What is Hypertrophy?

Muscle hypertrophy is the increase in the size of skeletal muscle, driven by the growth of its component cells. In fitness literature, hypertrophy is often broken down into two categories:

• Myofibrillar hypertrophy: An increase in the size and number of the contractile proteins (myofibrils) within the muscle cell.
• Sarcoplasmic hypertrophy: An expansion of the sarcoplasm—the fluid, energy substrates, and non-contractile proteins surrounding the myofibrils.

While both processes contribute to overall muscle size, the primary driver of functional, dense muscle tissue is myofibrillar hypertrophy. For the purposes of this book, when we talk about hypertrophy we are focusing on myofibrillar hypertrophy, which is the most effective way to build both size and strength that lasts long term.