I. Introduction
Parkinson's Disease (PD) is a common neurodegenerative disorder that as of 2005 was estimated to affect over 4 million people worldwide, with this number expected to double by 2030 [10]. Common symptoms of PD include bradykinesia, tremors, rigidity, instability, and changes in gait, including freezing of gait [25], [30]. Patients with PD walk more slowly than healthy individuals and the stride amplitude is also less than normal, although cadence is within normal parameters. In addition, they tend to have shorter and slower steps [17].