Myotonia is a symptom of a small handful of certain neuromuscular disorders characterized by delayed relaxation (prolonged contraction) of the skeletal muscles after voluntary contraction or electrical stimulation.

Myotonia is the defining symptom of many channelopathies such as myotonia congenita, paramyotonia congenita and myotonic dystrophy.

Generally, repeated contraction of the muscle can alleviate the myotonia and relax the muscles thus improving the condition, however, this is not the case in paramyotonia congenita. This phenomenon is known as the "warm-up" reflex and is not to be confused with warming up before exercise, though they may appear similar. Individuals with the disorder may have trouble releasing their grip on objects or may have difficulty rising from a sitting position and a stiff, awkward gait.

Myotonia can affect all muscle groups; however, the pattern of affected muscles can vary depending on the specific disorder involved.

People with disorders involving myotonia can have life-threatening reactions to certain anaesthetics called anaesthesia-induced rhabdomyolysis.