The accurate description of limb mechanics is paramount to developing an understanding of the neural control of movement in humans. The mechanics of upper limb movement can be described by stiffness, viscous and inertial parameters. Both stiffness and viscosity can be readily modulated by the central nervous system through neural input to the muscles of the limb. While there is no physiological mechanism to directly alter the inertial properties of a single limb segment, there is evidence that the CNS can plan multi-segmental limb movement with respect to inertia by altering limb configuration (Sabes et al., 1999).

We sought to investigate the influence of altering the inertial properties of the limb on the stability of rhythmic circling movements. Six participants were asked to perform a rhythmic circling task in various planar inertial force fields generated by a two degree of freedom servo-controlled robotic manipulandum. Subjects grasped a handle fixed to the end-point of the manipulandum with their left hand. A cursor on a computer screen placed in front of the subject provided feedback of the handle position as they traced around a target ellipse. The target ellipse was oriented in one of two configurations; one in which the major axis was 45 degrees from horizontal and the other in which the major axis was 135 degrees from horizontal. The movements were performed in three different inertial force fields, null, coincident and orthogonal to the target ellipse and were paced by an auditory metronome.

Results indicated that the inertial properties of the limb have a profound influence upon not only the stability of the movement but also the preferred orientation of the movement ellipse. This study demonstrates the powerful constraints that limb mechanics have on the movement options of the central nervous system.