Primary orthostatic tremor is a rare, progressive movement disorder that causes unsteadiness when standing still, due to a rapid tremor affecting the legs and trunk. The tremor is sometimes described as having 'shaky legs,' and it improves or disappears when walking, sitting, or lying down. This may cause people with Primary orthostatic tremor to attempt to sit again or walk immediately after standing, for fear of falling. People with POT may also experience tiredness, physical exhaustion, muscle stiffness or weakness, and/or pain. The diagnosis of POT relies on a clinical exam as well as a specialized test called a surface electromyogram (EMG), which measures electrical activity in the muscles. The cause of POT is not completely understood, but the disorder is thought to involve dysfunction of the brain's ability to regulate nerve signals that control muscle activity.