If salt consumption, resting after exercise and even sudden noises trigger extreme weakness in your muscles or near-total (but temporary) paralysis, a genetic condition called periodic paralysis might be to blame. Fortunately, only about 1 in 100,000 people know the feeling.
It's caused by faulty ion channels in the walls of nerve cells. Potassium isn't able to properly pass through the channels that would allow it to cross through the cell wall when the muscle cell gets the "Go!" message from the brain. The muscles can't contract, and the body feels weak. (There are different types of periodic paralysis and, depending on which type a person has, potassium levels may be too high or too low in the cell.)
When an attack is about to occur, a person with this condition feels weakness or heaviness in the legs. Sometimes, small, random muscles are affected; other times, his or her arms and legs become immobilized. The incident may last for a few hours, or as long as a few days. By quickly beginning to exercise, a full attack may be prevented, but there is no cure. Dietary restrictions and avoiding excessive exercise or fasting are common treatments.
Though muscles will operate at full strength between attacks, years of periodic paralysis can permanently diminish muscle strength.