A new perspective on the development of motor variability during middle childhood

However you look at it, children’s movements are variable. Variability is an inherent characteristic of motor development. This thesis examines motor variability in middle childhood development (5-10 years of age) in typically developing children as well as in children with Developmental Coordination Disorder (DCD), a movement disorder that is characterized by high levels of variability. Doing so, this thesis puts the spotlight on the developmental period of middle childhood, therewith rejuvenating the ‘forgotten years’ of development. Until now, variability in motor activities in this period has been viewed as something negative, reflecting error of execution and therefore hindering performance of everyday life activities. The results of this thesis indicate that variability in middle childhood may play a different role than previously assumed. It is proposed that variability is a form of exploration to find suitable solutions to perform a particular task. That is, this thesis changes the connotation of variability in middle childhood from negative to positive. This is an exciting finding for research concerning DCD, because broadening the concept of motor variability in DCD may offer new understanding about possible underlying coordination mechanisms of this disorder. This thesis paves the way for a new perspective on variability in middle childhood in typically as well as in atypical developing children

Adjustments in end-effector trajectory and underlying joint angle synergies after a target switch:Order of adjustment is flexible

Goal-directed reaching adapts to meet changing task requirements after unexpected perturbations such as a sudden switch of target location. Literature on adaptive behavior using a target switch has primarily focused on adjustments of the end-effector trajectory, addressing proposed feedback and feedforward processes in planning adjusted actions. Starting from a dynamical systems approach to motor coordination, the current paper focusses on coordination of joint angles after a target switch, which has received little attention in the literature. We argue that joint angles are coordinated in synergies, temporary task-specific units emerging from interactions amongst task, organism, and environmental constraints. We asked whether after a target switch: i) joint angles were coordinated in synergies, ii) joint angles were coordinated in a different synergy than the synergy used when moving to the original target, and iii) synergies or end-effector trajectory was adjusted first. Participants (N = 12) performed manual reaching movements toward a target on a table (stationary target trials), where in some trials the target could unexpectedly switch to a new location (switch trials). Results showed that the end-effector curved to the switched target. Joint angles were synergistically organized as shown by the large extent of co-variation based on Uncontrolled Manifold analyses. At the end of the target switch movement, joint angle configurations differed from the joint angle configurations used to move to the original stationary target. Hence, we argue, a new synergy emerged after the target switch. The order of adjustment in the synergies and in the end-effector was flexible within participants, though most often synergies were adjusted first. These findings support the two-step framework of Kay (1988) to understand the coordination of abundant degrees of freedom and to explain adaptive actions. The flexibility in the order of adjustments of synergies suggests that the coordination of DOF emerges from self-organization

What the Dynamic Systems Approach Can Offer for Understanding Development:An Example of Mid-childhood Reaching

The Dynamic Systems Approach (DSA) to development has been shown to be a promising theory to understand developmental changes. In this perspective, we use the example of mid-childhood (6- to 10-years of age) reaching to show how using the DSA can advance the understanding of development. Mid-childhood is an important developmental period that has often been overshadowed by the focus on the acquisition of reaching during infancy. This underrepresentation of mid-childhood studies is unjustified, as earlier studies showed that important developmental changes in mid-childhood reaching occur that refine the skill of reaching. We review these studies here for the first time and show that different studies revealed different developmental trends, such as non-monotonic and linear trends, for variables such as movement time and accuracy at target. Unfortunately, proposed explanations for these developmental changes have been tailored to individual studies, limiting their scope. Also, explanations were focused on a single component or process in the system that supposedly causes developmental changes. Here, we propose that the DSA can offer an overarching explanation for developmental changes in this research field. According to the DSA, motor behavior emerges from interactions of multiple components entailed by the person, environment, and task. Changes in all these components can potentially contribute to the emerging behavior. We show how the principles of change of the DSA can be used as an overarching framework by applying these principles not only to development, but also the behavior itself. This underlines its applicability to other fields of development

A new perspective on the development of motor variability during middle childhood

However you look at it, children’s movements are variable. Variability is an inherent characteristic of motor development. This thesis examines motor variability in middle childhood development (5-10 years of age) in typically developing children as well as in children with Developmental Coordination Disorder (DCD), a movement disorder that is characterized by high levels of variability. Doing so, this thesis puts the spotlight on the developmental period of middle childhood, therewith rejuvenating the ‘forgotten years’ of development. Until now, variability in motor activities in this period has been viewed as something negative, reflecting error of execution and therefore hindering performance of everyday life activities. The results of this thesis indicate that variability in middle childhood may play a different role than previously assumed. It is proposed that variability is a form of exploration to find suitable solutions to perform a particular task. That is, this thesis changes the connotation of variability in middle childhood from negative to positive. This is an exciting finding for research concerning DCD, because broadening the concept of motor variability in DCD may offer new understanding about possible underlying coordination mechanisms of this disorder. This thesis paves the way for a new perspective on variability in middle childhood in typically as well as in atypical developing children

Task constraints act at the level of synergies and at the level of end-effector kinematics in manual reaching and manual lateral interception.

International audienceTo coordinate the redundant degrees of freedom (DOF) in the action system, synergies are often proposed. Synergies organi e DOF in temporary task-specific units emerging from interactions amongst task, organism, and environmental constraints. We examined whether task constraints affect synergies, end-effector kinematics, or both. To this end, we compared synergies and end-effector kinematics when participants (N = 15) performed discrete movements of identical amplitude in manual reaching (stationary targets) and manual lateral interception (moving targets, with different angles of approach). We found that time-velocity profiles were roughly symmetric in reaching, whereas they had a longer decelerative tail and showed an angle-ofapproach effect in interception. Uncontrolled Manifold analyses showed that in all conditions joint angle variability was primarily co-variation, indicating a synergistic organi ation. The analysis on the clusters of joint angle configurations demonstrated differences between reaching and interception synergies, whereas more similar synergies were used within interception conditions. This implies that some task constraints operate at the level of synergies while other task constraints only affect end-effector kinematics. The results support a two-step process in the organi ation of DOF, consisting of synergy formation and further constraining of synergies to produce the actual movement, as proposed by Kay (1988)

The cost of simplifying complex developmental phenomena:a new perspective on learning to walk

Researchers can study complex developmental phenomena with all the inherent noise and complexity or simplify behaviors to hone in on the essential aspects of a phenomenon. We used the development of walking as a model system to compare the costs and benefits of simplifying a complex, noisy behavior. Traditionally, researchers simplify infant walking by recording gait measures as infants take continuous, forward steps along straight paths. Here, we compared the traditional straight-path task with spontaneous walking during 20 minutes of free play in 97 infants (10.75-19.99 months of age). We recorded infants' footfalls on an instrumented floor to calculate gait measures in the straight-path and free-play tasks. In addition, we scored videos for other critical aspects of spontaneous walking-steps per bout, shape of walking path, and step direction. Studying infant walking during free play incurred no cost compared with the straight-path task, but considerable benefits. Straight-path gait was highly correlated with spontaneous gait and both sets of measures improved with walking age, validating use of the straight-path task as an index of development. However, a large proportion of free-play bouts were too short to permit standard gait measures, and most bouts were curved with omnidirectional steps. The high prevalence of these "non-canonical" bouts was constant over development. We propose that a focus on spontaneous walking, the phenomenon we ostensibly wish to explain, yields important insights into the problems infants solve while learning to walk. Other areas of developmental research may also benefit from retaining the complexity of complex phenomena

Individual Differences in Learning a Novel Discrete Motor Task

Many motor learning studies focus on average performance while it is known from everyday life experience that humans differ in their way of learning new motor tasks. This study emphasises the importance of recognizing individual differences in motor learning. We studied individual tool grasping profiles of individuals who learned to pick up objects with a novel tool, a pair of pliers. The pair of pliers was attached to the thumb and the index finger so that the tip of the thumb and the tip of the index finger were displaced to the beaks of the pair of pliers. The grasp component was manipulated by varying the location of the hinge of the pair of pliers, which resulted in different relations between beak opening and closing and finger opening and closing. The Wider Beak group had the hinge at 7 cm, the Same Beak group had the hinge at 10 cm (i.e., in the middle), and the Smaller Beak group had the hinge at 13 cm from the digits. Each group consisted of ten right-handed participants who picked up an object with one of the pairs of pliers 200 times on two subsequent days. Hand opening, plateau phase, hand closing, grasping time and maximum aperture were analyzed. To characterize individual changes over practice time, a log function was fitted on these dependent variables and the ratio of improvement was determined. Results showed that at the beginning stage of tool use learning the characteristic grasping profile consisted of three phases; hand opening, plateau phase and hand closing. Over practicing individual participants differed in the number of phases that changed, the amount of change in a phase and/or the direction of change. Moreover, with different pliers different learning paths were found. The importance of recognizing individual differences in motor learning is discussed