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.

When you lift weights, your muscles make more of the tiny molecular machines that build proteins, which helps them grow bigger.

When you first start lifting weights, your body makes more of the tiny machines that build muscle proteins, which helps your muscles grow. But after a while, your body shifts focus and gets better at using those machines more efficiently instead of making more of them.

When you lift weights, your muscles grow bigger because your body makes more tiny protein-making machines inside muscle cells, which helps build more of the proteins that make muscles strong and bulky.

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.

When rats lift weights with quick, short reps, their leg muscles make more of the cellular machinery needed to grow bigger, compared to slow, long reps—and this might be why quick reps help muscles grow more.