What is muscle hypertrophy?

In one line

Muscle hypertrophy is the growth of a muscle that happens when each of its cells becomes thicker, for example through training.

What is muscle hypertrophy?
What exactly grows in 'hypertrophy training'?
At the gym you often hear 'hypertrophy routine' or 'hypertrophy training'. People use the word when they talk about training to build muscle. But what actually changes inside the body when a muscle gets bigger?
In one line
A muscle getting thicker and bigger
Muscle hypertrophy means a muscle getting thicker and bigger. In biology, hypertrophy means a tissue grows because its cells get bigger. The number of muscle cells does not go up; each cell gets thicker.
What is muscle made of?
Bundles of long, thread-like cells
If you split a muscle into tiny parts, you find countless long, thread-like cells. These cells are called muscle fibers. Many muscle fibers are tied into bundles, and the bundles together make one muscle.
Inside a muscle fiber
Packed with protein strands that pull
One muscle fiber holds hundreds to thousands of very thin strands made of protein. These strands are called myofibrils. When the myofibrils shorten, the whole fiber shortens, pulls on the bone, and the body moves.
How it grows
More myofibrils make a thicker fiber
Strength training, where you lift or push heavy things, makes the fiber build more myofibrils. With more strands, the fiber gets thicker, and the whole muscle gets bigger. That is muscle hypertrophy.
Thickness, not number
No new muscle fibers are made
When a muscle grows, new fibers are not added. The fibers you already have each get thicker. A tissue growing because its cells multiply is a different process with a different name.
Building and breaking down
Build more than you break down
Muscle protein is always being made and broken down at the same time. Making new protein is called synthesis, and breaking it down is called breakdown. When synthesis stays higher than breakdown, the muscle grows, and training and eating shift this balance.
The opposite is atrophy
Unused muscle shrinks
The opposite of hypertrophy is atrophy. When a muscle is not used, protein is lost from the fibers and the muscle gets smaller. When muscle mass and strength both drop a lot with aging, the condition is called sarcopenia.
How long does it take?
Seen on scans by about 3 weeks, mixed with swelling
About 3 weeks after starting strength training, the front thigh muscle was measurably thicker on scans. But part of the growth at 3 weeks was swelling of the muscle after training. After 10 weeks, the muscle was still thicker once the swelling had gone.
A common myth
Does muscle turn into fat when you stop training?
No. Muscle is made of muscle fibers, and fat is made of fat cells that store oil; they are different tissues. When you stop training, muscle shrinks through atrophy, and fat grows separately if you eat more than you use.
Summary
Hypertrophy is muscle fibers getting thicker
Muscle hypertrophy is a muscle getting thicker because strength training adds myofibrils inside its fibers. Muscle does not turn into fat when you stop; it only shrinks through atrophy. Related terms: muscle protein synthesis, sarcopenia, muscle fiber types.