Software (Actigraph).
Differences in descriptive characteristics among groups were examined using one-way ANOVA for continuous variables and chi-square for categorical variables. HRF differences were examined using ANCOVA, controlling for age in months, sex, height, and weight where appropriate. VPA group differences were also examined using ANCOVA and adjusted for age, sex, and daily wear time. Significant group effects were then tested using post hoc tests with Bonferroni correction for multiple comparisons (P < 0.017). To examine whether sex moderated the effect of group on HRF outcomes, separate linear regressions were conducted for each HRF variable including the main effects of group and sex, and an interaction term for sex–DCD group.
For HRF outcomes in which significant group differences were found, separate mediation analyses were conducted to determine if the differences were mediated by levels of VPA. The tests for indirect (mediation) effects were conducted using the PROCESS software macro for SPSS (29), with DCD group entered as the independent (X) variable, VPA as the (M) variable, and HRF outcomes as the dependent variable (Y), with all aforementioned covariates included. As recommend by Hayes (29), bootstrapping was set to 10,000 samples. All analyses were conducted using SPSS v20.
Of the 1330 eligible families, 1225 provided verbal consent, with 594 children invited into the longitudinal cohort based on their MABC-2 scores. One participant withdrew, 1 participant was unable to complete the baseline appointment, and 3 additional children were excluded due to medical reasons, leaving 589 children included in the final CATCH cohort (301 TD, 177 rDCD, and 111 DCD). Potential intellectual disability (IQ < 70) was identified in 7 children (2 rDCD, 5 DCD); however, removing these participants did not significantly affect the results, and therefore, they were included in all subsequent analyses. Participant characteristics are presented in Table 1. rDCD children were slightly younger than TD children, with a higher percentage of boys in both DCD groups compared with the TD group.
Results of the body composition analyses are presented in Table 2. All children had valid height, weight, and waist circumference measurements. Six children were missing body fat percentage measurements due to refusal/an inability to lie still (n = 5) or due to a skin rash preventing placement of the electrodes (n = 1). Although there was a trend for the DCD group to have higher waist circumference and absolute BMI, no statistically significant differences in any of the body composition outcomes were found among the three groups. This pattern of results was similar for boys and girls as no significant group–sex interactions were found.
Cardiorespiratory and musculoskeletal fitness
Results of the physical fitness assessments are presented in Table 3. Overall, there was a large main effect of DCD group on both musculoskeletal and aerobic fitness performance; children in the DCD group had the greatest fitness deficits, and children in the DCD group performed better than the DCD group but worse than the TD group. Thirty-one children (11 DCD, 7 rDCD, 13 TD) were excluded from the aerobic fitness (time to exhaustion) analysis due to an inability to reach a heart rate of 180 bpm. Children in each group reached an average maximal heart rate of 198 bpm (F = 0.41, P = 0.66); however, the DCD group reached this maximal heart rate almost 1.5 min faster than did the TD group. Thirteen children were excluded from all of the Wingate analyses due to an inability/refusal to pedal >25 rpm (n = 7; 4 DCD, 1 rDCD, 2 TD) or equipment malfunction (n = 6; 1 DCD, 2 rDCD, 3 TD); an additional 4 children with DCD were excluded from the mean power and fatigue analysis due to a refusal to continue pedaling for the entire duration of the test. Children in the DCD group had the lowest peak and mean muscle power on the Wingate test, and fatigued to a greater extent over both the first 10-s and the complete 30-s test compared with both the rDCD and TD groups. Ten children were not able to complete a long jump with two feet and were excluded from the analysis (4 DCD, 5 rDCD, 1 TD). TD children were able to jump significantly farther than both the rDCD and DCD groups, with children in the DCD group demonstrating the shortest long jump distance. No difference in flexibility among the three groups was found. The linear regression analyses found no significant group–sex interactions, suggesting that the effect of group on all physical fitness outcomes did not differ between girls and boys.
Role of physical activity
There were no significant differences among the three groups for daily minutes of VPA (DCD 31.2 min·d−1, rDCD 31.5 min·d−1, TD 32.3 min·d−1; F = 1.4, P = 0.24). The first mediation analysis was conducted with time to exhaustion as the dependent variable. There was a nonsignificant direct effect of group on VPA (95% confidence interval (CI), −2.4 to 0.35; P = 0.14) and a significant direct effect of VPA on time to exhaustion (95% CI, 0.34–1.69; P < 0.01). There was no significant indirect mediation effect of VPA on the DCD group to time to exhaustion relationship (95% CI, −2.9 to 0.18). A similar pattern of results was found for peak power (indirect effect: 95% CI, −0.71 to 0.11), 10- and 30-s mean power (indirect effect: 95% CI, −0.80 to 0.09 and −0.73 to 0.06, respectively), 10- and 30-s fatigue (indirect effect: 95% CI, −0.02 to 0.44 and −0.03 to 0.56, respectively), and long jump distance (indirect effect: 95% CI, −0.81 to 0.16). Overall, although VPA had a significant direct positive effect on all aerobic and musculoskeletal fitness outcomes (P < 0.05), it did not explain the large differences in HRF among the three groups.
This was the first large-scale study to comprehensively examine HRF using objective laboratory measures in preschool-age children with DCD or rDCD. With regard to body composition, children with DCD and rDCD, on average, did not significantly differ on measures of waist circumference, BMI, or body fat percentage compared with TD children. Although there is a trend for larger BMI and waist circumference among children in the DCD group, the relative difference in BMI and waist circumference among the groups is quite small and may have minimal clinical relevance, especially considering that the overall rates of overweight/obesity in all of the groups are considerably lower than the current estimated Canadian prevalence (30). Although these findings contradict the consistent, significant body composition differences in older samples, longitudinal evidence has shown that the differences in waist circumference and BMI between children with and without DCD widen over time from middle childhood to adolescence (6). Although these data are not sufficient to show causation, the minimal differences in body composition in this early childhood period are consistent with the hypothesis that overweight/obesity may be a secondary consequence of DCD that emerges in middle childhood and/or adolescence, possibly due to decreased physical activity, and that these trends may become significant as children reach school age (31).
There were no group differences in flexibility as measured by the sit-and-reach test, which may be due to the heterogeneity in flexibility profiles exhibited by children with DCD (32). Young children with DCD and rDCD demonstrated poorer aerobic and musculoskeletal fitness compared with TD children, which is consistent with the extant DCD literature in older samples of children (3). When comparing our results with previous work conducted with young children, our findings are inline with Hands and Larkin (11), who found both aerobic and musculoskeletal fitness differences between children with and without motor learning difficulties at age 5 to 8 yr. Although our results confirm previous findings of lower anaerobic power in young children with DCD (9,10), Schott and colleagues (10) did not find the same deficit in cardiorespiratory fitness as found in the current study. This may be a result of their smaller sample size or reliance on a field-based aerobic fitness assessment, which may be more heavily influenced by motivational or environmental factors. Overall, we found that young children with DCD and rDCD were not able to produce the same power outputs and fatigued faster both over an incremental, progressive aerobic endurance test and during the all-out short-term anaerobic test. These physical fitness deficits may hinder their ability to keep up with their peers in both free play and organized activities, and may be a contributor to why children with DCD begin to withdraw from physical activity as they get older.
The difference in aerobic and musculoskeletal fitness between groups was not mediated by levels of daily VPA. These findings are inline with longitudinal results from the Physical Health and Activity Study Team (PHAST) study, which followed a large sample of children from ages 9 to 14 yr. The PHAST group found that, although children with DCD had steeper rates of decline in cardiorespiratory fitness (measured using field-based tests) and greater increases in BMI and waist circumference from middle to late childhood, this widening gap in HRF was not explained by differences in self-reported physical activity (6,7). In contrast, Silman and colleagues (33) examined a subsample of the PHAST cohort at age 12–13 yr using laboratory-based measures of cardiorespiratory fitness and found that objectively measured physical activity significantly mediated the differences in maximal oxygen uptake (V˙O2max) between children with and without DCD. These discrepancies may be due to the different methodologies used or the age of the samples, whereby the mediating effect of physical activity on cardiorespiratory fitness may not emerge until children are nearing adolescence. As described in Wall’s skill gap hypothesis (34), the motoric demands of play are low in this early-age group, which may be why children with DCD are not yet withdrawing from physical activity to an extent that would significantly impact fitness levels.
Since daily VPA levels did not explain why children with DCD had lower HRF, alternative explanations are required. Tests of fitness and motor coordination, although distinct, measure related movement components. An underlying deficit in neuromotor development, such as increased levels of co-contraction of agonist and antagonist musculature (35), may therefore hinder performance on both tests. Increased co-contraction would interfere with a child’s ability to perform well-timed coordinated contractions, such as cycling at a fast cadence and propelling the body forward during jumping, and may also contribute to a reduced economy of locomotion in young children with DCD. Oxygen cost of locomotion has been studied in older children with DCD; although no significant differences were found, children with DCD had poorer quality of locomotion during treadmill running and perceived themselves as working harder (36). Impaired neuromuscular control may, therefore, also contribute to an inflated heart rate response and increased sense of perceived exertion at a given intensity compared with TD children, which may explain why children with DCD in the current study reached exhaustion on the treadmill almost 90 s earlier than TD children. Psychological factors have also been linked to fitness performance in children. For example, Cairney and colleagues (37) found that generalized self-efficacy toward physical activity explained a significant proportion of the difference in performance on the Leger 20-m shuttle run between children with and without DCD. Therefore, low confidence in their physical abilities may have prevented children with DCD from performing to their true maximal potential. In addition, it is possible that potential differences in patterns of accumulation of VPA throughout the day, instead of overall daily VPA, may explain some of the observed HRF differences. For example, children with DCD may accumulate their VPA in shorter bout durations, which may partially explain their reduced performance on physical fitness tests.
A diagnosis of DCD is not recommended for young children unless motor assessments are repeated and consistent skill delays are found (2); therefore, it is possible that not all of the children in the DCD group, especially those in the rDCD group, will meet the criteria for DCD in future years. Because the CATCH study will conduct repeated annual motor assessments, we will be able to determine how potential changes in motor coordination affect changes in fitness outcomes through the early childhood period. Because of the young age of the sample, measurement of V˙O2max, the gold standard in aerobic fitness testing, was not possible; however, time to exhaustion using the Bruce protocol has been found to be strongly correlated with directly measured V˙O2max in children (38), and, therefore, provides a good indicator of aerobic fitness in preschool children. Furthermore, the total test duration on the Bruce protocol in the TD sample was similar to the reference values published for preschool children (23), which further support the validity of this test and our findings.
Future research is necessary to elucidate the specific causes of fitness deficits in children with DCD. Although coactivation has been studied using isometric and isokinetic movements in older children (35), this needs to be examined in younger children during more functional tasks, such as cycling and walking/running. In addition, the role of psychological factors on fitness performance has not yet been studied in preschool-age children and will be important to examine. Although the fact that these physical fitness deficits already exist at such a young age is concerning, Farhat and colleagues (39) were able to show that an 8-wk motor skill intervention was able to improve motor skills, cardiorespiratory endurance, and exercise tolerance in 8-yr-old children with DCD. These findings hold promise that motor interventions at an early age may help to improve fitness levels; however, longer, well-controlled randomized trials are necessary to determine if early motor interventions can lead to increases in fitness both acutely and over time in children with DCD.
Young children with DCD and at risk for DCD have poorer aerobic fitness and short-term muscle power compared with TD children, which is not explained by levels of daily VPA. These fitness differences may persist or widen over time, putting children with DCD at risk for poor health outcomes as they get older. Early interventions targeting perceived and actual motor competence as well as physical fitness may help to prevent the trends of unhealthy weight gain and physical inactivity observed through middle childhood and into adolescence in children with DCD.
The authors would like to thank all of the CATCH study participants and their families as well as acknowledge the trainees and research staff who assisted with recruitment, scheduling, and data collection.
The CATCH study is funded by the Canadian Institutes of Health Research (MOP 126015). S. K. D. is funded by an Ontario Women’s Heath Scholars Award. B. W. T. is supported by a Canada Research Chair in Child Health and Exercise Medicine. C. M. is supported by the Lillie Chair in Childhood Disability Research.
The authors have no conflicts of interest to declare. The results of the present study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation and do not constitute endorsement by the American College of Sports Medicine.
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