Patients often describe myotonia as stiffness that is worse at the onset of activity and that improves with repeated muscle contractions. Myotonia tends to worsen in cold weather. Myotonia can be elicited by voluntary muscle contraction of by muscle percussion (e.g., by tapping the thenar eminence). During physical examination, myotonia may be elicited by asking the patient to make a tight fist and then quickly open the hand. Myotonia is caused by an abnormality in the muscle membrane, i.e., of the ion channels that control the contraction of muscle fibers.