What's Your Muscle Type?

 By Dan McCarthy//High Performance Consultant

Is it possible for muscle fiber types to change with training? An important variable when classifying an athlete as a sprinter, or an endurance athlete, is having a good idea what their dominant muscle fiber type may be. Identifying the ideal training for an athlete’s events is dependent upon choosing the training most specific to their fiber types. Given the willingness to train like a sprinter, could a world-class endurance athlete become a world-class sprinter, or vice versa?


The short answer is, probably not. Humans have three main fiber types:

Muscle Fiber Chart


Katie Ledecky swims the 1500 at the 2012 Olympics in London (Small).Fifty-two percent of the average person’s muscle fibers are Type I (Slow Twitch), 33% are Type IIa (Fast Twitch Oxidative), and 15% are Type IIb (Fast Twitch Glycolitic). It doesn’t appear as if humans can completely switch a fiber type from Type I to Type II, or Type II to Type I. Basically, because of the characteristics of a Type I fiber at the cellular level, it cannot become a Type II fiber; however, with specific training it does appear that a Type IIa fiber can develop more sprint-like characteristics, and a Type IIb can become better at using oxygen.

When considering world-class athletes, the picture becomes clearer. Both world-class endurance athletes and world-class sprinters have extremely dominant fiber types. Up to 80% of a world-class marathoners muscle fibers will be Type I and 75% of a world-class sprinters muscle fibers will be Type II. Given an athlete with such a clear predisposition for a specific event, it would be unreasoned to train them to excel at an event on the other end of the spectrum.

Muscle fiber type most likely doesn’t change in humans with training, but the capacity of those fiber types will adapt with appropriate exercise. In other words, instead of trying to make a world-class sprinter more like a marathoner, the best bet is to maximize the training which makes that sprinter so fast.