Freezing of gait appears to result from a number of fundamental problems in patients with Parkinson disease. Automaticity is impaired, putting more stress on voluntary mechanisms. Internal drivers of movement are impaired, likely because of deficient basal ganglia function. Deficiency of internal forces to initiate movement is a major factor in freezing. This deficiency gives a greater influence to external or sensory factors. The sensory factors can both help or hinder freezing. Analogous to the problem with set-shifting, there is also some difficulty in regulation of internal versus external factors and in regulation of different external factors.