Parkinson's disease also called PD is a progressive disorder of the nervous system. With approximately 20 new cases per 100,000 people. There are 1,000,000 or more people with PD in the United States.

PD is characterized by four main features: rigidity or stiffness of the arms, legs or neck; tremors, usually of the hands; slowness and reduction of movement; and loss of balance. Other symptoms may accompany the main features, including depression, confusion, postural deformity, speech and swallowing difficulty, drooling, dizziness on standing, impotence, urinary frequency and constipation.

While seldom the main symptom, rigidity is one of the four primary symptoms and is experienced as a stiffness of the limbs. In PD, rigidity is greater in the limbs whereas in PD+, rigidity is greater in the neck and trunk.

Tremor, at rest, is usually the earliest and most prominent symptom of PD. It is usually the symptom that brings the patient to the doctor. Patients with tremor usually have a longer and more "benign" course than patients without tremor. The tremor, initially, can involve one side more than the other and the hands more than the feet. The tremor is usually present when the limbs are resting; when the patient is seated with his/her hand supported or when the patient is walking with hands hanging loosely. The tremor usually stops when the muscles are activated. For some patients, the tremor may be more prominent when maintaining a posture (Postural tremer). Occasionally the tremor may increase during movement (kinetic tremor) or the tremor may be prominent during writing. Postural or kinetic tremor are more common in Essential Tremor (ET) than in PD. Several types of tremor can coexist in PD including resting, postural, kinetic and writing tremor.

Bradykinesia is the most disabling symptom of PD. Bradykinesia includes slowness and loss of movement, delays in starting to move, frequent stoppages of movement, fatigue and inability to perform two movements at once, e.g. swinging the arms while walking. The PD patient who is bradykinetic differs from the patient who is weak or paralyzed. Weakness or paralysis is an inability to move because of a lack of power. The PD patient has enough power to move, but cannot move rapidly.

Postural instability results from impairment of the balance reflexes that are responsible for correcting equilibrium in response to positional changes. In PD, as a result of postural instability, patients fall easily. Postural instability may be experienced when a patient attempts to turn or enter a doorway.

The disturbance in walking in PD is characterized by short steps and results from a combination of rigidity, bradykinesia and postural instability.