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Muscle hypertrophy involves an increase in size of muscle through a growth in size of its cells. Two factors contribute to hypertrophy: sarcoplasmic hypertrophy, which focuses more on increased muscle glycogen storage; and myofibrillar hypertrophy, which focuses more on increased myofibril size.

Muscle hypertrophy involves an increase in size of muscle through a growth in size of its cells. Two factors contribute to hypertrophy: sarcoplasmic hypertrophy, which focuses more on increased muscle glycogen storage; and myofibrillar hypertrophy, which focuses more on increased myofibril size.

In the bodybuilding and fitness community and even in some academic books skeletal muscle hypertrophy is described as being in one of two types: Sarcoplasmic or myofibrillar. According to this hypothesis, during sarcoplasmic hypertrophy, the volume of sarcoplasmic fluid in the muscle cell increases with no accompanying increase in muscular strength, whereas during myofibrillar hypertrophy, actin and myosin contractile proteins increase in number and add to muscular strength as well as a small increase in the size of the muscle.

Sarcoplasmic hypertrophy is greater in the muscles of bodybuilders while myofibrillar hypertrophy is more dominant in Olympic weightlifters.[1] These two forms of adaptations rarely occur completely independently of one another; one can experience a large increase in fluid with a slight increase in proteins, a large increase in proteins with a small increase in fluid, or a relatively balanced combination of the two.

[1] Kraemer, William J.; Zatsiorsky, Vladimir M. (2006). Science and practice of strength training. Champaign, IL: Human Kinetics. p. 50. ISBN 0-7360-5628-9.

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