The textbook sports-physiology description of muscles includes two major muscle fiber types: fast-twitch fibers for sprinting and slow-twitch fibers for endurance. Very little is known about another type of fiber, the IIX muscle fiber. Unlike the predominant fiber types, this type is dispersed throughout the different muscles in the body and therefore nobdy really knew what their specialization was. A recent Nature news article now reports a significant breakthrough in understanding the function of this rare muscle type. The research is published in the journal Cell Metabolism. It appears that despite their relative scarceness, type IIX fibers contribute significantly to athletic performance.
Obviously, a drug or treatment which could increase the proportion of IIX fibers in human muscle would be the ultimate doping treatment.