Myotonia 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. SNOMEDCT_US:3434004 An involuntary and painless delay in the relaxation of skeletal muscle following contraction or electrical stimulation. Delayed relaxation of muscle fibers after contraction Delayed relaxation of muscle fibres after contraction HP:0002486 HP:0003792 UMLS:C0027125 HP:0003754 HP:0003632 Abnormal muscle physiology