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.

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

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.

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 in these protein-makers in strong responders, and lab studies back up that this process helps muscle grow.

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 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.