A fasciculation, or "muscle twitch", is a small, local, involuntary muscle contraction and relaxation. At times it may be visible under the skin. Deeper areas can be detected by EMG testing. A fasciculation can happen in any skeletal muscle in the body. Fasciculations can occur in normal healthy people. One uncommon reason for them is due to a lower motor neuron lesion that may be caused by any of several diseases. One example would be Amyotrophic Lateral Sclerosis.
- Sabyasachi Sircar, Principles of Medical Physiology (Stuttgart; New York: Thieme, 2008), p. 116