Simply it’s very lean muscle… years ago “Hybrid Type II” was the cool word.
Individual muscle is made up of three muscle fiber types: Type I, Type IIa, and Type IIb (or IIx depending what school you sit) in varying proportions. When these portions reach a certain balance (highly disputed percentages of course) the muscle provides both powerful and explosive movements coupled with aerobic endurance… and thus you have Hybrid Type III…
So Hybrid Type III in reality is actually just another fancy name for Hybrid Type II which is Type IIa (note: which can be both IIa and IIb/IIx) muscle fiber that has taken on the attributes and traits of a Type I muscle fiber.
To make it even more confusing… many sport physiologists/researchers/scientists believe Type IIb doesn’t exist in humans (it exists only in rodents and other small animals) and that we should use Type IIx instead… Type IIx being somewhere in between IIa and IIb in speed of muscle contraction (IIb being the highest or fastest). So, this would mean Type IIa muscle fiber is essentially a hybrid of Type I and Type IIx fibers; and thus, Type II fibers are faster than Type I and Type IIx is faster than Type IIa.
Type I = Slow Oxidative (SO)
Type IIa = Fast Oxidative (FO)
Type IIx = Fast Oxidative Glycolytic (FOG)???
Type IIb = Fast Glycolytic (FG)
This would mean that Type IIa is a HYBRID fiber mix between Type I and Type IIb orIIx fibers with very high contraction rate (making them very strong and explosive).
What has been found through research which is important to the athletes is that it appears that pure fast (Type IIb or IIx) fibers can transition and adapt to “hybrid” (Type IIa) fibers with heavy endurance training. Biopsies taken of elite endurance athletes have shown that they have almost no IIb fibers, but do have a significant percentage of the intermediate Type IIa fibers. It appears that the research suggests that Type IIa fibers do not transition into Type I fibers.