When rats lift weights with different numbers of reps, their muscles show the same early biological changes no matter if they do few or many reps—and those early changes don’t tell us which workout will make muscles grow bigger over time.

If rats lift weights slowly for longer periods, they grow less muscle than if they lift quickly or at a medium pace—even if the total effort is the same—because how fast they move matters for muscle growth.

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 rats do strength exercises for two weeks, their muscles don't get bigger or heavier, even though their muscle-building processes speed up. This might mean the muscles are just reorganizing themselves instead of growing in the short term.

When you lift weights, your muscles temporarily stop building protein, but right after you finish, they kick into high gear to repair and grow—this happens because key signaling molecules in your muscles get activated during recovery.

When rats do a short, intense workout, their muscle-building signals spike a lot right after but fade back to normal within two days.