New Findings Reveal the Truth Behind Muscle Growth
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. 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 near 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.
While type 2Bs have low fatigue resistance, 2As have intermediate fatigue resistance. Type 2B fibers last less than a minute before fatigue sets in, while 2As can go for five minutes. One reason is that 2A fibers contain more mitochondria—the cell’s energy factory—than type 2Bs and are more efficient at using oxygen, thus making fuel more available.
As for type 2Bs, the authors say that in bodybuilders they work more or less as reserve fibers. When recruited during exercise, they convert into the type 2A fibers that are so abundant in bodybuilders. Interestingly, high numbers of type 2B fibers are found in obese women and in those with spinal cord injuries, in the latter case likely as the body’s way of compensating for the injury.
One limitation of the study, the authors suggest, is that the bodybuilders might have been born with greater numbers of type 2 muscle fibers, thus having a genetic predisposition toward more muscle growth.
What does it all mean in a practical sense? More than anything that bodybuilders’ knowledge is empirical. They’ve discovered from years of experience that using weights that give them a range of eight to 12 reps yields the most muscle growth. Now we know why: Because of the middle-of-the-road type 2A fibers, which have characteristics of both aerobic and anaerobic muscle fibers. So for maximum training progress it’s best to gear your training for the type 2As. That doesn’t mean it’s useless to use heavy weights that give you six reps or fewer per set. They’re great for strengthening connective tissue and for activating the type 2B fibers that will transform into 2A fibers with continued training.
Kesiidis, N., et al. (2008). Myosin heavy chain isoform distribution in single fibers of bodybuilders. Eur J Appl Physiol. 103(5):579-83.
©,2015 Jerry Brainum. Any reprinting in any type of media, including electronic and foreign is expressly prohibited
