As people get older, their muscles might not grow as well after weight training because the cellular machinery that builds muscle proteins doesn’t work as hard after a workout.

After a tough workout, older people’s muscles might not build new protein-making machines as well as younger people’s, which could be why they don’t get as strong or muscular over time.

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.

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

Lifting weights makes your muscles bigger and stronger, and this happens because your muscle cells start making more of the tiny machines that build proteins.