Myoclonus is sudden jerking or twitching of a muscle or group of muscles. You can have one episode or many in a row. And they can last as long as a few minutes. Some types of myoclonus happen to healthy people. A hiccup or doing a double-take when something startles you are common examples. But some people have myoclonus because of a health problem like multiple sclerosis, dementia, or Parkinson's disease.