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Washington.edu reported that:

Muscle cell number is affected by exercise level while young

and elitefitness.com said:

a trainee in the early stages of training would increase cell number

Where an unreferenced page on wikipedia said the opposite:

Contrary to popular belief, the number of muscle fibres cannot be increased through exercise.

Which one is true, I'm a little bit confused?

Note: I'm not asking about the increase in the cells' myofibril count, but the increase in the numbers of cells themselves.

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  • The question seems confusing to me. The wikipedia claim you find contradictory seems to be not about muscle cells, but about muscle fibries, i.e. myofibrils you are specifically writing you are not interested about. Can you clarify?
    – Suma
    Commented Mar 30, 2015 at 7:43
  • Was there any reason you chose to post this as a new question, rather than fix the existing one?
    – Oddthinking ♦
    Commented Mar 30, 2015 at 8:43
  • I lost my account cookie, so I'm leaving a response to Suma as a comment. You said: "be not about muscle cells, but about muscle fibries, i.e. myofibrils". As far as I can tell, muscle fibers is another word for muscle cells, and myofibrils are different.
    – user25618
    Commented Mar 30, 2015 at 14:56
  • And, to @Oddthinker, I asked a new question because despite there being a notable claim, the other question was closed based on its motivation (homework). I didn't know how to fix that, since it will have always been asked to solve a homework problem.
    – user25618
    Commented Mar 30, 2015 at 15:00

1 Answer 1

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It is evident that muscle growth happens through exercise. This article, Muscle growth and exercise explains that there are three mechanisms involved:

  1. by an increase in muscle cell numbers,
  2. by an increase in muscle fiber diameter, and
  3. by an increase in fiber length.

So, your question is whether the first mechanism is affected by exercise. The others are not related to the count of the cells, but how large each cell is.

The abstract goes on to explain:

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.

So, the answer to the question is no, exercise does not increase the number of muscle cells.

Exercise does, however, increase muscle mass, by increasing the size and count of the components that make up each cell:

Thus, growth 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 appears to take place by splitting of the myofibrils. This 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.

What determines adding sarcomeres and myofibrils?

You guessed right: Exercise.

Exercise stimulates the increase in myofibrils which increase the overall size of muscle cells.

Well exercised muscles can not only add more size but can also develop more mitochondria, myoglobin, glycogen and a higher density of capillaries.

Nutrition (especially protein intake) also plays an important role in muscle growth, but growth cannot occur without exercise.

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