Here's a bit more about it:

* Hypertrophy is an increase in the size of muscle cells (also known as muscle fibers) due to an increase in the amount of protein within the cell. This happens in response to physical activity, such as weightlifting or endurance training.

* Hyperplasia is an increase in the number of muscle cells. This is less common in humans, but it does occur in some situations, such as during fetal development.