Muscles get bigger when they're put under tension during exercise - like when you lift weights. Only exercises that make your muscles work hard against resistance will help them grow.

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 you move your muscles through their full stretch and contraction during exercise—like fully lowering and raising a dumbbell—you build more muscle than if you only move partway, because the muscle gets stretched under load, which seems to trigger better growth.

When your muscles contract or get stretched, the physical pull they feel is the main reason they grow bigger — it's like the muscle gets a signal from the tugging to grow more muscle fibers.

Your muscles grow bigger not because of just one thing like lifting heavy weights or doing lots of reps, but because of how all the workout factors—like how hard you push, how often you train, and how well you recover—work together.

Your muscles don't grow while you're lifting weights—they grow later, while you rest, because that's when your body repairs tiny tears and builds more muscle tissue using the energy and nutrients you've eaten.