If you have a spinal cord problem, something that causes the spinal cord to be impinged at a certain level, you can get "clonus" spasms. These are electrical impulses that just get caught between your spinal cord impingment and where they originated, causing your legs, for example to just "jump". or, if it's in your neck, your arms could do the same thing.
These are perfectly normal and very controlable. Baclophen, a fast acting, very strong muscle relaxer is very effective in preventing these.
Lots of love and healing hugs,
Dan