Involuntary muscular contractions (6)
I believe the answer is:
spasms
'involuntary muscular contractions' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for spasms that I've seen before include "Cramps or contractions" , "Seizures and convulsions" , "Sudden convulsions" , "spells" , "Convulsions, fits" .)