If you lift free weights, you'll get better at lifting free weights. If you use machines, you'll get better at using machines. Practicing the exact same movement makes you better at that movement.

Lifting free weights and using weight machines both make you just as strong when you test your strength in new or different ways — neither one is better than the other.

If you lift weights with your own body and free weights, you'll get stronger at doing just that — not necessarily at using machines. And if you train mostly on machines, you'll get better at those machines. Your body learns the exact movement you practice.

If you train using weight machines and then test your strength on machines, you’ll get stronger than if you trained with free weights — but only because the test matches the training. It’s like practicing basketball shots from the free-throw line and then being tested there: you’ll do better than if you practiced elsewhere.

If you want to get stronger at lifting weights like dumbbells or barbells, training with those same free weights will help you gain more strength than using machines — because your body gets better at the exact movements you practice.

When it comes to building upper-body strength, machines might give you a little more gain than free weights, but for your legs, both work about the same.