Myotonia
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Myotonia is a neuromuscular disorder characterized by the slow relaxation of the muscles after voluntary contraction or electrical stimulation. Generally, repeated effort is needed to relax the muscles, and the condition improves after the muscles have warmed-up. 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.
The disorder can affect all muscle groups. It may be acquired or inherited, and is caused by an abnormality in the muscle membrane. Myotonia is a symptom commonly seen in patients with myotonic muscular dystrophy and in a group of disorders called channelopathies (hereditary diseases that are caused by mutations in the chloride, sodium or potassium ion transport channels in the muscle membrane). Myotonia arising from channelopathies can be exacerbated by exposure to cold, by eating foods that are potassium-rich (such as bananas), and with exertion.
Interestingly, myotonia is not always a disease-related or abnormal phenomenon. Humans and other animals often display myotonia when placed in situations of extreme stress or fear; a resultant increase in 'fight-or-flight' hormones such as epinepherine and cortisol may cause increased muscle tension throughout the body.
External link
National Institute of Neurological Disorders and Stroke: Myotonia (http://www.ninds.nih.gov/health_and_medical/disorders/myotonia.htm)