Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) myotonia congenita ?
    ### Response:
    Myotonia congenita is a disorder that affects muscles used for movement (skeletal muscles). Beginning in childhood, people with this condition experience bouts of sustained muscle tensing (myotonia) that prevent muscles from relaxing normally. Although myotonia can affect any skeletal muscles, including muscles of the face and tongue, it occurs most often in the legs.
    