Fasciculations are slight and involuntary muscular contractions that occur underneath the skin, but that do not produce any observable limb movement. Fasciculations are visible to the naked eye and are sometimes described as looking like small worms are moving within the muscle. These contractions occur because of spontaneous nerve discharges that fire within clumps of skeletal muscle fibers.
Fasciculations are typically benign, but sometimes they can be caused by neurological diseases, such as Amyotrophic Lateral Sclerosis. In the specific case of ALS, the fasciculations occur due to damage present in the lower motor neurons. They can be considered an early warning sign of the possible onset of ALS.