In the simplest terms, fast-twitch (type II) muscle fibers are built for short, powerful bursts of energy—that’s in contrast to slow-twitch (type I) muscle fibers, which are built for endurance activities like long-distance running or biking. Type II muscle fibers (IIa & IIb/IIx) are able to produce lots of force but they also tire quicker than slow-twitch muscle fibers.
