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Muscle Fiber Hypertrophy vs. Hyperplasia
Jose Antonio PhD
WHAT IS HYPERPLASIA?
Hypertrophy refers to an increase in the size of the cell while hyperplasia
refers to an increase in the number of cells or fibers. A single muscle cell is
usually called a fiber.
HOW DO MUSCLE FIBERS ADAPT TO DIFFERENT TYPES OF EXERCISE?
If you look at a good marathon runner's physique and compared him/her to a
bodybuilder it becomes obvious that training specificity has a profound effect.
We know that aerobic training results in an increase in mitochondrial
volume/density, oxidative enzymes, and capillary density (27). Also, in some
elite endurance athletes the trained muscle fibers may actually be smaller than
those of a completely untrained person. Bodybuilders and other strength-power
athletes, on the other hand, have much larger muscles (14,40). That's their
primary adaptation, their muscles get bigger! All the cellular machinery related
to aerobic metabolism (i.e., mitochondria, oxidative enzymes, etc.) is not
necessary for maximal gains in muscle force producing power, just more
contractile protein. We know that this muscle mass increase is due primarily to
fiber hypertrophy; that is the growth of individual fibers, but are their
situations where muscles also respond by increasing fiber number?
EVIDENCE FOR HYPERPLASIA
Scientists have come up with all sorts of methods to study muscle growth in
laboratory animals. You might wonder what relevance this has to humans. Keep in
mind that some of the procedures which scientists perform on animals simply
cannot be done on humans due to ethical and logistical reasons. So the more
convincing data supporting hyperplasia emerges from animal studies. Some human
studies have also suggested the occurence of muscle fiber hyperplasia. I'll
address those studies later.
DOES STRETCH INDUCE FIBER HYPERPLASIA?
This animal model was first used by Sola et al. (38) in 1973. In essence, you
put a weight on one wing of a bird (usually a chicken or quail) and leave the
other wing alone. By putting a weight on one wing (usually equal to 10% of the
bird's weight), a weight-induced stretch is imposed on the back muscles. The
muscle which is usually examined is the anterior latissimus dorsi or ALD (unlike
humans, birds have an anterior and posterior latissimus dorsi). Besides the
expected observation that the individual fibers grew under this stress, Sola et
al. found that this method of overload resulted in a 16% increase in ALD muscle
fiber number. Since the work of Sola, numerous investigators have used this
model (1,2,4-8,10,19,26,28,32,43,44). For example, Alway et al. (1) showed that
30 days of chronic stretch (i.e., 30 days with the weight on with NO REST)
resulted in a 172% increase in ALD muscle mass and a 52-75% increase in muscle
fiber number! Imagine if humans could grow that fast!
More recently, I performed a study using the same stretch model. In addition, I
used a progressive overload scheme whereby the bird was initally loaded with a
weight equal to 10% of the its weight followed by increments of 15%, 20%, 25%,
and 35% of its weight (5). Each weight increment was interspersed with a 2 day
rest. The total number of stretch days was 28. Using this approach produced the
greatest gains in muscle mass EVER recorded in an animal or human model of
tension-induced overload, up to a 334% increase in muscle mass with up to a 90%
increase in fiber number (5,8)! That is pretty impressive training
responsiveness for our feathered descendants of dinosaurs.
But you might ask yourself, what does hanging a weight on a bird have to do with
humans who lift weights? So who cares if birds can increase muscle mass by over
300% and fiber number by 90%. Well, you've got a good point. Certainly, nobody
out there (that I know of), hangs weights on their arms for 30 days straight or
even 30 minutes for that matter. Maybe you should try it and see what happens.
This could be a different albeit painful way to "train." But actually the
physiologically interesting point is that if presented with an appropriate
stimulus, a muscle can produce more fibers! What is an appropriate stimulus? I
think it is one that involves subjecting muscle fibers to high tension overload
(enough to induce injury) followed by a regenerative period.
WHAT ABOUT EXERCISE?
The stretch induced method is a rather artificial stimulus compared to normal
muscle activity. What about "normal" muscular exercise? Several scientists have
used either rats or cats performing "strength training" to study the role of
muscle fiber hyperplasia in muscular growth
(9,13,17,18,20-22,25,33,34,39,41,42). Dr. William Gonyea of UT Southwestern
Medical Center in Dallas was the first to demonstrate exercised-induced muscle
fiber hyperplasia using weight-lifting cats as the model (20,21,22). Cats were
trained to perform a wrist flexion exercise with one forelimb against resistance
in order to receive a food reward. The non-trained forelimb thus served as a
control for comparison. Resistance was increased as the training period
progressed. He found that in addition to hypertrophy, the forearm muscle (flexor
carpi radialis) of these cats increased fiber number from 9-20%. After examining
the training variables that predicted muscle hypertrophy the best, scientists
from Dr. Gonyea's laboratory found that lifting speed had the highest
correlation to changes in muscle mass (i.e., cats which lifted the weight in a
slow and deliberate manner made greater muscle mass gains than cats that lifted
ballistically) (33).
Rats have also been used to study muscle growth (25,39,47). In a model developed
by Japanese researchers (39), rats performed a squat exercise in response to an
electrical stimulation. They found that fiber number in the plantaris muscle (a
plantar flexor muscle on the posterior side of the leg) increased by 14%.
Moreover, an interesting observation has been made in hypertrophied muscle which
suggests the occurrence of muscle fiber hyperplasia (13, 17, 28, 47). Individual
small fibers have been seen frequently in enlarged muscle. Initially, some
researchers believed this to be a sign of muscle fiber atrophy. However, it
doesn't make any sense for muscle fibers to atrophy while the muscle as a whole
hypertrophies. Instead, it seems more sensible to attribute this phenomenon to
de novo formation of muscle fibers (i.e., these are newly made fibers). I
believe this is another piece of evidence, albeit indirect, which supports the
occurrence of muscle fiber hyperplasia.
EXERCISE-INDUCED GROWTH IN HUMANS
The main problem with human studies to determine if muscle fiber hyperplasia
contributes to muscle hypertrophy is the inability to make direct counts of
human muscle fibers. Just the mere chore of counting hundreds of thousands of
muscle fibers is enough to make one forget hopes of graduating! For instance,
one study determined that the tibialis anterior muscle (on the front of the leg)
contains approximately 160,000 fibers! Imagine counting 160,000 fibers (37), for
just one muscle! The biceps brachii muscle likely contains 3 or 4 times that
number!
So how do human studies come up with evidence for hyperplasia? Well, it's
arrived at in an indirect fashion. For instance, one study showed that elite
bodybuilders and powerlifters had arm circumferences 27% greater than normal
sedentary controls yet the size (i.e., cross-sectional area) of athlete's muscle
fibers (in the triceps brachii muscle) were not different than the control group
(47). Nygaard and Neilsen (35) did a cross-sectional study in which they found
that swimmers had smaller Type I and IIa fibers in the deltoid muscle when
compared to controls despite the fact that the overall size of the deltoid
muscle was greater. Larsson and Tesch (29) found that bodybuilders possessed
thigh circumference measurements 19% greater than controls yet the average size
of their muscle fibers were not different from the controls. Furthermore, Alway
et al. (3) compared the biceps brachii muscle in elite male and female
bodybuilders. These investigators showed that the cross-sectional area of the
biceps muscle was correlated to both fiber area and number. Other studies, on
the other hand, have demonstrated that bodybuilders have larger fibers instead
of a greater number of fibers when compared to a control population (23,30,36).
Some scientists have suggested that the reason many bodybuilders or other
athletes have muscle fibers which are the same size (or smaller) versus
untrained controls is due to a greater genetic endowment of muscle fibers. That
is, they were born with more fibers. If that was true, then the intense training
over years and decades performed by elite bodybuilders has produced at best
average size fibers. That means, some bodybuilders were born with a bunch of
below average size fibers and training enlarged them to average size. I don't
know about you, but I'd find that explanation rather tenuous. It would seem more
plausible (and scientifically defensible) that the larger muscle mass seen in
bodybuilders is due primarily to muscle fiber hypertrophy but also to fiber
hyperplasia. So the question that needs to be asked is not whether muscle fiber
hyperplasia occurs, but rather under what conditions does it occur. I believe
the the scientific evidence shows clearly in animals, and indirectly in humans,
that fiber number can increase. Does it occur in every situation where a muscle
is enlarging? No. But can it contribute to muscle mass increases? Yes.
HOW DOES MUCLE FIBER HYPERPLASIA OCCUR?
There are two primary mechanism in which new fibers can be formed. First, large
fibers can split into two or more smaller fibers (i.e., fiber splitting)
(6,25,39). Second satellite cells can be activated (11,16,17,43,44).
Satellite cells are myogenic stem cells which are involved in skeletal muscle
regeneration. When you injure, stretch, or severely exercise a muscle fiber,
satellite cells are activated (16,43,44). Satellite cells proliferate (i.e.,
undergo mitosis or cell division) and give rise to new myoblastic cells (i.e.,
immature muscle cells). These new myoblastic cells can either fuse with an
existing muscle fiber causing that fiber to get bigger (i.e., hypertrophy) or
these myoblastic cells can fuse with each other to form a new fiber (i.e.,
hyperplasia).
ROLE OF MUSCLE FIBER DAMAGE
There is now convincing evidence which has shown the importance of eccentric
contractions in producing muscle hypertrophy (15,24,45,46). It is known that
eccentric contractions produces greater injury than concentric or isometric
contractions. We also know that if you can induce muscle fiber injury, satellite
cells are activated. Both animal and human studies point to the superiority of
eccentric contractions in increasing muscle mass (24,45,46). However, in the
real world, we don't do pure eccentric, concentric, or isometric contractions.
We do a combination of all three. So the main thing to keep in mind when
performing an exercise is to allow a controlled descent of the weight being
lifted. And on occasion, one could have his/her training partner load more
weight than can be lifted concentrically and spot him/her while he/she performs
a pure eccentric contraction. This will really put your muscle fibers under a
great deal of tension causing microtears and severe delayed-onset muscle
soreness. But you need that damage to induce growth. Thus, the repeated process
of injuring your fibers (via weight training) followed by a recuperation or
regeneration may result in an overcompensation of protein synthesis resulting in
a net anabolic effect (12,31).
HAS THE DEBATE BEEN SETTLED?
In my scientific opinion, this issue has already been settled. Muscle fiber
hyperplasia can contribute to whole muscle hypertrophy. There is human as well
as rat, cat, and bird data which support this proposition
(1-3,5-8,13,17,20-22,25,29,35,37,47), a veritable wild kingdom of evidence. Does
muscle fiber hyperplasia occur under all circumstances? No. There are several
studies which show no change in fiber number despite significant increases in
muscle mass (4,18,19,23,26,30,36,41). Is it possible that certain muscles can
increase fiber number more so than others? Maybe. Can any Joe Schmoe off the
street who lifts weights to get in better shape increase the number of fibers
for instance in their biceps? Probably not. What about the elite bodybuilder who
at 5'8" tall is ripped at a body weight of 250 lbs.? Are his large muscles
purely the result of muscle fiber hypertrophy? I think it would be extremely
naive to think that the massive size attained by elite bodybuilders is due
solely to fiber hypertrophy! There is nothing mystical about forming new muscle
fibers. Despite the contention that fiber number is constant once you're born
(18,19), we now have an abundance of evidence which shows that muscle fiber
number can increase. Besides, there is nothing magical at birth which says that
now that you're out of the womb, you can no longer make more muscle fibers! A
mechanism exists for muscle fiber hyperplasia and there is plenty of reason to
believe that it occurs. Of course, the issue is not whether fiber number
increases after every training program, stress, or perturbation is imposed upon
an animal (or human). The issue is again, under which circumstances is it most
likely to occur. For humans, it is my speculation that the average person who
lifts weights and increases their muscle mass moderately probably does not
induce fiber hyperplasia in their exercised muscle(s). However, the elite
bodybuilder who attains the massive muscular development now seen may be the
more likely candidate for exercise-induce muscle fiber hyperplasia. If you are
interested in a comprehensive scientific treatise on this subject, read a
scientific review article that I wrote a few years ago (7).
KEY TERMS
anabolic - in reference to muscle, a net increase in muscle protein
catabolic - in reference to muscle, a net decrease in muscle protein
concentric - shortening of a muscle during contraction
eccentric - lengthening of a muscle during contraction
hyperplasia - increase in cell number
hypertrophy - increase in cell size
isometric - no change in muscle length during a contraction
mitochondria - is an organelle ("little organ") found within cells and is
involved in generating ATP via aerobic processes
muscle fiber - also known as a myofiber; is the multinucleated cell of skeletal
muscle
myoblast - an immature muscle cell containing a single nucleus
myogenesis - the development of new muscle tissue, esp. its embryonic
development
satellite cell - are the cells responsible in part for the repair of injured
fibers, the addition of myonuclei to growing fibers, and for the formation of
new muscle fibers.
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