Our key findings showed that children with DCD were less efficient than the control children when reaching and grasping a moving target. Children with DCD only successfully completed 65% of test trials, whereas control children had a 100% success rate. Within the successful trials, children with DCD had significantly longer RTs and MTs as well as larger PF than control subjects when reaching and grasping a moving toy car. However, when the angle of the slope increased from 8° to 15°, children with DCD adjusted their MT and PF in a similar manner as that of control children. Children with DCD appeared to have the ability to modify their ongoing movement in response to an increase in the speed of a moving target.
Previous data showed that children with DCD had prolonged RTs in response to static visual stimuli such as a green arrow on a computer screen9 or a light flash.16 We found that children with DCD had significantly longer RTs than control subjects to release a button when they detected that the toy car started to move. Results of previous studies and the present study imply that children with DCD could have delay in detecting static or moving visual stimuli. In addition, the previous finding of a prolonged premotor time found in children with DCD suggested that these children could have delays in central processing, that is, time taken for stimuli registration, coding, processing, and response programming.13 In the present study, the prolonged RT in children with DCD could be related to their difficulties in detecting the moving car, processing the visual-spatial information of the car, and/or planning for the reach-and-grasp action.9 , 17
In the present study, both control children and children with DCD did not modify their RT in response to the change in the angle of the slope. It was possible that the reach-and-grasp task was simple; therefore, no extra time was required to plan the movement strategy despite a variation in the speed. In contrast to RT, children in both the control and the DCD groups made significant changes in MT and PF when the slope increased from 8° to 15° (Table 1). The most intriguing finding was that children with DCD could modify their MT and PF in a similar manner to that of control children, suggesting that children with DCD were able to use the visual-spatial information (ie, the change in speed of the toy car) as feedback to modulate their ongoing movement. However, our finding was in contrast to previous data. Children with DCD were found to be less affected by visual feedback distortion in a center-out drawing task than control children, suggesting that visuomotor adaptation operated differently in children with DCD.22 Mon-Williams et al19 reported that children with DCD could not use visual cue to modify their movement trajectories when reaching to new target positions. Children with DCD could have difficulty in making an online correction of movement direction.19 Modification of MT, on the other hand, could be simpler than that of movement direction and therefore within the ability level of children with DCD. For PF, Smit-Engelsman et al13 reported that children with DCD could adjust an isometric force with their index finger to match a visual target. Even under time pressure as shown in the present study, children with DCD could modulate their peak grip force to secure a faster-moving target.
The present study had a number of limitations. First, the present study did not employ kinematic movement analysis of the reach-and-grasp task. Therefore, the trajectory, velocity, and acceleration pattern of the upper limb could not be examined. Second, the present study included a small sample of children with DCD, and the number of the control subjects and subjects with DCD was not equal. Results of the study can not be generalized to children with DCD of all ages and with different disability levels.
To conclude, this is the first study showing that reaching and grasping a moving object is impaired in children with DCD. There was a 35% failure rate in completing this task. Children with DCD had significantly longer RTs and MTs as well as larger PF than control subjects. However, with an increase in the speed of the target, children with DCD behaved like control children to scale their movement speed and PF. Children with DCD appeared to be able to use visual-spatial information as feedback to modify their ongoing movement. Findings from the study suggest that children with DCD could benefit from interventions to improve their response time and force control and to elicit more consistent and accurate motor strategies in reaching and grasping moving targets. However, the training of speed and force modulation might not be a primary interest. Further intervention study is required to prove to support this hypothesis.
I thank Dr Louisa Law, Dr Kevin Kwong, Mr Sik-cheung Siu, and Mr Yat-man Cheung for their thoughtful comments and technical support during the development of testing instrument. I also thank the children and their parents for their participation in this study.
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