INTRODUCTION
In a typical pediatric practice, physical therapists assess and treat children with a variety of disorders. One such disorder, known but not well understood, is developmental coordination disorder (DCD). 1 This acronym describes a condition of children that is anything but simple to understand and that poses a challenge for pediatric physical therapists. If asked, many physical therapists might not recognize that they have many children with DCD on their caseloads; these children tend, more often, to be referred to as those with “mild motor” or “low tone” problems or children who are “a little bit delayed.” As Missiuna and Polatajko 2 illustrated, different labels have been ascribed to these children, including the clumsy child syndrome, 3–7 physically awkward, 8–10 children with movement difficulties, 11 children with perceptual motor dysfunction, 12 developmental dyspraxia, 13–17 and children with sensory integrative dysfunction, 18,19 mostly on the basis of the perspective of the professionals who work with them.
Physical therapists can undoubtedly recall images of young children who have been assessed and found to be, for lack of a better word, “clumsy.” They might have struggled to find the right words to describe the observed clumsiness, awkward gait pattern, and unique way that these children have “fixed” or stabilized their joints. Although motor milestones might have been achieved within normal limits, motor skills that are learned might not generalize across settings and might appear to be present at one moment and absent the next. 20–23 Each child who is clumsy presents a little differently from others, making it difficult to develop and apply a treatment approach.
Until recently, the argument that children with DCD did not require intervention might have been acceptable because it was thought that children demonstrating these characteristics would outgrow their difficulties. 24 There is now compelling evidence that the motor difficulties of children with DCD continue to be present into adolescence and adulthood. 25–30 Should pediatric physical therapists assess and intervene with these children? Beyond some fairly basic developmental recommendations, what else can be done?
As children with DCD struggle to learn new motor skills, they demonstrate a pattern of avoidance of activity that results in fewer practice opportunities. 31 Watkinson et al 32 note the obvious implications of this withdrawal from typical activities on social interaction, fitness, and health status. Evidence is accumulating that demonstrates that children with DCD have poor motoric competence, spend less time in physical activity, 9,11,33 and eventually develop secondary impairments of reduced strength and endurance. 34,35 Clearly, early intervention is warranted.
The purpose of this article is to review some of the contemporary theories of motor learning and motor control that may facilitate physical therapists’ understanding of children with DCD. Current literature concerning the primary nature of their impairments and the secondary conditions that seem to arise will be examined in depth. The emphasis throughout this review will be on interpreting and applying theoretical knowledge to the observation and recognition of children with DCD. By understanding who these children are, how they present clinically, and the theoretical underpinnings of their difficulties, it will be possible to appreciate the unique contribution that physical therapists can make to their management.
WHO ARE THE CHILDREN WITH DCD?
DCD has been described as a “chronic and usually permanent condition characterized by impairment of motor performance that is sufficient to produce functional motor performance deficits that are not explicable by the child's age or intellect, or by other diagnosable neurological or spatial-temporal organizational problems.”36 A formal diagnosis of DCD is made by a physician when the criteria outlined in the fourth Diagnostic and Statistical Manual of Mental Disorders1 have been met. Other criteria of note are that a child's motor development must differ from their development in other areas and that the functional motor problems must have an impact on academics and/or daily life activities. Physicians must rule out difficulties in coordination that might be attributable to a neurological disorder or to pervasive developmental disorder. When cognitive impairments are also present, the motor performance limitations must be greater than would be expected for the child's mental age. 1
Children who demonstrate the characteristics of DCD form a heterogeneous population, 37–41 and this variation can be observed in many respects. Children might differ in the degree of involvement (from severe to mild) and in the extent to which the disorder affects daily tasks (affecting nearly every activity to affecting only specific activities). 42 There might be involvement of gross motor and postural functions or only of fine motor manipulative skills requiring eye-hand coordination. 43 Children with DCD also differ in the extent to which they display comorbid conditions, including nonverbal learning disabilities, speech/articulation difficulties, and attention deficit disorder. 40,44,45
The recognition of DCD might depend on the extent to which physical, social, and attitudinal factors have influenced motor skill acquisition. Although DCD must be considered to be, at least theoretically, present from birth, children differ with respect to the apparent age of onset. Many children with DCD do not display the full extent of their functional difficulties until they reach school age. For reasons that will be explored throughout this review, their coordination difficulties might not be easy to observe until the point at which the child learns and performs skills that require adaptation in the speed, timing, force, or distance of the movement. The developmental progression might also appear to vary depending on the environmental and task demands placed on the child in the early years.
Despite the fact that the presentation of DCD might seem varied, some primary movement impairments seem to be quite common across this population. When comparing children with DCD with children who are developing typically, researchers have consistently described a “slowness of movement” in studies of both reaction time and movement time. 46–48 Children with DCD seem to rely on the use of vision more heavily than other senses to control movement, well beyond the age at which children who are developing typically would rely on vision. 21,49,50 They seem to have difficulty selecting the best motor response for a task 51 and may repeat motor tasks in the same way regardless of their success with that task. 22
Of considerable concern are the secondary impairments associated with DCD, which include decreased strength and power 34,35,52 and withdrawal from physical activity. 9,11,32,33 Raynor 35 recently provided strong empirical evidence of the progressive decrease in strength and power of children with DCD between the ages of six and nine years. These secondary sequelae are precursors for decreased participation in sport or leisure activities, reduced opportunities for social interaction, and diminished physical fitness across the lifespan.
How do these characteristics manifest themselves in a practical sense? Children with DCD are likely to experience difficulty with the flexibility and adaptability required for activities such as running, jumping, hopping, skipping, throwing, and catching. They may also experience difficulties with printing and handwriting, and have difficulty managing fasteners such as buttons and zippers. 14,17,24,45 David 53 suggests that any functional activity that depends on adequate eye-hand coordination could be problematic. Their performance on these tasks will not be age appropriate, and the spatial and temporal uncertainty that exists for children with DCD will be subtly expressed in the quality of their movements. Movements might lack speed and will definitely lack fluidity. When motor skills are not attained at a time that is developmentally appropriate, a “snowball effect” is created in that these skills cannot then be applied flexibly and adaptively as subsequent skills are learned. The theoretical underpinnings of some of these commonly observed characteristics of children with DCD will now be explored in more depth.
UNDERSTANDING THE ROLE OF FEEDBACK IN CHILDREN WITH COORDINATION DIFFICULTIES
In the past, motor development was believed to arise from developmental changes in the brain (ie, neuromaturational theory of motor development). It is now clear that the central nervous system is influenced by the child's experiences. 54 Cooperation among many interacting systems is believed to contribute to motor performance and development 55 with the inherent understanding that the central nervous system must be flexible and adaptive since it constantly incorporates sensory information into the production of successful movement patterns. 56 The capacity to learn from solutions to previous motor problems is dependent on a child's ability to attend to and interpret feedback from their bodies and from the environment. Ultimately, their response to environmental cues results in adaptive, flexible motor behavior.
Many contemporary authors have discussed this capacity to select adaptive variations and produce the most efficient movement patterns. 57–59 Touwen 57 suggested that attending to feedback allows one to select the best strategy to apply in a particular movement situation. This idea of variable application of strategies and use of feedback corresponds with literature regarding the stages of acquisition of skilled movement. 60,61 The first cognitive stage of learning is characterized by variability as the child gets the general idea of the movement and selects the most efficient movement pattern to put into place. During this stage, the child needs to attend to and integrate past knowledge with feedback from the environment. 62 In the second, associative stage, the child learns to perform the skilled movement with more accuracy and should attend to feedback from their bodies as they detect and correct errors. Feedback should receive less conscious attention as the movement is automated in the third autonomous stage.
Children with DCD lack both adaptability and flexibility in their motor behaviour. 63 They demonstrate movements that are inaccurate and lack fluency and, thus, appear to be in an early stage of motor learning. 20 Fitts and Posner 61 suggested that as skills are learned, feedback requirements should lessen and change with an increasing reliance on proprioceptive and kinesthetic feedback rather than visual input. Children with DCD, however, may continue to rely predominantly on visual information, as if they were still in the early learning stages. As a result, in research studies that investigate motor control and motor response, these children appear to behave in a manner that is similar to younger children. 5,6,21,64 Once children have progressed to a stage of motor learning when cues from the environment have been incorporated into the selection of a movement strategy, they are in a state of readiness 20 and are prepared to adapt to new situations. This state of readiness is seen in premovement organization, or “postural biasing,” and is a necessary prerequisite for coordinated and efficient motor control. Anticipatory motor control encompasses preparatory movements that occur both before and during a task movement as well as those that serve to support that movement. 65,66 In the absence of such premovement preparation, the learner must continue to rely more heavily on feedback that is provided as the movement is occurring, making it a much more demanding and effortful task. Some research has suggested that children with DCD continue to rely heavily on visual feedback to guide task performance and do not automate movements to the same degree as children who are developing typically. 21,47,48,67 Other researchers have suggested that the problem might lie in a failure of children with DCD to use anticipatory control strategies for motor tasks; as a result, they might have to rely more heavily on feedback to control the movement. 67,68 In studies conducted by van der Meulen et al, for example, children were tested using a variety of goal-directed reaching tasks in which visual feedback was either present or occluded. Children with DCD were found to be less efficient in their reaching movements, both with and without vision, which the authors interpreted as reflecting impairment in anticipatory control. 47,48
As previously noted, children with coordination difficulties have also been described as repeating tasks the same way over and over again, regardless of their success with the task. 69 Two possible reasons related to the use of feedback have been suggested to explain this observation. Goodgold-Edwards and Cermak 20 proposed that children with DCD experience difficulty understanding the demands of a task and its component parts and do not effectively use the feedback originating from their knowledge of performance. These authors further postulated that children with DCD might attend to the wrong cues and not to the more salient aspects of the feedback. 20
In all of the above studies and explanations, it is clear that children with DCD do not interpret and utilize sensory feedback or feedback from task performance in the same manner as children who are developing typically.
UNDERSTANDING “FIXING” AS AN INEFFECTIVE STRATEGY
Children with DCD have frequently been described as tending to “fix” or freeze their joints during task performance. 23,35 The deliberate stabilization of their joints in this way leads to their stiff, awkward, and clumsy appearance 23; it also increases the time it takes them to adapt to changes in their movement environment. 21 Fixing has been defined as holding the body as a means of stabilizing so that another part can be moved with better control. 70 It can, therefore, be thought of as a strategy to control for excess degrees of freedom. We are all challenged to constrain the multiple degrees of freedom of joints and muscles so that we can function in an efficient way. Children with DCD who fix their joints during task performance, though, are more likely to become fatigued 71 and to demonstrate inconsistency in task performance. 21,23
Bernstein 72 pointed out long ago that there were far too many independent coordinates to be controlled during every single movement, so we must constrain groups of muscles across several joints to act as functional units. Fetuses have been noted to use all available anatomical degrees of freedom, which is viewed as essential for the development of normal structure and function of joints. 73 Early on, however, infants who are developing typically transition from using all available degrees of freedom to begin to develop “coordinative structures,” which Bernstein describes as groups of muscles and joints acting as functional units. In theory, these coordinative structures represent the most efficient way of performing a movement; in the case of children with DCD, however, we find them to be constraining joints awkwardly and stiffly. By constraining the degrees of freedom, the performer has fewer decisions to make. Overall, this postural fixation contributes to less efficient movement patterns and is, once again, reflective of an earlier, less skilled stage of movement acquisition.
Jordan and Rosenbaum 74 suggest that solving the degrees-of-freedom problem is not just a spatial issue (eg, the shoulder has three planes of motion); it is also a temporal issue. The impact of being too constrained and limited is seen clearly with difficulties in flexible, adaptive motor behavior as well as with acquiring new motor skills. It has been observed more frequently in infants born preterm, who are at high risk for developmental disorders, than in infants born at term, 70,75 and it has been noted to contribute to the relatively stereotypic (invariable) movement patterns often noted in infants born early. 76 Children with DCD are challenged not only with respect to control of the anatomical degrees of freedom but also with regard to the timing, duration, and sequencing of movements.
RATE-LIMITING FACTORS
Modern day motor theories suggest that the most efficient movement emerges through self-organization among all of the contributing subsystems within the child, the environment, and the task. 55 Motor developmental change might be constrained by any of these contributing subsystems. 77 For children with cerebral palsy, the rate-limiting factor for the acquisition of basic motor abilities might be spasticity. 78 For children with DCD, their inability to attend to feedback during performance of the movement or to increase or alter the pattern of the degrees of freedom to permit more efficient, flexible, and adaptable movement solutions might be rate-limiting factors. The type of feedback on which children with DCD are reliant (eg, visual rather than proprioceptive) may also be a factor that limits speed in that different senses provide information at varying speeds.
Another rate-limiting factor may be the manner in which children with DCD activate their muscles during tasks that require co-activation. Children with DCD use different neuromuscular strategies than their age-matched peers. 79 During unilateral reaching, when compared with children who are developing typically, children with DCD demonstrate delayed onset of antagonist muscle activity with a longer duration of agonist activity. With asymmetrical bilateral reaching, children with DCD use variable strategies, changing duration or onset of one or both agonist and antagonist muscle groups; their peers alter only the duration of antagonist muscle activity. These patterns of activation are believed to contribute to their slower and more variable movement times. 79 Raynor 35 also described an increased level of co-activation in a knee flexion/extension task in which children with DCD demonstrated a much less effective method of muscular organization than their peers. A similar picture emerged with regard to balance and postural control, in which it was found that children with DCD demonstrated a less effective pattern of muscle activation when their balance was challenged. Studies such as those described above have also demonstrated that this unusual pattern of activation does not improve with age for children with DCD. 35 Clearly further research is required to delineate and understand these rate-limiting factors.
IMPLICATIONS FOR CLINICAL PRACTICE
Physical therapists have contributions to make both in assessment of, and intervention with, these children. Detailed descriptions of assessment tools and a thorough discussion critically appraising intervention practices for children with DCD are reported elsewhere. The remainder of this article focuses on more general recommendations that might facilitate early identification and offers a brief discussion of interventions that might be appropriate to encourage participation and prevention of secondary impairments.
EARLY IDENTIFICATION
Physical therapists are skilled in the observation of gross motor task performance and can assist in accurately identifying and assessing children with DCD. In particular, physical therapists might be able to observe the lack of adaptive flexibility when movement strategies are executed, decreased postural control, reliance on visual feedback, lack of premovement organization, and the fixing that is so characteristic of children with DCD in the early years. These observations might lead to earlier identification than would typically occur and can help to prevent the development of some secondary impairments. In particular, early recognition of children with this disorder can lead to education and guidance that will encourage their engagement in the typical activities of childhood, thereby reducing the risk of decreased self-esteem, self-efficacy, and social participation that have been noted at an early age. 33,80
INTERVENTION
From the perspective of the World Health Organization, interventions can be considered as being directed toward remediation of impairment, reducing activity limitations, and improving participation. 81 Decreasing fitness and strength over the years, withdrawal from participation in physical activity, lack of competence, and lack of motivation have all been documented as serious secondary impairments for children with DCD. 11,32–35 Whether direct intervention targeting impairments is efficacious, or even appropriate, with younger children is not yet clear. There is, however, undoubtedly a role for physical therapy in preventing the long-term physical effects of inactivity in children with DCD. Early consultation with families and physical educators should take place to encourage appropriate leisure activities that emphasize participation and fitness without the risk of injury. 53 Watter 82 supports this view, suggesting that although some physical impairments (eg, low tone) cannot be “cured,” they might be improved through promotion of an active lifestyle.
It is possible that a larger role for the physical therapist in intervention will be to consider the impact of the child's impairments on activity; for example, the therapist might assist young children to learn strategies for managing feedback and organizing their bodies so they can attend to the most salient environmental cues. A recent meta-analysis of intervention studies directed at improving children's activity concluded that there was some evidence for motor skill interventions with children with DCD who are more than five years of age. 83 Movement educators have found task-specific intervention to be a useful way to teach children with DCD specific gross motor skills, 84 and they also emphasize its indirect effect in enhancing general participation in physical activity. 63 Preliminary evidence has also been shown for the effectiveness of cognitive approaches that guide the child in discovering verbally based strategies that help them solve problems during the learning of motor activities. 85 To date, all of these approaches have been used with school-aged children and/or adolescents rather than with preschool children.
The role of the physical therapist in enhancing participation hinges on understanding the relationship between children's impairments and performance of particular activities. It is possible to predict those types of functional tasks that are more likely to be problematic for children with DCD and to understand why certain sports and leisure activities may be more or less successful for them. First, it is important to make the distinction between two types of motor behaviors. Early milestones such as sitting, crawling, and grasping (which are considered basic motor abilities) seem to develop relatively spontaneously in these children without instruction. Coordination difficulties seem to be much more evident with learned skills, including such things as handwriting, playing the piano, kicking a ball, and playing baseball. Children with DCD experience particular difficulty with the skills that require greater precision, continuous adaptability, and eye-hand coordination. 86 It is also important to appreciate the requirements of individual tasks. There are those that require constant monitoring of feedback during task performance and those, that once learned, do not require adaptations in response to environmental feedback. As one might expect, tasks with a heavy reliance on integrating feedback from the senses will be difficult for children with DCD. 86
The type of task as well as the degree of teaching involved therefore need to be taken into account when making recommendations about participation in sporting and leisure activities. Activities like swimming, skating, skiing, and bicycling require some initial teaching of the skill. For children with DCD, these activities might be difficult during early learning of the skill because all novel skills are difficult for them and they don't generalize easily from previous learning. Without encouragement and individualized attention, children with DCD may express dissatisfaction with these activities. Parents can be helped to understand that, because these sports contain a sequence of movements that are very repetitive, children with DCD can become very successful. These are important “lifestyle” sports in which individuals with DCD can continue to participate throughout their lifetime. In contrast, activities such as hockey, baseball, football, basketball, and other ball-related sporting activities contain a high level of unpredictability. When the environment is changing or variable, the child has to not only learn the movement but also continuously monitor the environment to adapt to change. Any time a player is required to hit or catch a baseball, contact a hockey stick to a puck, move quickly around other players, changes must be made in the direction, force, speed, and distance of the movement. Even when the skill is learned, children must continue to adapt to changes in the environment and their place in it. For the child with DCD, participation in any activity that has a high degree of spatial and temporal uncertainty is not likely to be successful. 87
SUMMARY AND CONCLUSIONS
Physical therapists can play a unique role in the early management of children with DCD. As keen observers of movement, physical therapists can assist in early diagnosis by identifying those young children with inadequate anticipatory control, who use fixing (often ineffectively), and who have difficulties with adaptive movement solutions resulting from lack of ability to attend to internal feedback and demands of tasks involving variations in speed, timing, force, or distance of movement in a functional context. Once these children are identified, physical therapists can use their knowledge of the nature of the movement impairments, of potential secondary impairments, and of the impact of each of these on task performance. They can work with families to encourage their children to engage in activities involving continuous, repetitive movements to maintain strength and power and to promote the long-term physical, social, and psychosocial benefits of participation in physical activity.
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