Bi-stability of movement coordination as a function of skill level and task difficulty.

This study investigated whether the level of practice interacts with the initial conditions (here manipulated as preparatory movements) and task difficulty (ball angular velocity and friction) in determining the stability of movement coordination for a roller ball motor task. Practice level and task difficulty were manipulated as two control parameters that theoretically were hypothesized to change the threshold for the transition between task success and failure. The findings showed bi-stability and hysteresis in the coordination mode as a manifestation of dependence on the initial conditions. The transition from failure to success in the roller ball task as a function of practice time can be modeled as a saddle-node bifurcation corresponding to a first order phase transition. It is proposed that task difficulty acts as a control parameter that is a dual to skill level and that also effectively compensates for practice time.