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Somewhere in the back of my mind, I have the claim that a muscle never increases its amount of cells but, if the muscle gets bigger, it's simply because individual cells get bigger.

The book Anatomy Trains on page 36 cites "Changes in sarcomere length and physiological properties in immobilized muscle by Williams et al" when it makes the claim :

Stretched, a muscle will attempt to recoil back to its resting length before giving up and adding more cells and sarcomeres to bridge the gap.

Is that true? Do muscles increase the number of their cells in that way?

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  • $\begingroup$ This is what we currently know: gssrr.org/thesis/10.pdf . It suggests that both hypertrophy and hyperplasia happens. You might find the answer about whether stretch causes hyperplasia. $\endgroup$ – inf3rno Nov 11 '14 at 0:27
  • $\begingroup$ @inf3rno - I got to page 22 without seeing hyperplasia. Might you direct me to where in this paper hyperplasia is given as a means to increase muscle mass in adults? $\endgroup$ – anongoodnurse Nov 11 '14 at 22:31
  • $\begingroup$ @anongoodnurse page 7 - "One look at the current state of professional bodybuilding adds substantial evidence to support that hyperplasia actually occurs given the fact that bodybuilders keep adding substantial muscle mass that could not be otherwise gained through the mechanism of hypertrophy. There appears to be a threshold that governs hypertrophy and once the ceiling of adaptation has b een met, other mechanisms for growth such as hyperplasia must be considered (Engel & Franzini - Armstrong, 2004)." - I don't have the time now to read further that's why I did not wrote an answer. $\endgroup$ – inf3rno Nov 12 '14 at 5:32
  • $\begingroup$ @anongoodnurse ctrl+f and "hyperplasia", I run over the text, it says that by the adaptation of untrained people is due to hypertrophy, which has a threshold. after that threshold it is most likely that other mechanisms work, like increasing fiber length, or increasing cell numbers. it claims that currently there is no proof, because we lack of studies on professional bodybuilders. $\endgroup$ – inf3rno Nov 12 '14 at 5:50
  • $\begingroup$ @inf3rno - "One look at the current state of professional bodybuilding adds substantial evidence to support that hyperplasia actually occurs given the fact that bodybuilders keep adding substantial muscle mass that could not be otherwise gained through the mechanism of hypertrophy." I have any number of reputable papers in peer-reviewed journals that refute this. Even this thesis parer does not emphasize that theory (and muscle builders are not scientists). $\endgroup$ – anongoodnurse Nov 12 '14 at 6:22
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The "back of your mind" is correct: "if the muscle gets bigger, it's simply because individual cells get bigger."

Growth of muscle can occur in three ways:

  • by an increase in muscle cell numbers
  • by an increase in muscle fiber diameter
  • by an increase in fiber length.

However, growth in cell numbers is limited to the prenatal and immediately postnatal period, with the animals and man being born with or soon reaching their full complement of muscle cells.

[G]rowth occurs by either hypertrophy of the existing muscle fibers by adding additional myofibrils to increase the muscle mass or by adding new sarcomeres to the ends of the existing muscle fibers to increase their length. Both of these mechanisms occur during the growth process. Growth in the girth of the muscle fibers... may be stimulated by development of stress creating an unequal pressure with splitting at the Z-band and development of additional SR and T-tubule systems. This adds to the diameter or girth of myofibers without any hyperplasia. The growth in length occurs at either end of the fibers and results in addition of new sarcomeres. In both cases, new myofibrillar protein must be synthesized and deposited in the muscle cells.

Adding or removing sarcomeres and myofibrils is determined by exercise, that is, the degree of force a muscle can generate which is in turn dependent on the degree of overlap of the thick and thin filaments. Thus, the amount of tension would control the number of in-series sarcomeres and the number of myofibrils in a single muscle fiber.

Nutrition is also known to play an important role in muscle growth, but growth (that is, increase in girth) cannot occur without exercise.

The exact roles of GF and IGF-I are not completely clear.

Muscle size does not increase by the addition of new muscle cells:

The results show that the increase in muscle cross-sectional area from childhood to adult age is caused by an increase in mean fiber size. This is accompanied by a functional development of the fiber population: the proportion of type 2 fibers increases significantly from the age of 5 (approx. 35%) to the age of 20 (approx. 50%), which, in the absence of any discernible effect on the total number of fibers, is most likely caused by a transformation of type 1 to type 2 fibers.

Activation of satellite cells, a claim made by body builders, is mis-represented by them. The satellite cells seem only to aid severely damaged muscle cells to heal, not to increase in number.

Muscle growth and exercise
Muscle hypertrophy
Growth and development of human muscle: A quantitative morphological study of whole vastus lateralis from childhood to adult age
Regulation of muscle mass by growth hormone and IGF-I

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A tissue can undergo two types of quantitative growth:

  • hypertrophy - cells increase in size
  • hyperplasia - cells increase in number

There are tissues that grow as a result of one of the above processes and there are tissues that grow because both processes happen.

A Google search for muscular growth yields as first result Muscle hypertrophy. And this is the process that happens to muscle tissue of an adult body under physical exercise.

But muscle tissue can undergo hyperplasia too if stimulated by specific hormones like IGF-1 and human growth hormone (Wikipedia:Hyperplasia).

Muscle hyperplasia can occur also under physical activity in animals but this hasn't been proven for humans (Antonio J, Gonyea WJ. 1993). Stretch overload seems to be the most important trigger of hyperplasia (Kelley G., 1985).

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