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.

After nine weeks of weight training with either heavy or light weights to muscle failure, the chest muscles of men who exercise recreationally do not grow significantly larger.

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%.

Doing a lot of stretching can make you stronger without necessarily making your muscles bigger. This suggests the strength gains come from your nervous system and muscle structure working better together, rather than just from building more muscle mass.