Effects of repetition duration on skeletal muscle hypertrophy in a rat model of resistance exercise.

Your muscles don't grow while you're lifting weights—they grow later, while you rest, because your body uses that time to repair and build new muscle tissue after the workout stresses them.

The total amount of work your muscles do when lifting weights — how hard and how long you push — decides how strong the muscle-building signals get turned on.

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 with very slow, long reps, they don't push as hard as when they do faster reps—even if the total effort is the same—and that might be why their muscles don't grow as much.