The REAL charlie horse.

Hyperkalaemic periodic paralysis is a genetic disorder which occurs in both humans and horses. It is characterized by muscle hyperexcitability or weakness which, exacerbated by potassium or cold, can lead to uncontrolled shaking followed by paralysis. Onset in humans usually occurs in the twenties.
-http://en.wikipedia.org/wiki/Hyperkalaemic_periodic_paralysis