A fasciculation /fəˌsɪkjʉˈleɪʃən/, or "muscle twitch", is a small, local, involuntary muscle contraction and relaxation which may be visible under the skin or detected in deeper areas by EMG testing. They arise as a result of spontaneous depolarization of a lower motor neuron leading to the synchronous contraction of all of the skeletal muscle fibers within a single motor unit. Fasciculations can happen in any skeletal muscle in the body. Fasciculations have a variety of causes, the majority of which are benign, but can also be due to disease of the motor neurons. Fasciculations are commonly encountered in healthy people and are rarely bothersome. In some cases the presence of fasciculations can be annoying and interfere with quality of life. In such cases, where the remainder of the neurological exam is normal, and EMG testing does not indicate any additional pathology a diagnosis of benign fasciculation syndrome is usually made.

The most effective way to detect fasciculations may be surface electromyography (EMG). Surface EMG is more sensitive than needle electromyography and clinical observation in the detection of fasciculation in people with ALS.