People who build more muscle from weight training tend to make more ribosomes—the tiny machines in cells that build proteins—compared to those who don’t gain as much muscle. One study found big gains...

When people lift weights and their muscles get bigger, their cells make more of the tiny machines (ribosomes) that build muscle proteins — and the people who gain the most muscle tend to make the most of these machines.

Some people’s muscles grow way more than others when they lift weights, and this might be because their muscle cells make more of the tiny machines (ribosomes) that help build muscle protein.

When older people do strength training for four weeks, those who get much bigger muscles also see a big jump in the cellular machinery that helps build muscle, while people who don’t gain muscle don’t see this change—so it seems like making more of this machinery is tied to getting stronger and bigger.

People who gain more muscle from weight training also tend to have more RNA in their muscles, but this only explains a tiny part—about 8%—of why they got bigger. So while RNA levels go up, they’re not really the main reason muscles grow.

When you lift weights, your muscles grow bigger because your cells first make more protein-making machines (ribosomes) — and the more of these machines you build, the more your muscles grow.