A muscle cramp is a sudden, severe, and involuntary muscle contraction or over-shortening. It can cause mild-to-severe pain, and a paralysis-like immobility. Usually, it resolves on its own over several seconds, minutes, or in the worst scenario after several hours. In the healthy adult population, the incidence of muscle cramping is 50–60%.

Source Journal of Electromyography and Kinesiology , Volume 41, Pages 89-95

Cramps are usually classified into three types - the first two are so common that many consider them a 'normal' part of human existence.

1) those that occur during sleep without any obvious trigger.

2) those triggered by vigorous exercise.

The third type, which is very much less common, is as a result of pathological disorders of some kind (.e.g. diabetes etc )

Scientific research attempting to establish the exact cause of cramp began more than 100 years ago. Leading to two widespread theories : the 'dehydration hypothesis' and 'electrolyte depletion hypothesis'.

It's now generally considered, however, that neither of the two theories offers a full explanation (see the review paper linked above).

Research is hampered by the fact that cramps tend to be very intermittent, and are difficult (and possibly unethical) to spontaneously generate.

Note that there is also no proven effective drug treatment for the prevention of cramp.

General details :Wikipedia

Further info.on excercise related cramp ^{[ paywalled ]} Muscle & Nerve, 54,177–185

Also see : Writer's Crampplugin-autotooltip__plain plugin-autotooltip_bigWriter's Cramp

Writer's cramp, also called mogigraphia and scrivener's palsy, is a disorder which involves cramps or spasms of muscles of the hand and/or forearm.

"Writer's cramp is a focal dystonia of the hand in which an individual, usually someone whose occupation requires a significant amount of writing, has paresis or even paralysis when attempting to write despite having no weakness in the hand on standard clinical testing, no musculoskeletal problems in the arm that could account for t…