Amyotrophic lateral sclerosis (ALS) is a degenerative neurological disease affecting only the motor system. It is also referred to as motor neuron disease (MND) and is characterized by muscle wasting, weakness and fasciculations. Fasciculations refer to involuntary contractions/twitching of affected muscles. Affected muscles are wasted (thin) and weak.
In this video, you can see fasciculations in deltoid, biceps and triceps muscles, more on the left side. The affected muscles are wasted too.
Fasciculations can also can occur in another condition, called as benign fasciculation syndrome (BFS). BFS, as the name suggests, is not a serious condition. Fasciculations can occur in any part of body, however, there is no weakness or wasting of muscles in BFS.