Just stretching your muscle under light weight won’t make it grow — you need to push hard while it’s stretched out, like doing a bicep curl slowly at the bottom where it’s fully extended.

Even if you lift heavier weights with your muscles mostly bent, you don’t grow bigger muscles than if you lift lighter weights with your muscles stretched out — the stretch matters more than how heavy the weight is.

Lifting weights when your muscles are stretched out more (like at the bottom of a squat or a full bicep curl) tends to make them grow bigger than lifting with your muscles mostly shortened, as long as there's still resistance pushing back when they're stretched.

When you do strength exercises where your muscles are stretched the most (like lowering a weight fully), you build more muscle than when you only move partway through the motion, no matter which muscle you're working.

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.

If you're already strength-trained, doing full lifts (like fully curling a dumbbell) won't make your muscles grow more than just doing the stretchy part of the movement—so maybe stretching your muscles under load is the real secret to getting bigger.