The Kinematics Consequences of Invariant Dynamics during Growth in Children (6-18 Years)

The human growth between age 6 to 18 is characterized by an approximately geometrically similar growth of segments’ dimensions. Geometrical and mechanical properties of the limb segment in the gravitational field and the control properties of the muscles describe the limb and muscle dynamics respectively. The repeatability and smoothness of kinematics patterns in the variety of motor task, in the respect to velocity profiles implies the complex optimal control processes with the application of different criteria (e.g. minimum jerk, Hogan 1984). Alternative theories emphasize the role of limb dynamics (Neilson, 1992) and intrinsic muscle properties in the trajectory formation (Krylow and Rymer, 1977). It is unclear to what extend the limb dynamics affects the trajectory formation. Are the properties of muscle fibres (Asai et al. 1996, Lin et.al. 1996) or limb dynamics (Neilson, 1972) responsible for the limitation of maximum velocities of voluntary movements? It was found previously that the segment’s moment of inertia, passive stiffness and damping of a joint increase according to the fifth power of body height, thus the dynamics of the body segments remains invariant (Lebiedowska et al. 1999) in children 6-18 years old. Thus, there is no need to adapt the control patterns to the changes in body size. The aim of this study was to determine what are the kinematics consequences of the invariant limb dynamics on the execution of selected movement tasks in children 6-18 year olds.