Ex Phys Home
Hypertrophy is an increase in size. With the muscle fiber this size is an increase in diameter of the muscle fiber.
or cross-sectional area of the muscle fiber. Therefore the changes causing this are as follows;
In addressing this question, always refer to what increases, what doesn't change and what is debateable.
1) actin and myosin increase in diamter size
2) actin and myosin also increase in number
3) there is also an increase in the sarcoplasm in the muscle cell
4) In humans there is no change in the number of muscle fibers
5) Hyperplasia (the longitudinal splitting of one fiber into two fibers) from training is a debatable issue in Exercise Science

In the first ~8 weeks of resistance training there are mimimal hypertrophy changes. However,a person is getting stronger.
This is reflective of neural drive. With neural drive, 1) more motor units are firing, 2) synergistic (other helper muscles)
are helping with the force production, and 3) themotor units are firing with greater synchronicity
(i.e., simultaneousness or harmony).