Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Primary orthostatic tremor ?
    ### Response:
    Primary orthostatic tremor is a movement disorder characterized by rhythmic muscle contractions that occur in the legs and trunk immediately after standing. It may be perceived more as an unsteadiness than an actual tremor. The tremor may disappear or improve when a person is sitting or walking.
    