1. Introduction
Developmental coordination disorder (DCD) is a neurodevelopmental disorder characterized by impaired ability to acquire and execute coordinated motor skills which interferes with the daily activities and academic performance [1]. While lack of motor coordination skills and clumsiness are widely studied among children with DCD, the knowledge of mechanisms underlying this disorder is still limited. Commonly used screening methods are result-oriented which solely measure the outcome of the movement. Process-oriented methods can provide information regarding the strategies used to perform a movement and help design effective interventions.
Evaluating motor variability is an important method to know the underlying process of coordination, and can be used for understanding the central mechanisms of the motor system [12]. Few studies have examined motor variability in children with DCD. This study aims to evaluate the motor variability of children with DCD using a dynamic approach based on a contralateral hand-foot coordination task. Moreover, the symmetry of the coordination pattern was examined in two groups of DCD and non-DCD children.
2. Materials & Methods
Using the Movement Assessment Battery for Children- Second Edition (MABC-2), 15 children with DCD and 20 children without DCD aged 7-10 years were assessed. To assess their coordination and variability, they were asked to perform a contralateral hand-foot marching task (e.g., coordination between right hand and left foot) in the sagittal plane (in-phase coordination). Three trials of 30 seconds were performed and participants were asked to carry out the task at a preferred frequency.
Movements were recorded and analyzed using VICON motion capture system; the continuous relative phase (CRP) and standard deviation of CRP were calculated to measure coordination and variability, respectively. According to the defined task, the CRP close to zero represents greater coordination ability (Figure 1). 3. Results
Figure 2 illustrates the mean of CRP and variability of the CRP in DCD and Non-DCD groups. Children with DCD showed significantly higher variability (P=0.028). However, the CRP was not significantly different between the groups (P=0.739). 
Moreover, the results revealed a significant difference between the variability of right hand-left foot and left hand-right foot coordination in children with DCD (Z=-2.10, P=0.036) (Table 1).


Finally, we found a significant negative correlation between participants’ coordination variability and their motor competence score in MABC-2; variability could explain 32% of the variance in the motor competence.
4. Discussion & Conclusion
Children with DCD showed more variability in coordinating their contralateral hands and foots, and their coordination had asymmetry. Moreover, children with more coordination variability had lower level of motor competence in MABC-2. Increased variability has been observed in patients with DCD under different tasks [17, 18, 19, 20] and several explanations have been provided. For instance, it is suggested that the increased variability in these children is due to a significant noise in their neuro-motor system [21]. However, in the current study no difference was found between the performance of children with and without DCD in the marching task. According to the dynamic systems theory, the increased variability can be a compensatory strategy for children with DCD to explore the solution for executing the task properly [26]. This can explain the higher variability in children with DCD, although they were not significantly different with non-DCD children in performing the task.  
The asymmetry of the variability in children with DCD is associated with their deficit in integrating the sensory information such as proprioceptive feedback [20]. However, the research in this area is still sparse, and more studies are needed to explore the underlying mechanisms. The negative correlation between the coordination variability and motor competence in children highlights the importance of process-oriented measures and dynamic approach for comprehensive understanding of the motor difficulties in children with DCD.

Ethical Considerations
Compliance with ethical guidelines
The current study was approved by the Ethics Committee of the Faculty of Physical Education and Sport Sciences, University of Tehran (Code: IR.UT.SPORT.REC.1396030).

Funding
This study was extracted from the PhD. Dissertation of the first author at the Department of Motor Behavior, Faculty of Physical Education and Sport Sciences, University of Tehran.

Authors' contributions
All authors met the standard criteria for writing based on the recommendations of the International Committee of Publishers of Medical Journals (ICMJE).

Conflicts of interest
The authors declared no conflicts of interest.

Acknowledgements
The authors would like to thank all parents and children who participated in this study for their cooperation.

 
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