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 you lift weights, your muscles grow because of the physical pulling and stretching forces they feel—this is thought to be the main reason they get bigger.

Lifting weights makes your muscles grow because the longer and harder you work them, the more they get the signal to build new muscle fibers.

Lifting weights makes your muscles bigger, and the main reason is how much total work you do—like how heavy the weights are, how many sets and reps you do, and how long you hold the tension. More total work = bigger muscles.

When people lift weights until they can't anymore, their muscles grow just as much whether they use heavy or light weights, as long as they push to their max. This shows that how hard you try matters more than how heavy the weights are.

Using external momentum to lift heavier weights during resistance training may still lead to muscle growth in people new to exercise, even if their form is less strict, because the muscles may still experience sufficient tension.