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According to this website, the number of myofibrils in a muscle can increase with exercise:

Thus, the muscle cells’ myofibrils will increase in thickness and number.

I had a look on google scholar and I could find very limited evidence confirming this. Is it true that exercise can increase the number of myofibrils in a human (I found one article, but it referenced rats not humans)?

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2 Answers 2

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According to the well respected Medical Textbook Guyton and Hall, Textbook of Medical Physiology, Unit XV, "Sports Physiology", under the subheading "Muscle Hypertrophy",

The changes that occur inside the hypertrophied muscle fibers themselves include (1) increased numbers of myofibrils, proportionate to the degree of hypertrophy, (2) up to 120 per cent increase in mitochondrial enzymes ...

I would therefore say that the number of myofibrils do increase due to resistance training.

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The References contains a list of review articles supporting the written text. The one that probably fits with this section on muscle growth is "Rennie, Control of the size of human muscle mass. Annu Rev Physiol 66:799, 2004." I found this online, but only abstract is available without subscription. –  Mew Jan 17 '13 at 3:54
    
DOI link to that Rennie paper –  EnergyNumbers Mar 1 '13 at 14:03
    
Thanks for that –  Mew Mar 1 '13 at 14:09
    
"Muscle fibers are permanently differentiated; therefore, they are incapable of mitotic activity to produce additional myonuclei in times of increased protein synthesis and muscle growth. Yet, myonuclear number increases during skeletal muscle hypertrophy, thereby maintaining the myonuclear domain ... The predominant source of the additional myonuclei is satellite cells that are localized in indentations in the sarcolemma beneath the basal lamina. Less commonly, satellite cells may possibly fuse with each other to form new fibers (hyperplasia)" –  EnergyNumbers Mar 1 '13 at 14:39
    
I think that's how the new fibres come about - more news shortly. Might be worth asking over on Biology –  EnergyNumbers Mar 1 '13 at 14:42

Text before "Thus, the muscle cells’ myofibrils will increase in thickness and number." is talking about satellite cells fuse with myofibrils, and nothing relates to the number of myofibrils increased is mentioned. No relevance citation support his statement so we should ignore it.

For your reference, excerpt from an article of the same research group says:

It is important to emphasize the point that this process is not creating more skeletal muscle fibers (in humans), but increasing the size and number of contractile proteins (actin and myosin) within the muscle fiber.

It is sad that no evidence for human's myofibrils can divide. Even rat's heart muscle cells divide but it only occur in vitro, not in vivo.

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By the way muscle fibers does not equal muscle myofibrils, so your quote is useless. Muscle fibers are composed of myofibrils, so it's certainly possible that the number of myofibrils increase, while the number of muscle fibers remains the same. –  Mew Jan 16 '13 at 23:27
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Furthermore, wikipedia states "Satellite cell activation has also been demonstrated on an ultrastructural level following exercise.", which I'm sure can be verified with simple research. Activation of satellite cells surely increases the number of myofibrils. Your answer seems to focus on cell division, however new myofibrils formed by satellite cells is still new myofibrils, and my question doesn't ask specifically about cell division. –  Mew Jan 16 '13 at 23:32

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