The development of coordination for reach-to-grasp movements in children.

When adults reach to grasp stationary targets, movement kinematics (endpoint trajectories, interjoint coordination) are highly stereotyped and stable. The emergence of an optimal coordination for reaching involves mastering the redundant number of degrees of freedom while the body grows. Reaching has been well studied in healthy children under the age of 3 years. We characterised the development of coordination during reaching in children over the age of 3 years and identified age ranges in which stable patterns emerge. A random sample of 38 healthy children aged 4-11 years and nine adults participated in the study. They reached from the seated position with the dominant arm and grasped a cone placed at three distances in the forward sagittal plane in front of the body. Kinematic data from markers placed on the arm, head and trunk were recorded at 100 Hz (Optotrak Motion Analysis System). Immature patterns of reaching were characterised by increased variability in younger compared to older children. Hand trajectories became smoother and less variable with age. Interjoint coordination became more consistent, while trunk displacement and variability decreased with age. Only children between 8 and 10 years old had variability similar to adults. Our data suggest that different aspects of movement kinematics mature at different rates. However, our data do not support the idea of a sequential maturation of different biomechanical variables.