Head, arm and trunk coordination during reaching in children

During postural and locomotor tasks, the orientation of the head with respect to space is maintained in order to serve as an egocentric reference value for maintaining balance. In young children during locomotor tasks, task difficulty determines the coordination of movements between head-trunk segments: the more difficult the task, the more the child limits the head on trunk movement ("en bloc") rather than letting the head move freely in space. For reaching tasks, however, there are no data about the development and maturation of coordination between the head and trunk movements and when the pattern of coordination is considered mature. The goal of this study was to characterize the development of head-trunk coordination during reaching from a sitting position in typically developing children. Forty-four typically-developing (TD) children aged from 2.8 to 11.8 years and six healthy adults participated. Children were divided into five groups (G1-G5) according to their age: 2-3, 4-5, 6-7, 8-9 and 10-11 years old. The task involved reaching towards and grasping a piece of food in the younger group or a wooden block in the older children and adults with the dominant hand, adequate to the grip size of each participant, and returning it to the mouth area to simulate self-feeding. The object was placed in line with the midline of the body at three different distances from the trunk according to the participant's arm length (two within and one beyond arm's length). Rotational movements of the head and trunk in three planes; yaw, roll and pitch, were recorded using three-dimensional tracking systems (Optotrak, Northern Digital, Model 3010 or Ariel Performance Analysis System). The variables analysed were relative head and trunk angle, absolute head and trunk angle, the anchoring index (AI) and initial direction of head and trunk rotation (direction index: DI). Patterns of head-trunk coupling were different along different axes of rotation and across groups. For the AI, a head-stabilized-on-trunk (HST) or "en bloc" pattern was observed with approximately the same frequency as a head-stabilized-in-space (HSS) pattern in the youngest children in the yaw plane for reaches within arm's length. In all other planes and for reaches of all distances, a HSS pattern was evident in the youngest children and remained consistent across the groups of children. Compared to the children, adult reaching was characterised by fixed head-trunk coordination (HST) in the roll plane at all reach distances, and greater decoupling in yaw plane motion for the two closest distances. There were no age-related differences in the pitch plane strategy which was mainly HSS. The DI patterns matured by 2-3 or 4-5 years of age, except for reaches to T1 in the pitch plane. In addition, in the roll plane, there was evidence of a two-step maturation that was not complete until adulthood. Maturation of strategies used to stabilize the head and trunk relative to each other and to the reaching arm differ across movement planes for a seated reaching task. Our data suggest that different aspects of head and trunk coordination during reaching movement mature at different rates, like for locomotor tasks previously described, and that the maturation follows a non-chronological and protracted course. These results can serve as a comparative database with which to contrast head and trunk coordination in children with movement disorders. However, in terms of typical development, these data should be considered specific for the task studied and may not reflect general principles of motor development.