Motion variability--its definition, quantification, and origin.

The concept of motion variability is discussed and a normalized measure for its quantification is introduced. An example demonstrates that this new measure constitutes a global indicator of the current state of a motor learning process. The causes of motion variations are briefly discussed. They include initial perturbations of the skeletal, muscular, and neural systems as well as perturbations due to incremental changes, during motion execution, of external forces, muscular parameters (fatigue), afferent sensory inputs, and of the motor programs controlling the execution of the motion.