develops from fast muscle cell Fast muscle cells, also known as type II muscle fibers, are a particular type of muscle cell that specializes in conducting rapid, high-intensity contractions. These cells are primarily found within skeletal muscle groups that are directly involved in gross motor activities, such as running, jumping, or lifting heavy weights. The designation "fast" refers to their speed of contraction in response to neural stimuli, which is significantly quicker when compared to other muscle cell types such as slow-twitch or type I muscle fibers. The primary function of fast muscle cells revolves around their role in anaerobic metabolism, which provides the energy needed for short, forceful bursts of power. They contain a high concentration of glycolytic enzymes that facilitate this process, leading to a swift break down of glucose to generate ATP (adenosine triphosphate), the primary energy currency in biological systems. On the downside, the byproduct of this very rapid, anaerobic metabolic process is lactic acid, which can build up and cause muscle fatigue. Fast muscle cells can be further classified into two subtypes based on their metabolic characteristics: type IIa and type IIb/x. Type IIa cells, also known as fast oxidative-glycolytic fibers, possess a good oxygen supply and can function in both anaerobic and aerobic conditions, exhibiting moderate resistance to fatigue. On the other hand, type IIb/x cells, also known as fast glycolytic fibers, primarily rely on anaerobic metabolism and tire out a lot quicker. Both these subtypes can adapt to changing demands due to exercise and conditioning, highlighting the plasticity that is a key characteristic of these versatile muscle cells. (This extended description was generated by ChatGPT and reviewed by the CellGuide team, who added references, and by the CL editors, who approved it for inclusion in CL. It may contain information that applies only to some subtypes and species, and so should not be considered definitional.) A muscle cell that can develop high tension rapidly. It is usually innervated by a single alpha neuron. https://cellxgene.cziscience.com/cellguide/CL_0000190 ZFA:0009117 glycolytic muscle fiber fast muscle myoblast extrafusal muscle fiber