By: Arthur Anderson

You may have caught a coach talking about different muscle types in class, or maybe it was in a health and fitness article you read. Essentially, we have been told that there are three different types of muscle fiber; Type I (slow twitch), Type IIa (fast twitch) and Type IIb (superfast twitch). Okay so what does the this mean. A Type I, slow twitch, muscle fiber is more efficient at using oxygen to generate ATP and can go a long time before fatigue sets in; they are designed for endurance. A Type II, fast twitch and superfast twitch, muscle fiber is more efficient at anaerobic metabolism and provide for powerful bursts of energy. It has been believed for some time that you were born with fixed distribution of these fibers in your muscles. So, if you were born with a higher portion of slow twitch muscle fibers you have the inherent advantage to be a triathlete. Or if you had a higher portion of fast twitch muscle fibers you could have the advantage of being a great power lifter or sprinter. If this theory were true wouldn’t you expect that those with predominantly slow twitch muscle fibers be the first ones culled out of the herd back when we were still being chased by big predators? That would mean today we should only have a population of power lifters and sprinters.

A recent article in the January 2018 edition of the Men’s Journal, “Inside the Muscle Cell”, regarding muscle fiber research dispels this theory of fixed muscle fiber distribution. The new thinking is that muscles can be changed over time. The study conducted by Andy Galpin, Ph.D. at Cal State Fullerton has discovered that we actually have six types of muscle fiber. Still the slow twitch, fast twitch and superfast twitch. The additional three types of muscle fiber are a mixture, or hybrid, of these three; slow/fast, fast/superfast, slow/fast/superfast and it’s these hybrid muscle fibers that can be converted from a hybrid to a single type of muscle fiber, (i.e. slow/fast to a fast twitch muscle fiber). These hybrid muscle fibers wait on the sidelines until they experience enough stimulus to change to a single type of muscle fiber and join the team.

How much stimulus do you need to entice these hybrid muscle fibers to change? In the research an average person may have a muscular makeup of about 30% hybrid muscle fibers. Hybrid muscle fibers can begin to change in a couple weeks, which is interesting because that is usually the time period I see beginning Cross fitters start to feel subtle changes in their workouts, a little stronger, a little faster. Within a year the Galpin study has seen up to a 20% conversion of hybrid muscle fibers. Is there a downside to all this? Yes and no. These hybrid muscles will revert back to their original hybrid state within a couple weeks of inactivity.

Just another amazing thing that your body does to meet the demands put upon it. Andy Galpin’s site, provides free podcasts about human physiology, nutrition and human performance, some pretty interesting stuff if you want to learn more about the human body.