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.

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.

If you stretch your chest muscles regularly for about 15 minutes, 4 times a week, for 8 weeks, you’ll likely get more shoulder flexibility—more than if you just did strength training.

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.

If you're a young guy who hasn't trained before and you do pec deck exercises for 12 weeks, your chest muscles will get thicker—about 17–18% thicker in the upper part and 21% thicker in the lower part—no matter if you do just one set or three sets each workout.