When young men do a certain type of tough exercise, their fast-twitch muscle fibers show more chemical changes than other fibers, which means different muscle types react differently to hard workouts.

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 guys who don’t normally lift weights do a tough leg workout on an empty stomach, their muscles show a big spike in two specific signals that tell the body to build more muscle, and this might mean doing more sets makes those signals even stronger.

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 young men do a specific type of hard weight training that focuses on lowering weights slowly, it triggers more muscle-building signals in their bodies compared to other common workout styles, even if the workout time is the same.

If untrained guys lift weights on an empty stomach, doing 5 sets of exercise really ramps up a key muscle-building signal in their cells—but just 1 set doesn’t do much, suggesting you need more than a little effort to kickstart muscle growth.