Clonus (from the Greek for “violent, confused motion”) is a series of involuntary muscular contractions due to sudden stretching of the muscle. Clonus is a sign of certain neurological conditions, and is particularly associated with so-called upper motor neuron lesions such as amyotrophic lateral sclerosis (ALS), stroke, multiple sclerosis or spinal cord damage. Unlike the small, spontaneous twitching known as fasciculations (usually caused by lower motor neuron pathology), clonus causes large motions that are usually initiated by a reflex.

Clonus is most common in the ankles, where it is tested by rapidly flexing forward (dorsiflexing) the relaxed ankle. It can also be tested in the knees by rapidly pushing the patella towards the toes. Only sustained clonus (5 beats or more) is considered abnormal. Clonus could be caused by the use of magnesium sulfate