Muscle Fiber Fact vs. Fiction
Jerry BrainumNew Findings Reveal the Truth Behind Muscle Growth
It’s no secret that having favorable genetics gives you a significant head start on bodybuilding success. Muscle fibers can be among those genetic advantages. Longer muscles, which have more muscle fibers, can produce greater rates of muscular growth. The number of fibers is determined at birth, although the types of fibers may be subject to change to a certain extent. Take, for example, the calf muscles. Having high calves usually means you have fewer muscle fibers in your calves, which translates into less potential for growth. That doesn’t preclude muscle growth; it just means that the odds of obtaining massive calves are stacked against you.
Human muscle fibers come in three main types:
1) Type 1, or slow-twitch, are smaller and generally more suited to endurance, or aerobic, activity.
2) Type 2A are intermediate, showing some of the characteristics of types 1 and 2B fibers.
3) Type 2B are most the amenable to growth. They’re the “strength and size” fibers. They have a low resistance to fatigue, as they lack the extensive blood vessels and mitochondria present in type 1 fibers. They work mainly through anaerobic metabolism. On the other hand, type 2B fibers have the thickest motor neuron connections, which means they produce greater force than the other kinds of fibers.
Muscle fibers are recruited in a certain order, with type 1 fibers being activated first, followed in order by the type 2As and 2Bs. Most exercise physiology textbooks say that type 2B fibers can be recruited only by heavy weight and high intensity. For years bodybuilders have been told that they need heavy weights and high intensity to achieve gains in muscle size and strength. That’s because they need to activate type 2B muscle fibers.
Years ago a study was published that compared the muscle fibers in elite competitive bodybuilders to those of unathletic physical education students. Because the bodybuilders had arm circumferences that averaged 19 to 20 inches, the researchers fully expected the bodybuilders to show far larger muscle fibers than the students had. After all, muscle growth involves a thickening of muscle fibers as a result of intense exercise, which causes increased muscle protein synthesis. Yet when viewed under the microscope, the muscle fibers of the massive bodybuilders weren’t all that different from those of the far less muscular students. How could that be?
The researchers suggested that years of heavy and intense training had encouraged a process called hyperplasia, which is a splitting of muscle fibers. So while the bodybuilders’ individual muscle fibers weren’t larger than normal, they’d produced far more of them. As the fibers themselves weren’t counted, the hyperplasia hypothesis remained speculative, but how else to explain the notable disparity in muscle size between the bodybuilders and the students?
A more recent study also produced a number of surprises. Once again, champion bodybuilders were compared to ordinary college students. The bodybuilders all consumed a high-protein diet, averaging 200 to 220 grams daily, and none had used anabolic steroids for two years prior to the study—or so they said. They trained regularly four to six times a week, for three to four hours at each session. The muscles examined in the study were the front thighs, and the bodybuilders’ leg routines averaged two sessions a week. They did 12 sets of 10 to 20 reps, using 70 to 90 percent of one-rep-maximum weights.
Note that the study examined single muscle fibers. Since the type 2B fibers are the muscle fibers most likely to grow, it stands to reason that the bodybuilders in the study would have an abundance of such fibers, or at least more of them than the other kinds of muscle fibers. The reality was that they showed a higher portion of types 1 and 2A fibers, with a complete absence of type 2Bs. How could that be?
Training. Typical bodybuilding training isn’t characterized by using maximum weight for six reps or fewer but instead involves higher reps, less rest time between sets and high training intensity. It turns out that this style of training favors the transformation of fibers into type 2As, which have some of the characteristics of both strength and endurance, exactly matching the way most bodybuilders train. The body adapts by spurring the development of the muscle fibers most efficient for the purpose: type 2A fibers.