If you stretch or lift weights for 8 weeks, your chest muscle might grow a little, but that growth doesn’t explain much of why you get stronger—other factors are probably doing more of the work.

After nine weeks of lifting weights—whether heavy or light—men who work out casually don’t get bigger pectoral muscles, even though other muscles in their body do grow. This might mean some muscles just don’t respond the same way, or maybe they’re not doing the exercises right.

Stretching your chest muscles regularly might make them stronger—almost as much as lifting weights does—if you do it for 15 minutes, 4 times a week for two months.

Stretching your chest muscles for 15 minutes, 4 times a week for 8 weeks, might build muscle just as much as lifting weights — studies say it could increase muscle thickness by around 6%.

If you're someone who exercises regularly, doing chest workouts 3 times a week for 8 weeks might make your chest muscles about 10% stronger, but it won’t change how far you can move your shoulders.

If young guys lift heavy weights with fewer reps, they’ll likely build more chest and back muscle than if they lift lighter weights with more reps — at least over 8 weeks.