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.

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.

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.

If untrained young guys do pec deck workouts three times a week for three months, following a specific plan, their chest muscles get noticeably thicker—by about 17 to 21%—in both the upper and lower parts of the chest.

If you're new to working out, doing just one set of chest machine exercises three times a week gives about the same muscle growth as doing three sets — both lead to around a 20% bigger chest muscle after 12 weeks.

Doing push-ups for four weeks won’t make your chest muscles thicker, even if you get stronger at doing them.