This study was undertaken in an effort to ascertain whether or not the coarse fibrillation seen in the skeletal muscles of man is in the same category as tremor and to learn what factors influence it. The term "coarse fibrillation" is used advisedly in order to avoid confusion with the type of fine muscle action which physiologists call "fibrillation." Denny-Brown and Pennypacker1 have studied a type of fine muscle contraction following complete muscle denervation. These fibrillations are invisible through the skin. They call these contractions "true fibrillations" and refer to the fibrillations visible through the skin as "fascicular contractions." In this paper we are dealing with the phenomenon of irregular muscle contractions visible through the skin as "rippling," which clinicians have called fibrillation.
Brain2 states: "Fibrillary twitching of muscles is seen in its most typical form as a result of chronic degeneration of the anterior horn cells." This
DE JONG H, SIMONS DJ. A COMPARATIVE STUDY OF FIBRILLATION AND TREMOR. JAMA. 1942;118(9):702-705. doi:10.1001/jama.1942.02830090020005